Do our sun and moon have names?

Do our sun and moon have names?

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We seem to have named every moon orbiting other planets. Why haven't we named our own moon? And for that matter, why doesn't our sun have a name since we name or number stars?

"The Sun" is fine as long as you're not leaving our solar system; less so "the Moon" when there are hundreds of planetary satellites in this system alone.

In science fiction as varied as Isaac Asimov and"Star Trek", the names are Sol and Luna.

EDIT: My point is that sci-fi authors are writing from the point of view of societies with many "suns" and "moons", and those societies have adopted the classical terms as the "current" official names.

Well, even though one may encounter some "poetic" names such as Sol or Luna, I believe these bodies are referred to as "the Sun" and "the Moon", and that is the only formal name I've heard of.
As for the reason, I'd say it's because there is no need for special names like Sol because they aren't really used in practice (we've known of the Sun and the Moon for quite a bit), while it is quite important to name newly discovered objects so we would know what to refer to them as.

What is a name?

A name is a word, that is reasonably unique, that is used to identify a person or thing. When a child is born there is no word that identifies it, and so its parents have to choose a "name". Similarly when a new astronomical object, such as an asteroid, is discovered, it has no word that identifies it and so it is assigned first a number and later a name.

But there is already a word that identifies our sun and moon: "the Sun" and "the Moon". There is no need for a new name because they already have a name. Notice that English (unlike some other languages) gives us a typographical clue that these are names, because the first letter is capitalised. The Moon is a moon of the Earth and the Earth is a planet orbiting the Sun.

Earth, Moon and Sun are the correct names in English.

Of course the Sun and the Moon have names. The names of those objects in English are "the Sun" and "the Moon". Note the use of a definite article ("the") and the capitalization of the names themselves to indicate a proper name. The Sun and the Moon have many names, at least one name for each for almost every language ever spoken. The translations of those myriad names into English are inevitably "the Sun" and "the Moon". Being the two most obvious celestial objects, those names have been around for a long, long time, almost certainly predating writing.

On the other hand, the knowledge that the Earth is a planet that orbits the Sun is rather new, less than 500 years old. The knowledge that other planets have moons is newer yet, about 400 years old. The knowledge that the Sun is a star is even newer, less than 200 years old. Giordano Bruno was burnt at the stake in 1600 for suggesting that the Sun is a star. It wasn't until the 1830s when scientists were able to measure the enormous distance to even the closest stars.

Because there's no need to. "Sun" and "Moon" are relative terms and it's clear which bodies we mean (as long as we say the Sun / the Moon) when on Earth. If you stood on Proxima b's surface, Proxima Centauri would be your Sun, Proxima b's spherical satellite (if it has one) would be your Moon, and the Earth's Sun would be an average star in the night sky we'd have to find a name for in such case (e.g. Cassiopeia VI since it would enlarge the constellation of Cassiopeia by another star). But as long as you're on Earth, it doesn't matter.

You can even use the term "Earth" relative for another planet you're on in the sense of "land", just like we talk about a "geology of Mars" (geos is Greek for 'Earth').

It is named 'Sol' and the moon was named 'Luna'. Hence the term Lunar explorer and 'Solar' System…

Other cultures have used different names for our Sun and Moon (for thousands of years) - So actually they have multiple names each.

Most people refer to them simply as "The Sun" & "The Moon".

I did not find any definite reference, but if the Sun and the Moon had any other official name, it would have come from the International Astronomical Union (IAU). The only reference I found was a mention in passing of Sun and Moon (capitalized). There may be more information hiding in some publications. But if they had another official name for these two celestial bodies, it would seem reasonable to expect that it would be featured fairly prominently on their Web site.

It would in fact make some sense to have distinctive names, to avoid statements like "Alpha Centauri is the sun of another planetary system" where sun does not refer to our Sun. That may have been the idea behind the notion that there is a distinction between "Sun" and "Sol". But it does not look like the IAU backs this up.

As best as I can tell, that idea may have originated in Hollywood, specifically Star Trek. Since I do not have any definite information to back that up, take that with a grain of salt.

There isn't a general naming scheme. However, we can leverage a pattern for naming the apsis of orbits. We historically named these on a per-host body basis, so apogee and perigee were the names for the furthest and closest an orbit gets to the Earth, respectively. The -gee suffix is derived from Gaia, so that would be the name for our planet. The solar apsis are apohelion and perihelion, the -helion suffix being derived from "Helios," the Greek god of the sun. Lunar apsis have used -lune, -cynthion and -selene, deriving from names Luna, Cynthia, and Selene.

These are, of course, just one of many names that could be used, but the pattern of using historical gods would have value in such a situation. We might talk of a "new Earth" or a "new Sun" to describe a planet or a star that is similar in characteristics to our planet or star, so those names might shift from nouns to adjectives, describing planets or stars. However, there is little reason for a historical deity to be associated with a celestial body that was never seen by that historical society. It is unlikely names like Luna or Helios would ever be applied to bodies outside our solar system. Whether they would get applied to bodies in our solar system is a question for the linguists. It would depend highly on how we adapt other words like "sun" and "sol" in a multi-solar-system society.

And that linguistic adaptive approach would answer your question as to why we don't have names for them. Currently "sun" is sufficiently descriptive that it acts like a noun in our speech and needs no additional context. "The moon," with the pronoun "the" included is sufficiently descriptive. For the ancient Greeks, there wasn't even a need to identify the Earth's moon from a moon of Jupiter, because they didn't know Jupiter had moons.

For all I know, we'll name "the Sun" "Sun_00001" to disambiguate. But there does seem to be a history of leveraging the ancient deities when disambiguation is needed in a language.

Do our sun and moon have names? - Astronomy

How do planets and their moons get their names?


The official names of planets and their moons are governed by an organization called the International Astronomical Union (IAU). The IAU was established in 1919. Its mission is "to promote and safeguard the science of astronomy in all its aspects through international cooperation". Its individual members are professional astronomers from all over the World. The IAU is the internationally recognized authority for assigning names to celestial bodies and any surface features on them.

The IAU recognizes that astronomy is an old science and many of its names come from long-standing traditions and/or are founded in history. For many of the names of the objects in the solar system, this is especially so. Most of the objects in our solar system received names long ago based on Greek or Roman mythology. The IAU has therefore adopted this tradition in its rules for naming certain types of objects in the solar system.


With the exception of Earth, all of the planets in our solar system have names from Greek or Roman mythology. This tradition was continued when Uranus, Neptune, and Pluto were discovered in more modern times.

