# What is a "fundamental location", and why is Mosting A defined as one for the moon?

The crater Mosting A was chosen to be the "fundamental location" for the Moon's coordinate system (latitude and longitude). But what exactly is a "fundamental location"? What does it have to do with the establishment of a coordinate system? Is it just so that observers have a way of easily projecting a spherical grid on the moon? And why was Mosting A chosen for the job?

Every coordinate system needs one point defined as exact and unambiguous - the fundamental location which all other location measurements in the vicinity are relative to.

This can be the origin of the coordinate system (like 0° longitude and 0° latitude) - but it can also be any other point instead. You choose that (or one) point which you can revisit the easiest in a reproducable manner.

The given point in the center of the Mösting A crater previously was one of the fundamental points for the Moons reference frame as it was/is easy to identify: it is very round, has a sharp edge, thus the mid-point is easy to identify. There seems to be some original correspondance on the topic available from the late 19th century on ADS and discussions on improving the map accuracy prior to the moon landing which all refer to these properties as being the reason of choice.

Currently it's defined more accurately via the retroreflectors placed on the moon by the Apollo missions. As such the coordinate for that were calculated such that 0°/0° is at the center of the on average visible disk of the Moon and then the location for that retroreflector was defined to be the resulting coordinates. Without such fundamental point, different measurements might use (slightly) differently-defined coordinate systems and comparison of data would become difficult. The retroreflector on that location is the largest one, so the easiest to find.

It's basically similarily arbitrary that the 0° latitude on Earth pass through Greenwich observatory.

• Ah, so it's being used as a single point to define everything. I was thinking that it was being used in conjunction with another point (like 0°, 0°) to set up the grid. This at least answers the first part of my question, but I still can't find resources about why Mosting A was initially chosen. Apr 26 at 20:44
• In principle you need additional to one coordinate a direction to define the rotation of the coordinate system around that point. But in these kind of coordinate systems the North direction is usually defined towards the intersection of the rotational axis with the geoid (or selenoid,...). Apr 27 at 7:41
• No, the retroreflectors are at different locations. That of course means the fundamental point was changed at some time (that is when a more accurate point via the reflectors could be defined) Apr 27 at 8:22
• As I write in the answer and supported by the page I link: """The given point previously was one of the fundamental points for the Moons reference frame as it was/is easy to identify: it is very round, has a sharp edge, thus the mid-point is easy to identify.""" Apr 27 at 8:25
• You know, that is my bad. I must be blind. I'm going to delete my comments asking. Do you happen to have a source for that, though? I'd like to read about it some more. Apr 27 at 8:40