Does a word already exist for the anniversary of an event as measured by the orbital period of another planet? For example, (as of today on Earth) Curiosity Mars Rover has been on Mars for one Mars year, but that's 687 Earth days – nearly two Earth "anniversaries".

  1. Does the term "anniversary" have an explicit "on Earth" denotation? In other words does the prefix "ann(o)-" mean Earth year, or can it mean "year" for any given orbital body?
  2. Is the simplest way to express this "Mars anniversary", or is there a suitably understandable term?
  • It's a "Martianniversity", of course. Jun 24 '14 at 21:08
  • The term is "Mars Year" for Mars, or Planet Year in general.
    – Oldcat
    Jun 24 '14 at 21:08

It is known as a return.

Astrologers refer to the return of a planet to its sidereal position at the time of a person's birth.

So while an astrologer will refer to a Saturn Return, for example, as a significant event in a person's life, the term is used more widely to refer to the orbital period of a planet.

As others have said, a Martian Year is also widely used, especially by astronomers.

  • Is this usage common in astronomy, or is it confined to astrology?
    – phenry
    Jun 24 '14 at 21:11
  • But I don't think there is any phrase which would convey Mars anniversary in the sense you mean it, without explanation. The concept simply does not exist in the language or in most people's minds, at least up to now.
    – Colin Fine
    Jun 24 '14 at 21:21

According to English for Students: anni, annu and enni are the Latin roots for year. As such, I believe that the word anniversary would refer to a year of the item preceding it.

  1. Lunar Anniversary - Commemoration of the Apollo 11 Moon Landing, July 24, 1969
  2. Anniversary Clock - A Clock that requires winding once a year based on a torsion pendulum

In case 1, the anniversary is used in the traditional sense. In case 2,the anniversary is about 400 days, which would be synonymous with your measure of time regarding orbit of Mars.

  • Speaking of orbital bodies, lunar periods are especially ambiguous. Is a lunar year the orbital period of the moon around Earth, or the Earth year as measured in ... months?
    – kojiro
    Jun 24 '14 at 22:14


1 - "Anniversary" does not have an explicit earth-centric origin. It comes from Latin annum, which means year, and is thus generic.

2 - There appears to be no generally agreed-upon term that would encapsulate anniversary with respect to the Martian year. See the article Timekeeping on Mars for some background. I tend to think that Mars anniversary fits the bill, but if you were writing a science-fiction story you might want to coin a term in order to achieve some kind of verisimilitude. @PeterShor's "Martianniversary" in question comments is very interesting in this regard.

Some astronomical terms can be body- or body-type specific. For example, perigee is the closest approach of a body to earth (geos), whereas perihelion is the closest approach of a body to the sun (helios). On the other hand, periapsis is the generic term for closest approach of one body to another. See Apsis on Wikipedia.

Incidentally, I have a suggested coinage for Martian anniversary that I just now coined:

Mars in the Greek language is Ares, making the term for the closest approach to Mars of the moon Deimos, for example, into periareion. By this token, a Martian year could be called an Areannum, making a Martian anniversary an areanniversary.

You heard it here first, folks!

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