  • Mercury is the god of commerce, travel and thievery in Roman mythology. The planet probably received this name because it moves so quickly across the sky.
  • Venus is the Roman goddess of love and beauty. The planet is aptly named since it makes a beautiful sight in the sky, with only the Sun and the Moon being brighter.
  • Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, many other names for our planet in other languages.
  • Mars is the Roman god of War. The planet probably got this name due to its red color.
  • Jupiter was the King of the Gods in Roman mythology, making the name a good choice for what is by far the largest planet in our solar system.
  • Saturn is the Roman god of agriculture.
  • Uranus is the ancient Greek deity of the Heavens, the earliest supreme god.
  • Neptune , was the Roman god of the Sea. Given the beautiful blue color of this planet, the name is an excellent choice!
  • Pluto is the Roman god of the underworld in Roman mythology. Perhaps the planet received this name because it's so far from the Sun that it is in perpetual darkness.

Natural Satellites

For those moons have been known for a long time (such as the Galilean moons of Jupiter), the names were assigned from mythological characters. For example, the moons of Jupiter were named for characters who had roles in the life of Zeus (the Greek mythology counterpart of the Roman God Jupiter).

For recently discovered natural satellites of the planets, they are first given a "provisional" or temporary name while additional observations are made to confirm their existence. This temporary name (usually consisting of the year of discovery and some number indicating the order of discovery in that year) is assigned by an organization called the Central Bureau for Astronomical Telegrams (CBAT). For example, when Voyager 2 found a bunch of new moons in its 1989 Neptune encounter, they were named S/1989 N 1, S/1989 N 2, etc. When the existence of the object is confirmed (and its orbit determined), it is given a final name. The name is suggested by the discoverer(s), but following tradition is strongly encouraged.

Note that the moons of Uranus are a special case in our solar system. They are named after literary characters (from works by William Shakespeare and Alexander Pope) rather then characters from mythology.

Surface Features

Landscape features on planets and natural satellites follow a set of complicated conventions set by the IAU Nomenclature Committee. The rules set restrictions on allowable names such as: a planetary feature may not bear the name of a living person or of a political or religious figure from the last 200 years.

Naming of Solar System Objects and Features

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919 (also see Resolution 13 of the Fourth United Nations Conference on the Standardization of Geographical Names, held in Geneva in 1982). The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.

Major Planets and the Moon

The eight major planets in our Solar System and Earth's satellite have official IAU names. The names of the major planets were already in common use when the IAU formed in 1919 (e.g. scientifically, in professional and amateur astronomy literature, in nautical almanacs, etc.). However, the names of the planets have been included in wording for IAU resolutions multiple times since the IAU's founding and these names can be considered formally adopted by the IAU membership. While there are cultural names for the planets and Earth's satellite in other languages, there are classic names for the major planets and Moon which appear in English language IAU resolutions and the IAU Style Manual (which was approved by an IAU resolution in 1988).

What follows is a partial list of instances of use of these planet names, but it is by no means exhaustive. This compilation demonstrates, however, that the names of the planets and Moon have appeared in IAU resolutions (or in wording of documents approved by IAU resolution) approved by IAU General Assemblies multiple times, and continue to be in ubiquitous use.

1976: The names of the then major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) and the Earth's satellite (Moon) appear in IAU Resolution No. 10, which was approved by the XVIth General Assembly of the IAU in Grenoble, France in 1976.

1988: The IAU XXth General Assembly — which met in Baltimore in 1988 — approved IAU Resolution A3 on the Improvement of Publications, which recognized "the importance of identifying astronomical objects by clear and unambiguous designations" and recommended "that the authors and editors of the astronomical literature adopt the recommendations in the IAU Style Manual". The "IAU Style Manual (1989): The Preparation of Astronomical Papers and Reports" by George A. Wilkins (President of IAU Commission 5) was published in December 1988, and reprinted as Chapter VIII ("IAU Style Book") in the "Transactions of the International Astronomical Union Vol. XXB: Proceedings of the Twentieth General Assembly Baltimore 1988" (1990 ed. Derek McNally Kluwer Academic Publishers Dordrecht). The IAU Style Manual Sec. 5.25 lists the names of the "principal planets" as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

2006: IAU Resolution B5 (Definition of a Planet in the Solar System) explicitly lists the eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. While Resolution B5 defined the category of dwarf planet, Resolution B6 explicitly lists Pluto as an example.

One also sees these planet names are used ubiquitously by modern IAU working groups (e.g. IAU Working Group for Planetary System Nomenclature, the IAU Working Group on Cartographic Coordinates and Rotational Elements in their recent report by Archinal et al. 2011 Celestial Mechanics and Dynamical Astronomy, Vol. 109, Issue 2, pp. 101-135, etc.).

So the IAU does recognize official names for the major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and Earth's satellite (Moon).

Planetary Features

Planetary nomenclature, like terrestrial nomenclature, is used to uniquely identify a feature on the surface of a planet or satellite so that the feature can be easily located, described, and discussed. The procedure is as follows:

  • When the first images of the surface of a planet or satellite are obtained, themes for naming features are chosen and names of a few important features are proposed, usually by members of the appropriate IAU task group.
  • As higher resolution images and maps become available, names for additional features may be requested by investigators mapping or describing specific surfaces or geological formations.
  • At this point, anyone may suggest that a specific name be considered by a Task Group, but there is no guarantee that the name will be accepted. Please submit name requests via this form.
  • Names successfully reviewed by a task group are submitted by the task group chair to the Working Group for Planetary System Nomenclature (WGPSN).
  • Upon successful review by vote of the members of the WGPSN, names are considered approved as official IAU nomenclature, and can be used on maps and in publications. Approved names are immediately entered into the Gazetteer of Planetary Nomenclature, and posted on its web site. Any objections to these names based on significant substantive problems or inconsistent application of normal IAU naming convention must be forwarded in writing or email to the IAU General-Secretary within three months from the time the name was placed on the web site. The General-Secretary will make a recommendation to the WGPSN Chair as to whether or not the approved name(s) should be reconsidered. The General-Secretary, in concurrence with the IAU President, may seek the advice of external consultants.
  • Approved names are also listed in the transactions of the IAU.
  • The categories of the planetary features are listed here.

