According to the answer to this question, the 0° longitude of the Moon is that which points toward Earth. On Earth it goes through Greenwich (although I was amused to hear that the French disagreed), which is a fairly arbitrary (albeit historical) decision, and on Mars it was chosen apparently at random.

As the exploration of space continues, and more and more planets, moons, planetoids and asteroids are being mapped, (and with the rapid discovery of swarms of exoplanets, which further discoveries will no doubt eventually allow us to map in detail as well), is there a system for setting up coordinate systems on each? Some traditional or systematic structure for choosing a Prime Meridian on alien worlds?

Or are such decisions made more haphazardly, perhaps left up to whoever first chooses to make the first map? If so, is there any standard approval body, or official way of settling disputes between competing coordinate systems?


1 Answer 1


One need look no further than the Moon.

There were (and to some extent still are) some rather severe internecine wars within the Jet Propulsion Laboratory on the "right" way to represent the Moon's orientation. One group insists that the right way to describe the Moon's orientation is via its principal axes (the MOON-PA point of view). Per this point of view, any other approach is not just dumb, it's dumber than dumb. Another group insists that the right way to describe the Moon's orientation is via its orientation with respect to the Earth (the MOON-ME point of view). Per this point of view, that other description is even dumberer.

As a disinterested third party observer of these internecine wars, that these wars exist to me is what is dumb. I'd much rather have one agreed-upon frame that in one swell foop describes the Moon's rotation, it's gravity field, and it's topology. That desire is apparently a "good luck with that" kind of proposition.

As TildalWave mentioned in a comment, there is an interdivisional working group within the International Astronomy Union (IAU) that is dedicated to describing the rotational states of the planets, satellites, asteroids, and comets that comprise the solar system. This is the Cartographic Coordinates & Rotational Elements Working Group, described here and here. This group can only make recommendations. It has no binding powers that say "this is how thou shalt do it." In fact, this group to some extent operates at the whims of other IAU committees.

For example, the IAU definition of the "north pole" is different for a planet than for a small solar system body. The "north pole" of a planet or the satellite of a planet always has a positive +z component with respect to the solar system invariant plane. Oh the other hand, the north pole of some non-planetary / non-satellite body is that direction from which the body appears to be rotating counterclockwise. There has been one case where a planet has been relegated to a non-planet status, and that one case was troublesome with regard to the direction in which the "north pole" points. Pluto appears to rotate clockwise when observed from above the invariant plane, counterclockwise when observed from below. This means that the 2006 decision to designate Pluto a non-planet status meant that the description of Pluto's rotation had to be totally revamped.

  • $\begingroup$ Re Pluto losing its planetary status, here's my so far favorite take on how that happened (YouTube video, David A. Aguilar, Harvard-Smithsonian Center for Astrophysics during the CfA Observatory Nights: What is a Planet?). Sound is a bit clanky and way too loud but audible if you turn the volume down. $\endgroup$
    – TildalWave
    Oct 14, 2014 at 13:51
  • 2
    $\begingroup$ After reading the debates, I personally stand on the side of Pluto not being a planet. It is fun, however, to make fun of Pluto being delegated from planetary to dwarf planetary status. "Dwarf planet" stands along with a few other "adjective noun" terms used in science where the adjective relegates the noun to not-quite-a-noun status. My favorite is "red herring", which oftentimes are neither fish nor reddish. $\endgroup$ Oct 14, 2014 at 14:48

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