# How is the Moon's coordinate system defined?

In the Apollo 17 Wikipedia article, the coordinates of the descent stage are given: 20.19080°N 30.77168°E. What are the specifics of the Moon's coordinate system? Where is 0°?

## 2 Answers

0° is the line of longitude that contains the point on the surface closest to Earth. In other words, (0,0) is at about the center of the face of the Moon we see.

The details are a little more complicated, since the "closest point" is really a mean position. This document has much more information on the coordinate systems and other lunar standards.

• Any idea why would some NASA/JPL documents, like the one you link to, leave whole pages blank, with the only content in them a box saying "This page is intentionally left blank."? I've noticed such pages before and, for the better of me, can't think of a good reason why would they keep them? :O Dec 27, 2013 at 9:31
• Such documents used to be rendered into a physical form by impressing carbon and iron oxide particles onto flat sheets of tightly compacted cellulose fibers. Both sides of those sheets would be used for economy and to reduce the size of those objects. The sheets would be tied together at one end to keep them in order, and would be read by opening the sheets at the opposite end. It was considered good practice for new sections to always start on the right sheet. So if the previous section had an odd number of sheet faces used, a blank face would permit the next to start on a right face. Dec 27, 2013 at 16:41
• This answer is (very) close to correct. The issue is "this document". That group at JPL uses the lunar principal axis frame. That frame differs from the lunar mean Earth frame by about a kilometer. There apparently is a bit of an internecine war at JPL regarding which system is "right". JPL's SPICE resolves this conflict by letting the user choose between IAU_MOON, MOON_ME, and MOON_PA. The first is essentially a low resolution version of the second that involves truncating the IAU lunar rotation model. JPL's moon ME frame is a fixed rotation from its moon PA, which is what is in DE421. Feb 28, 2014 at 18:02
• I'm shocked. JPL is always just one big happy family. :-) Feb 28, 2014 at 18:53

They used selenographic coordinates. A small yet well-preserved crater called Mösting is used as a fundamental reference point, and given these coordinates:

Latitude:   3° 12' 43.2" South
Longitude:  5° 12' 39.6" West


All coordinates are defined relative to this location, so there's no need to rely on a magnetic field, although the Moon does have one. For more information, please read this RAND corporation report of 1987, and the fundamental IAU/IAG paper.

• Coordinates of the Earth do not rely on a magnetic field, north and south poles are defined by the rotation of the Earth. The magnetic poles are at different locations hundreds of kilometers away from the geographic poles.
– Uwe
Aug 5, 2018 at 14:31