Definition of a Planet

We invite you to consult the IAU Resolutions B5 and B6 (PDF file, 92KB) adopted on August 2006, at our XXVIth General Assembly in Prague, as well as the press release published on the occasion. The following theme article may also be of interest:

Dwarf Planets

Dwarf planets are planetary-mass objects orbiting the Sun that are massive enough to be rounded by their own gravity, but are not planets or satellites. Unlike planets, these bodies have not cleared the neighbourhood around their orbits, and their paths sometimes cross with other, often similar, objects.

There are currently five identified dwarf planets in our Solar System, each named after a God from Greek, Polynesian, or Roman mythologies. These five bodies are Ceres, Pluto, Haumea, Makemake, and Eris. In addition all of these but Ceres are also classified as plutoids, meaning that they are dwarf planets that orbit beyond Neptune and have an absolute magnitude H greater than 1.

There are several stages before a proposed name is accepted:

  • When a body is initially sighted it is given a provisional name, which is later superseded by a permanent numerical designation once its orbit has been well determined.
  • The discovery team suggests a suitable name to the two relevant IAU groups — the working groups for Small Body Nomenclature (WGSBN) and Planetary System Nomenclature (WGPSN) — who together are responsible for naming dwarf planets. The name is intended to reflect the characteristics of the body itself, and be an appropriate moniker derived from mythology. Objects, including dwarf planets, far beyond the orbit of Neptune are expected to be given the name of a deity or figure related to creation for example Makemake, the Polynesian creator of humanity and god of fertility, and Haumea, the Hawaiian goddess of fertility and childbirth.
  • The IAU finally decides on the assignment of the name, priority given to the ones proposed by the discoverers.
  • Dwarf planets may not share a name with any other small Solar System bodies.

The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration:

  • Names for the Underworld from the world's mythologies.
  • Gods, goddesses, and dwarfs associated with the Underworld.
  • Heroes and other explorers of the Underworld.
  • Writers associated with Pluto and the Kuiper Belt.
  • Scientists and engineers associated with Pluto and the Kuiper Belt.
  • Destinations and milestones of fictional space and other exploration.
  • Fictional and mythological vessels of space and other exploration.
  • Fictional and mythological voyagers, travellers and explorers.

Satellites of Planets in the Solar System

The WGPSN is responsible for naming of satellites of planets. With the agreement of the WGPSN, the WGSBN will assume responsibility for the naming of satellites of minor planets. The WGPSN is responsible for naming of satellites of planets.

Modern technology has made it possible to discover satellites down to 1 km in size or even smaller. The greatly increased discovery rate of satellites has made it necessary to extend the existing name categories for the satellites of Jupiter and Saturn whose names are drawn from the Greco-Roman mythology. The Jovian satellites have previously been named for Zeus/Jupiter's lovers and favorites but now Zeus' descendants are also included as an allowable source of names. The satellites of Saturn have so far been named for the Greco-Roman Titans, descendants of the Titans, Giants and the Roman god of the beginning. In order to internationalize the names, we now also allow names of giants and monsters in other mythologies (so far Gallic, Inuit and Norse).

The process of naming newly discovered natural satellites is as follows:

  • When reported to the IAU Central Bureau for Astronomical Telegrams, the object is assigned a provisional name, consisting on the letter S followed by the year of discovery and a number indicating the order of discovery within that year.
  • When the satellite is confirmed, the discoverer suggests a final name. Expanding on past practice, satellites of minor planets will, where possible and appropriate, receive names of mythological characters closely related to the name of the primary and suggesting the relative sizes. For example, binary transneptunian objects of comparable size should receive the names of twins or siblings, consistent with the current principle of using names of gods of creation or the underworld. As another example, satellites that share Pluto's orbital rhythm should take the name of underworld deities, as Pluto itself is named after the Roman god of the underworld who was able to render himself invisible.
  • The IAU finally decides on the assignment of the name, priority given to the ones proposed by the discoverers.

More information:

  • Public Naming of Planets and Planetary Satellites: Reaching Out for Worldwide Recognition with the Help of the IAU (PDF file, 128KB)
  • More information on planetary nomenclature can be found on the USGS FAQ section
  • Explanation regarding the names given to planets and satellites can be found in:
  • The latest updates on nomenclature of features on the surface of satellites (craters, mountains, valleys, etc) can be found on the Planetary Surface Feature News page

Minor Planets

The assignment of a particular name to a particular minor planet is the end of a long process that can take many decades:

  • It begins with the discovery of a Minor Planet that cannot be identified with any already-known object. Such Minor Planets are given a provisional designation. The provisional designations are based on the date of discovery and are assigned by the Minor Planet Center (MPC) according to a well defined formula that involves the year of discovery, two letters and, if need be, further digits (for example 1989 AC or 2002 LM60).
  • When the orbit of a Minor Planet becomes well enough determined that the position can be reliably predicted far into the future (typically this means after the Minor Planet has been observed at four or more oppositions), the Minor Planet receives a permanent designation - number issued sequentially by the Minor Planet Center, for example (433), (4179) or (50000).
  • When a Minor Planet receives a permanent number, the discoverer of the Minor Planet is invited to suggest a name for it. The discoverer has this privilege for a period of ten years following the numbering of the object. The discoverer writes a short citation explaining the reasons for assigning the name according to the guidelines of the IAU.
  • All proposed names are judged by the fifteen-person Working Group for Small Body Nomenclature (WGSBN) of the IAU, comprised of professional astronomers with research interests connected with Minor Planets and/or comets from around the world.
  • 16 characters or less in length
  • preferably one word
  • pronounceable (in some language)
  • non-offensive
  • not too similar to an existing name of a Minor Planet or natural Planetary satellite.

The names of individuals or events principally known for political or military activities are unsuitable until 100 years after the death of the individual or the occurrence of the event.

  • names of pet animals are discouraged
  • names of a purely or principally commercial nature are not allowed.

There are more detailed guidelines for unusual Minor Planets in certain dynamical groups, for example:

  • Trojan asteroids (those that librate in 1:1 resonance with Jupiter) are named for heroes of the Trojan War (Greeks at L4 and Trojans at L5).
  • Trans-Jovian Planets crossing or approaching the orbit of a giant Planet but not in a stabilizing resonance (so called Centaurs) are named for centaurs.
  • Objects crossing or approaching the orbit of Neptune and in stabilizing resonances other than 1:1 (notably the Plutinos at the 2:3 resonance) are given mythological names associated with the underworld.
  • Objects sufficiently outside Neptune's orbit that orbital stability is reasonably assured for a substantial fraction of the lifetime of the solar system (so called Cubewanos or "classical" TNOs) are given mythological names associated with creation.
  • Objects that approach or cross Earth's orbit (so called Near Earth Asteroids) are generally given mythological names.

