When we first observe a new heavenly body (it could be a new star, asteroid, etc.,or a minor planet in our own solar system), are there any procedures set in place for establishing a system of longitudinal meridians?

Being that a prime meridian is an arbitrary concept that you can pick and establish anywhere, is a location decided based on a physical feature, or perhaps from the first points of data we gathered when making detailed observations of the heavenly bodies for the first time?

What about heavenly bodies with no easily discernible or non-stationary features (Gas Giants)? Also does the IAU regulate this process?

I have always wondered if the USSR and the US shared common reference points for locations such as on the moon.


1 Answer 1


As an example consider the prime meridian for Vesta.

Based on Hubble images, the IAU had established a prime meridian based on the observation of the Obler's regio (a dark region). The prime meridian was defined as passes through the middle of this region.

When the Dawn mission arrived at Vesta, it was discovered that the asteroid's pole was not in the same place that it had been assumed to be (based on Hubble data) and that Obler's regio was not a convenient basis for a prime meridian, as it was not even discernable from up close. Instead they chose a small crater, Claudia, that was close to the equator, and defined the prime meridian based on that (the prime meridian is defined as a passing 4 degrees from the centre of Claudia) This is now the generally accepted coordinate system, although it has not been "officially" adopted by the IAU.

In general, if you are the first to map a new world, you get to choose where do draw your meridian.

  • $\begingroup$ Thank you for the very helpful response and example. Do you know about any rules or procedures regarding meridians for gas giants? $\endgroup$
    – Sudachi
    Jan 8, 2017 at 22:49
  • $\begingroup$ I would guess that no meridians are defined for gas giants. All visible features are cloud-tops and move relative to one another. With no visible surface, there isn't a fixed point from which to define a meridian. When "maps" of Jupiter are made, the centre is just an an ad-hoc choice. Googleing maps of Jupiter seems to confirm this. $\endgroup$
    – James K
    Jan 9, 2017 at 17:05

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