Accepted names become official when they are published, along with their accompanying citations, in the Minor Planet Circulars, issued monthly by the Minor Planet Center.

The WGSBN recognizes the need to limit the numbers of Minor Planets named, and it requests individual discoverers and teams to propose no more than two names each two months.

Contrary to some recent media reports it is not possible to buy a name for a minor planet. If you have a name you would like to apply to a minor planet, the best advice is "Go out and discover one!".

The alphabetic list of all names is available at the Minor Planet Center including the discovery circumstances.

More information:

  • MPC page on naming minor bodies:
  • MPC guide to minor body astrometry:
  • WG Small Bodies Nomenclature Bulletins:


A comet is a body made of rock and ice, typically a few kilometres in diameter, which orbits the Sun. Comets may pass by the Sun only once or go through the Solar System periodically. A comet’s tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapours into space.

During the 19th century, comets were only given names after their second apparition, while those that had only appeared once were designated by a combination of year of discovery, numbers (both Arabic and Roman) and letters. Sometimes, the name of the discoverer was referred to in parentheses. It was not until the 20th century that comets were routinely named after their discoverers.

Today, the IAU’s Division F Working Group on Small Body Nomenclature (SBN) is the responsible body for strategic matters related to comet naming. When a comet is discovered and confirmed, the Minor Planet Center (MPC) announces it on behalf of the IAU. It is then given a designation according to the following pattern (see Resolution C.5 approved by the IAU in 1995 on p.32 of this PDF):

  • A prefix, alluding to the type of comet, which can be any of the following:
    • P/ for a periodic comet.
    • C/ for a comet that is not periodic.
    • X/ for a comet for which a meaningful orbit cannot be computed.
    • D/ for a periodic comet that no longer exists or is deemed to have disappeared.
    • I/ for all interstellar objects, whether comets or asteroids.

    As an example, the third comet discovered in the second half of January 2013, and classified as periodic, would be designated as P/2013 B3. The precise method, including exceptions and special cases, is described in the Cometary Designation System IAU resolution.

    When a periodic comet is observed after its second apparition, the IAU’s Minor Planet Center (MPC) gives it a sequential number indicating the order of the discovery.

    To complete the designation, comets are either given the name of the discovery team or of one or two individual members of the team (last name for an individual or one word or acronym for a team of astronomers). Discoveries by individuals are named for up to three independent discoverers. The names appear in chronological order and separated by a hyphen. In very rare cases the title can consist of three discoverers, or can even be generic.

    Examples of complete titles for comets (whether provisional or final) are 119P/Parker-Hartley, C/1995 O1 (Hale-Bopp) or 146P/Shoemaker-LINEAR.

    More detailed guidelines explaining the process of assigning the names of the discoverers to a comet can be found in this IAU document. Also read this essay on the web pages of the International Comet Quarterly.

    Do our sun and moon have names? - Astronomy

    My students keep asking where does the name Moon come from. Also why do we call the moon, moon. They say all the other planets that have moons have names for their moon.

    The Moon is called many things from different mythologies. For example, Luna by the Romans, Selene by the Greeks. Because the Moon is so obvious it has been known since prehistoric times, so tracing its name is difficult. It developed with different languages rather than being chosen conciously.

    See the Nine Eight Planets Moon Page for more information.

    The moons around other planets are collectively called moons as they hold the same position relative to their planet as The Moon does to ours - ie. they are named after the Moon. However in order to distinguish between them they also have individual names. The first moons to be discovered around another planet where the 4 largest moons of Jupiter discovered in 1609 by Galilleo Galillei so I suppose we can blame him for calling them moons! Names were then decided for them, unlike objects which have been known since prehistoric times whose names develop with a culture and language.

    This page was last updated on July 18, 2015.

    About the Author

    Karen Masters

    Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

    We Name Other Planets' Moons, So Why Haven't We Named Ours?

    To revist this article, visit My Profile, then View saved stories.

    To revist this article, visit My Profile, then View saved stories.

    Well, the moon does technically have a name: “the moon.” You’re assuming its name is generic for the category. But the moniker is actually exquisitely specific until Galileo discovered the moons of Jupiter four centuries ago, ours was one-of-a-kind. That is, its name became the name of the category and only started to seem like a lame non-name over time: We forgot how very beautiful and wonderful it is.

    So in short, your question is based on an ignorant and, frankly, kind of disrespectful misunderstanding. But don’t feel bad! It happens all the time, according to Pamela Redmond Satran, who, with Linda Rosen­krantz, founded Earth’s premier baby-naming website,, and has authored multiple books on the subject, including The Baby Name Bible and the very avant-garde-sounding Beyond Jennifer & Jason.

    When I spoke with Satran, she asked me to consider a baby-name equivalent of “the moon”—a name like John. It has been used so widely, for so long, even spun off into labels for other things, like a toilet, that it has started to feel utterly lackluster and unspecific. “It just becomes a generic word designating a male person,” she said. A guy looking for a prostitute is a john. An anonymous dead guy is John Doe. Often there’s a cyclical nature to this it takes about a hundred years for those names to get rediscovered and feel fresh again, Satran said. But, then again, recent generations of parents seem to be getting more and more intolerant of that sort of ordinariness.

    It feels like there’s more riding on a name now. Just look around, Satran said. “We’re a name-happy culture!” Every zoo in America seems to be holding a contest to name its new baby giraffe, she told me. Weather services are naming every conceivable kind of storm. Meanwhile, we are reaching deeper for unique names to give those things—often into esoteric, overlooked storehouses of names. A prominent source right now happens to be astronomy.

    All of a sudden, Satran said, names like Andromeda and Cassiopeia are flaring up on her radar. Venus Williams has made the name Venus “feel more possible now.” Erykah Badu has a kid named Mars. Actor Chris Noth named his kid Orion. An hour after we got off the phone, Satran sent me a news story about people in Russia naming their children Moon.

    So what Satran helped me understand is that your superficial dissatisfaction with our moon’s name, and your yearning for something more expressive, is totally hip and zeitgeisty. Wanting to rebrand our moon is as thoroughly modern a preoccupation as bone broth. The only way forward, it seems, would be to call the moon Grayson or Tate or Hermione or Richard, then rename it again a hundred years from now when someone feels that name’s too boring and raises the issue again. As for me, I find something wonderfully reassuring about the moon just being the moon while our opinions of it, down here, wax and wane.

    Do our sun and moon have names? - Astronomy

    All the other planets (Mercury, Venus, Mars, etc.) have names. What's the Earth's name? The other moons in the solar system have names too (Phobos, Io, Titan, etc.). What's our moon's name? Our galaxy has a name: the Milky Way. So what's the name of our solar system?

    The name of our planet is the Earth. The name of our moon is the Moon. The name of our solar system is the Solar System.

    Notice that I capitalize them, because when used as names, they are proper nouns. This also helps us distinguish between the planet Earth and earth (meaning soil), between the Earth's Moon and moon (meaning the natural satellite of a planet), and between our Solar System and any other solar systems (since any system containing a star and a planet or a planet-forming disk can be called a solar system.)

    This is the English language usage approved by the International Astronomical Union, the body in charge of naming celestial objects. It may seem odd that these important objects don't have names, but if you think about it, it just reinforces their importance. For example, the Moon is the Moon, not just any moon. It requires no other name, because it's the most important moon!

    You may read or hear people using Luna for the Moon, or Terra or Gaia for the Earth, or Sol for the Sun, but in English-speaking countries, these are poetic terms, often seen in science fiction stories, but not used by astronomers in scientific writing. In some countries where Romance languages are spoken, these terms are the official names.

    It's also interesting to note that most astronomers do not call our galaxy the Milky Way in technical writing--they call it the Galaxy.

    This page was last updated January 28, 2019.

    About the Author

    Britt Scharringhausen

    Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

    Of the roughly 10,000 stars visible to the naked eye, only a few hundred have been given proper names in the history of astronomy. [a] Traditional astronomy tends to group stars into constellations or asterisms and give proper names to those, not to individual stars.

    Many star names are, in origin, descriptive of the part of the constellation they are found in thus Phecda, a corruption of Arabic: -فخذ الدب- ‎ fakhdh al-dubb ('thigh of the bear'). Only a handful of the brightest stars have individual proper names not depending on their asterism so Sirius ('the scorcher'), Antares ('rival of Ares', i.e., red-hued like Mars), Canopus (of uncertain origin), Alphard ('the solitary one'), Regulus ('kinglet') and arguably Aldebaran ('the follower' [of the Pleiades]) and Procyon ('preceding the dog' [Sirius]). The same holds for Chinese star names, where most stars are enumerated within their asterisms, with a handful of exceptions such as 織女 ('weaving girl') (Vega).

    In addition to the limited number of traditional star names, there were some coined in modern times, e.g. "Avior" for Epsilon Carinae (1930), and a number of stars named after people (mostly in the 20th century).

    In 2016, the International Astronomical Union (IAU) organized a Working Group on Star Names (WGSN) [2] to catalog and standardize proper names for stars. The WGSN's first bulletin, dated July 2016, [3] included a table of 125 stars comprising the first two batches of names approved by the WGSN (on 30 June and 20 July 2016) together with names of stars adopted by the IAU Executive Committee Working Group on the Public Naming of Planets and Planetary Satellites during the 2015 NameExoWorlds campaign [4] and recognized by the WGSN. Further batches of names were approved on 21 August, 12 September, 5 October, and 6 November 2016. These were listed in a table of 102 stars included in the WGSN's second bulletin, dated November 2016. [5] The next additions were done on 1 February 2017 (13 new star names), 30 June 2017 (29), 5 September 2017 (41), 17 November 2017 (3), 1 June 2018 (17), and on 10 August 2018 (6). All 336 names are included in the current List of IAU-approved Star Names, last updated on 10 August 2018. [1]

    In addition, in 2019 the IAU organised its IAU 100 NameExoWorlds campaign to name exoplanets and their host stars. The approved names of 112 exoplanets and their host stars were published on 17 December 2019. [6] [7]

    In the table below, unless indicated by a "†" or "*", the "modern proper name" is the name approved by the WGSN and entered in the List of IAU-approved Star Names [1] or otherwise approved by the IAU. The WGSN decided to attribute proper names to individual stars rather than entire multiple-star systems. [8] Names marked with a "†" are no longer approved, while names marked with a "*" are names that were proposed or accepted since the last update to the list on 10 August 2018.

    What are the full moon names?

    In the Northern Hemisphere, and especially here in North America, we often refer to full moons by special names. Some almanacs assign full moon names by the month. Other almanacs like to reference full moons relative to seasonal markers, as defined by equinoxes and solstices. Is one way better than the other? No. Both have their roots in folklore. Below, we list full moon names that are commonly used in North America. Most of these names were derived from Old English and/or Native American sources. We list them first by the month, and then by the season. Around the middle of this post, we talk about Blue Moons. Toward the bottom, we have a word about moon names in the Southern Hemisphere.

    North American full moon names by month:

    January: Wolf Moon, Old Moon, Moon After Yule
    February: Snow Moon, Hunger Moon, Wolf Moon
    March: Sap Moon, Crow Moon, Lenten Moon
    April: Grass Moon, Egg Moon, Pink Moon
    May: Flower Moon, Planting Moon, Milk Moon
    June: Rose Moon, Flower Moon, Strawberry Moon
    July: Buck Moon, Thunder Moon, Hay Moon
    August: Green Corn Moon, Grain Moon
    September: Fruit Moon, Harvest Moon
    October: Harvest Moon, Hunter’s Moon
    November: Hunter’s Moon, Frosty Moon, Beaver Moon
    December: Cold Moon, Moon Before Yule, Long Night Moon

    About once every 19 years, February has no full moon at all. The last time that happened was in 2018. Read more: Why no full moon in February 2018?

    North American full moon names by season:
    After the winter solstice:
    Old Moon, or Moon After Yule
    Snow Moon, Hunger Moon, or Wolf Moon
    Sap Moon, Crow Moon or Lenten Moon

    After the spring equinox:
    Grass Moon, or Egg Moon
    Planting Moon, or Milk Moon
    Rose Moon, Flower Moon, or Strawberry Moon

    After the summer solstice:
    Thunder Moon, or Hay Moon
    Green Corn Moon, or Grain Moon
    Fruit Moon, or Harvest Moon

    After the autumnal equinox:
    Harvest Moon, or Hunter’s Moon
    Hunter’s Moon, Frosty Moon, or Beaver Moon
    Moon Before Yule, or Long Night Moon

    Full moon setting. Photo via Carl Galloway.

    What about Blue Moons?

    Blue Moons are a special case, whether they come by the month or by the season. Sky watchers in recent years have come to recognize both calendar-month and seasonal Blue Moons.

    Calendar-month Blue Moons happen when two full moons fall within a single calendar month. The second of the month’s two full moons is popularly called a Blue Moon. These sorts of Blue Moons happen seven times in every 19 years. In a year where February has no full moon at all, as in the year 2018, you can have two full moons in January and two full moons in March: two Blue Moons in single year.

    Let’s take a look at the 8 calendar-month Blue-Moons in the present 19-year Metonic cycle:

    1. March 31, 2018
    2. October 31, 2020
    3. August 31, 2023
    4. May 31, 2026
    5. December 31, 2028
    6. September 30, 2031
    7. July 31, 2034
    8. January 31, 2037

    Seasonal Blue Moons happen seven times in 19 years, too. There are usually three full moons between an equinox and a solstice, or vice versa. But sometimes four full moons fall in a single season. In that case, the third of a season’s four full moons is the Blue Moon. The last Blue Moon by this definition happened on May 18, 2019. The 7 seasonal Blue Moons in the current 19-year lunar cycle are:

    1) May 18, 2019
    2) August 22, 2021
    3) August 19, 2024
    4) May 20, 2027
    5) August 24, 2029
    6) August 21, 2032
    7) May 22, 2035

    Moon names in the Southern Hemisphere?

    As you probably know, the seasons for Earth’s two hemispheres are opposite. When it’s summer in the Northern Hemisphere, it’s winter in the Southern Hemisphere, and so on. When we originally published this article, we suggested that the full moon names might be likewise reversed. For example, the Green Corn Moon or Grain Moon – moon name for the August full moon in North American skylore – might work for the February full moon in the Southern Hemisphere.

    But assigning full moon names to moons in the Southern Hemisphere in that way – by flipping the names around to match the seasons – doesn’t really work. For example, our name for February’s full moon in North American skylore is the Snow Moon, Hunger Moon, or Wolf Moon. You wouldn’t expect a Wolf Moon for the Southern Hemisphere, because what most of us think of as a “wolf” – a gray wolf, canis lupus, isn’t native to the Southern Hemisphere.

    We asked several EarthSky friends in the Southern Hemisphere if they knew of full moon names for that part of the world. They all said no. We did see some full moon names for the large island of New Guinea, near Australia, at the website, but we have no idea if those names are still in use today, or if they come from historical records. Check them out if you’re interested!

    Bottom line: North American full moon names, listed first by month and then by season.

    Indigenous Astronomy and the Solar System

    Aboriginal and Torres Strait Islander people maintain detailed knowledge systems about objects in the Solar System, including the Sun, Moon, planets, comets, and meteors.

    Indigenous Astronomy and the Solar System

    Aboriginal and Torres Strait Islander people maintain detailed knowledge systems about objects in the Solar System, including the Sun, Moon, planets, comets, and meteors.

    These traditions also describe the movements of these objects. In this module, students will learn about the motions of the planets across the sky, with special reference to a phenomenon called retrograde motion. These traditions show how Aboriginal and Torres Strait Islander people carefully observe the movements of celestial object, explain their motions, and pass that information on to new generations. This is how science works, and this module will show students that science was developed long ago by Indigenous peoples and is as integral a part of Indigenous cultures as music and art.

    Aboriginal and Torres Strait Islander Knowledge of the Solar System

    Aboriginal and Torres Strait Islander people are keen observers of the night sky, having detailed knowledge systems built around the Sun, Moon, and planets visible to the eye (as a distance from the Sun: Mercury, Venus, Mars, Jupiter, and Saturn). For countless generations, they studied the motions of Solar System bodies through detailed observation, which was recorded and passed to successive generations through oral tradition. Aboriginal and Torres Strait Islander people distinguished planets from the background stars, noted the changing positions of planets in the sky over days and months, observed their changing positions relative to each other, and characteristics of their journey across the sky.

    In many Aboriginal traditions, the planets are seen as children of the Sun and Moon. They represent ancestor spirits walking across the sky, connecting ceremony and Law to various groups of stars. In Wardaman Aboriginal traditions, Uncle Bill Yidumduma Harney describes the planets moving across the sky as ancestral beings walking along a road. Just as you or I walk down the street, sometimes we stop and turn back before moving forward again. Sometimes we slow down and chat with other people during our journey. Uncle Yidumduma says the ancestral beings are coming back for another &lsquoyarn&rsquo with other planets as they travel across the sky. 1 Sometimes they come close together, in what is called a conjunction.

    The Aboriginal people of the Great Victoria Desert observe how Jupiter and Venus always followed one another along the &lsquoDreaming Road&rsquo which the planet-ancestors had made. These planets are seen as ancestral beings with heads, but no bodies. 2 The Dreaming Road described by some Aboriginal communities is equivalent to what Western astronomers call the zodiac. This is the region of the sky nine degrees on either side of the ecliptic (the path of the Sun). Since the Earth and all the planets orbit the Sun in one direction in a relatively flat plane, they will all appear to move along the zodiac (Fig. 1), or &ldquoDreaming Road&rdquo.

    In Western astronomy, the constellations found within the zodiac comprise the twelve star signs used by astrologers (although there are actually more than twelve). These are constellations through which the Sun, Moon, and planets pass. In Aboriginal and Torres Strait Islander traditions, these stars and constellations often have special relationships and connections to the Sun, Moon, and planet ancestor spirits. In Tiwi Lore of Bathurst and Melville Islands in the Northern Territory, the Sun-Woman carries her torch across the sky each day from East to West, reflecting the Sun&rsquos diurnal motion from dawn to dusk. The Moon-Man follows the same path, illuminating his way with a smaller torch. He is often attended by his four wives: the planets Mercury, Venus, Mars, and Jupiter.

    Fig.1 (top) The planets forming a line along the Ecliptic. Image: Stellarium. (bottom) The apparent motion of the Sun through the 12 Zodiac constellations along the Ecliptic. Image: Peter Christoforou (Astronomy Trek)

    Retrograde Motion

    The Sun, Moon, stars, and planets all move diurnally (from East to West) over the course of a night. But if we observe their motions over the course of days, weeks, and months, we notice they have strange motions. Observers, including Indigenous people, know the positions of the planets with respect to the background stars gradually move from West to East night after night. Each planet is a different distance from the Sun, and this means they orbit at different periods. Mercury, the planet closest to the Sun, orbits the Sun in just 88 days. Saturn, the farthest planet we can see with the unaided eye, takes 29 years to go around the Sun once. This means the planets closer to the Sun than us are always relatively close to the Sun. Mercury is seen either just before sunrise or just after sunset, but not for very long. Venus is similar, but is further from the Sun, meaning it can be a bit higher in the sky. The other planets can be visible anywhere along the zodiac as they orbit the sun farther than Earth.

    There are times when two or more planets come close together (conjunction) and the faster orbiting planet will overtake the slower, outer planet. From our perspective, it can appear as if a planet slows down in its gradual West to East motion, stops, then moves backwards. This is called Retrograde Motion. After a period the planet will slow down, stop, and resume its normal motion against the stars (Fig. 2). The Wardaman traditions about planet spirits moving back and forth during their journey along the Dreaming Road is a description of retrograde motion, showing us how Aboriginal people long ago observed the complex motions of the planets and incorporated that knowledge into oral traditions, which were passed to younger generations.

    Fig. 2: (top) how a slower planet appears to move retrograde against the background stars as the faster planet (observer) overtakes it. Image: Wiki Commons License. (bottom) Mars making a loop in Virgo as a result of retrograde motion as seen from Earth. Image: Tunç Tezel (The World At Night/NASA Image of the day 28 Oct 2014).


    In the past, Comets were first given a provisional designation, consisting of the year and a lowercase letter indicating the order of discovery in the year (e.g., 1994a was the first comet discovered or recovered in 1994, 1994b the second, etc.). The name was also assigned at an early stage. Up to three (preferably independent) discoverers may have been attached to the comet. Some time later, the comets that had passed perihelion in a given year were assigned Roman numeral designations indicating the order of perihelion passage within the year. The Roman numeral designations for 1993 and 1994 are given in the Jan. 1995 batch of Minor Planet Circulars (MPCs).

    Note that whole comet designation system was revamped starting in the beginning of 1995. The main points of the new scheme are:

    1. the provisional designation system now closely matches the designation system for minor planets. The first comet discovered in the first half of 1995 Jan. is designated 1995 A1, the second 1995 A2, etc.
    2. long-period comets and one-apparition periodic comets receive only a provisional designation — there is no equivalent of the Roman numeral designation.
    3. upon recovery at a second apparition (or following through aphelion) periodic comets receive a sequential number. E.g., P/Halley is 1P.
    4. routine recoveries of periodic comets do not receive provisional designations.
    5. the nature of the comet orbit is indicated by a prefix: P/ for periodic comets, C/ for long-periodic comets, D/ for defunct comets (e.g., 1993e) and X/ for uncertain comets. Additionally, A/ is used to indicate that the object is a minor planet.
    6. comets continue to be named in general terms for their discoverers ensuring fairness and simplicity.
    7. Provisional comet designations are assigned by the CBAT. Permanent comet numbers are assigned by the Minor Planet Center.

    The new scheme was backdated, so old comets received new-style designations. There is an interactive converter between old- and new-style comet designations.

    Some examples of new comet designations:

    Here is a copy of the official IAU resolution.

    Do our sun and moon have names? - Astronomy

    • In addition to the stars, the Sun, and the Moon, there are several other objects in the sky which are easily visible at night.

      From the ancient perspective, a planet is a point of light in the sky that moves relative to the stars, much as the Sun and Moon do.

    • With the naked eye, one can see five planets: Mercury, Venus, Mars, Jupiter, and Saturn.

    Extra: the Sun, Moon, and planets are associated with ancient gods, and their number is the basis of our seven-day week.

    • Like the Sun and the Moon, the planets all move near the ecliptic, never being more than a few degrees away.

    In the photo at the right, you can see (from top to bottom) Saturn, Venus, Jupiter, and Mercury in alignment with the recently set Sun.

      The planets move slowly enough that their positions change only slightly from night to night.

    They therefore rise in the east and set in the west as part of the sky's diurnal motion.

    Their speeds vary, but Mercury is the fastest, followed by Venus, Mars, Jupiter, and then Saturn, the slowest.

    This reverse motion is known as retrograde motion.

    Retrograde motion can last from weeks (Mercury) to months (Saturn).

    5.2 Geocentric Cosmology

    (Discovering the Universe, 5th ed., §2-0)

    • According to the laws of physics, there is no preference between saying that the Sun revolves around the Earth or the Earth revolves around the Sun.

    Each are equally true, although one perspective may be more useful than the other for a particular purpose, as we have seen.

    The former produces the retrograde motion, while the latter is primarily responsible for the direct motion.

    In the adjacent animation, the arrow points towards the stars we see behind the planet watch where the arrow points as the planet moves around its epicycle.

    For each planet, Ptolemy determined the sizes of its deferent and epicycle, and the speed of revolution of its epicycle and the planet itself.

    By projecting his model forward in time, Ptolemy was then able to correctly predict where the planets would be located centuries into the future.

    Because of the success of his model, Ptolemy's treatise on the subject, which became known as the Almagest ("the Greatest"), was the bible of astronomers through the Middle Ages.

    Other astronomers tried to correct it by adding additional levels of epicycles, but the result was exceedingly complex.

    5.3 Heliocentric Cosmology

    (Discovering the Universe, 5th ed., §2-1)

    • An alternative to the geocentric cosmology was actually suggested a century before Hipparchus by Aristarchus (3rd C. B.C.).

    The Earth also orbits the Sun, but the Moon still orbits the Earth.

    Mercury and Venus (the inner planets ) have smaller orbits than the Earth, while Mars, Jupiter, and Saturn (the outer planets ) have larger orbits.

    As a result, the Earth will regularly overtake and pass the outer planets, and the inner planets will do the same to the Earth.

    Like one car passing another on the highway, the second car will appear to "move backward".

    This is another example of parallax.

    The heliocentric cosmology was forgotten for almost 2000 years, until the 16th century, when the Polish astronomer Nicolaus Copernicus rediscovered it.

    He was also able to make very accurate predictions of the planets' relative distances from Sun (see the table below).

    Finally, in 1543, Copernicus' book On the Revolutions of the Celestial Spheres appeared, shortly before he died.

    The book was widely read in Europe, and gained enough support that it seriously threatened the geocentric model, which was virtually an article of faith in the Catholic Church.

    5.4 Planetary Configurations

    (Discovering the Universe, 5th ed., §2-1)

    • For millenia, observational astronomers have described the positions of planets and other celestial bodies using several special configurations, which can be easily understood in terms of the Copernican model.

    Question : what other type of celestial angular measurement is elongation similar to?

    At the right, the Sun and the red planet are in conjunction.

    Question : what is the phase of the Moon when it is in conjunction with the Sun?

    The image at the right shows a "triple" conjunction that occurred on April 23, 1998, between the Moon,Venus, and Jupiter.

    • When two celestial bodies are at right angles in the sky, they have an elongation of 90°, and they are said to be in quadrature .

    At the right, the Sun near the western horizon and the red planet near the meridian are in quadrature.

    Question : what is the phase of the Moon when it is in quadrature with the Sun?

    At the right, the Sun near the western horizon and the red planet near the eastern horizon are in opposition.

    Question : what is the phase of the Moon when it is in opposition to the Sun?

    Maximum elongation of a planet is not readily explained using the geocentric model, but it arises naturally out of the heliocentric model, simply by assuming that the orbits of Mercury and Venus lie inside the Earth's orbit.

    When an inner planet is at maximum eastern elongation it will only be visible shortly after sunset (an "evening star") when it is at maximum western elongation it will only be visible shortly before sunrise (a "morning star").

    Question : as observed from Earth, what is the phase of the inner planet at each of the four positions in the picture?

    In the heliocentric model, their orbits must therefore lie outside the Earth's orbit.

    Question : as observed from Earth, what is the phase of the outer planet at each of the four positions in the picture?

    5.5 Orbital Period

    (Discovering the Universe, 5th ed., §2-1)

    • The time it takes for a planet to complete one orbit is called the orbital period of revolution , often simplified to "orbital period" or just "period".

      As can be seen in the table at the right, the farther a planet is from the Sun, the longer is its sidereal period.

    The synodic period doesn't have a simple behavior, however it initially increases, and then decreases until it is slightly larger than one year.

    Planet Average
    Distance Sidereal
    Period Synodic
    Period Mercury 0.3871 AU 0.2408 y = 87.97 d 115.88 d Venus 0.7233 AU 0.6152 y = 224.70 d 583.92 d Earth 1.0000 AU 1.0000 y = 365.26 d ---- Mars 1.5237 AU 1.8809 y = 686.98 d 779.94 d Jupiter 5.2028 AU 11.862 y 398.9 d Saturn 9.5388 AU 29.458 y 378.1 d Uranus 19.1914 AU 84.01 y 369.7 d Neptune 30.0611 AU 164.79 y 367.5 d Pluto 39.5294 AU 248.5 y 366.7 d

    • For an inner planet, the sidereal period is shorter than the synodic period, because when the planet returns to its original position the Earth has moved in its orbit, so the planet must travel further to catch up to the Earth.

    In the animation at the right, Venus actually completes two sidereal periods (225 d) before it finally catches up with the Earth after the synodic period (584 d).

    5.6 Tests of the Heliocentric Model

    (Discovering the Universe, 5th ed., §2-2, §2-4)

    • Tycho Brahe (1546 - 1601), a Danish nobleman, was renowned for his development of astronomical instruments and his use of them to make measurements of the positions of stars and planets.

    His data were the most accurate available prior to the introduction of the telescope into astronomy, shortly after his death.

    Amongst other discoveries, he made accurate measurements of a supernova in 1572, and showed that it was in the realm of the stars, which was believed to be unchanging.

    Tycho attempted to measure this parallax, but he was unable to do so, and therefore concluded that the premise that the Earth moves around the Sun was wrong.

    Actually, it was Tycho's confidence in his own measurements which was ill-founded!

    As we have already seen, the stars do exhibit parallax, but because they are so far away, a telescope is required to observe it!

    • The telescope was invented by a Dutch optician late in the 16th century.

    Galileo Galilei (1561-1642), a professor of mathematics at the University of Padua, heard about the telescope in 1609.

    Recognizing the telescope's possibilities, Galileo immediately built one of his own, based only the sketchy details he had heard.

    Galileo then improved the design of the telescope to the point where it could be used for astronomy.

    Galileo quickly made several important astronomical discoveries, which were published in 1610 in his book The Starry Messenger .

    Galileo noticed that Venus' phases were related to its angular diameter and elongation: it is smaller (farther away from us) at the gibbous phase and larger (closer to us) at the crescent phase, with the extremes occurring at small elongations.

    The Galilean satellites were obviously orbiting Jupiter, which was contrary to a basic assumption of the geocentric model, viz. everything in the heavens orbited the Earth.

    Nevertheless, in 1632 Galileo published Dialogue Concerning the Two Chief World Systems--Ptolemaic and Copernican , which was such a masterpiece of exposition of the heliocentric model that readers ignored the ordained conclusion.

    Galileo was then brought before the Inquisition and forced to publicly recant his Dialogue was banned, and he spent the last eight years of his life under house arrest.

    The ban on Galileo's Dialogue wasn't lifted until 1822, and the Vatican's censure of Galileo himself wasn't removed until 1992!

    5.7 Kepler's Laws

    (Discovering the Universe, 5th ed., §2-3)

    • Although the heliocentric model worked just as well as the geocentric model, to make it work over a millenium Copernicus still had to add epicycles.

    Kepler didn't believe planetary orbits were necessarily circles, but could instead be other closed curves, such as the ellipse or oval.

    The semimajor axis therefore describes the overall size of the ellipse.

    It can be shown that a is the average distance of the ellipse from one focus.

    Because c is always less than a , the value of e varies between 0 and 1.

    When e = 0, c = 0, the foci coincide, and we have a circle.

    When e =1, the foci approach the opposite ends of the ellipse the result is so elongated that, from one focus, both the center and the other focus are infinitely far away, forming a curve called a parabola .

    Planetary orbits are ellipses, with the Sun at one focus.

      As can be seen in the table at the right, the eccentricity of the planets' orbits is generally quite small, except for Mercury and Pluto, whose orbits are noticeably elongated.

    This is why circles initially worked well in describing planetary orbits.

    The orbital inclination is usually quite small, except for Pluto.

    Question : where did we see orbital inclination previously?

    Question : why doesn't a planet usually disappear behind the Sun when they are in conjunction?

    • Kepler also noticed another characteristic of planetary motion: planets move fastest at perihelion, and slowest at aphelion.

      Kepler was able to quantify these varying speeds in what is known as Kepler's Second Law :

    Planets sweep out equal areas in equal times.

    Kepler's Third Law quantifies the observation that more distant orbits have longer periods:

    Here, the semimajor axis a is measured in A.U. and the orbital period P is measured in years.

    The graph at the right shows log P vs. log a the data falls along a straight line, with a slope of 3/2.