<p>Every coordinate system needs one point defined as exact and unambiguous - the <a href="https://de.zxc.wiki/wiki/Fundamentalpunkt" rel="nofollow noreferrer">fundamental location</a> which all other location measurements in the vicinity are relative to.</p>
<p>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.</p>
<p>The given point in the center of the <a href="https://planetarynames.wr.usgs.gov/Feature/11484" rel="nofollow noreferrer">Mösting A crater</a> 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 <a href="https://articles.adsabs.harvard.edu/pdf/1875AReg...13..268N" rel="nofollow noreferrer">original correspondance</a> on the topic available from the late 19th century on <a href="https://ui.adsabs.harvard.edu/" rel="nofollow noreferrer">ADS</a> and discussions on improving the map accuracy <a href="https://www.jstor.org/stable/44575047" rel="nofollow noreferrer">prior to the moon</a> landing which all refer to these properties as being the reason of choice.</p>
<p>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 <a href="http://lroc.sese.asu.edu/posts/153#extended" rel="nofollow noreferrer">largest one</a>, so the easiest to find.</p>
<p>It's basically similarily arbitrary that the 0° longitude on Earth pass through Greenwich observatory.</p>

CC BY-SA 4.0
2021-04-26T20:44:51.197Z
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.
Phiteros
CC BY-SA 4.0
2021-04-27T07:41:19.510Z
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,...).
planetmaker
CC BY-SA 4.0
2021-04-27T08:22:26.893Z
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)
planetmaker
CC BY-SA 4.0
2021-04-27T08:25:03.890Z
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."""
planetmaker
CC BY-SA 4.0
2021-04-27T08:40:40.163Z
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.
Phiteros
CC BY-SA 4.0
2022-07-05T00:24:48.973Z
Haha, Good catch @PM2Ring
planetmaker
CC BY-SA 4.0
2021-04-26T08:37:47.243Z
2022-07-05T00:23:51.690Z
2022-07-05T00:23:51.690Z
planetmaker
<p>The crater <a href="https://en.wikipedia.org/wiki/M%C3%B6sting_(crater)" rel="nofollow noreferrer">Mosting A</a> was chosen to be the &quot;fundamental location&quot; for the <a href="https://en.wikipedia.org/wiki/Selenographic_coordinates" rel="nofollow noreferrer">Moon's coordinate system</a> (latitude and longitude). But what exactly is a &quot;fundamental location&quot;? 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?</p>

<the-moon><coordinate><fundamental-astronomy>
What is a "fundamental location", and why is Mosting A defined as one for the moon?
Phiteros
# What is a "fundamental location", and why is Mosting A defined as one for the moon?

(Asked by: username_0 on 2021-04-26)

The crater [Mosting A](https://en.wikipedia.org/wiki/M%C3%B6sting_(crater)) was chosen to be the "fundamental location" for the [Moon's coordinate system](https://en.wikipedia.org/wiki/Selenographic_coordinates) (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?

**Tags:** the-moon, coordinate, fundamental-astronomy

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## 1 answers

### Answer by username_1 on 2021-04-26. Score: 6 | Accepted answer

Every coordinate system needs one point defined as exact and unambiguous - the [fundamental location](https://de.zxc.wiki/wiki/Fundamentalpunkt) 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](https://planetarynames.wr.usgs.gov/Feature/11484) 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](https://articles.adsabs.harvard.edu/pdf/1875AReg...13..268N) on the topic available from the late 19th century on [ADS](https://ui.adsabs.harvard.edu/) and discussions on improving the map accuracy [prior to the moon](https://www.jstor.org/stable/44575047) 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](http://lroc.sese.asu.edu/posts/153#extended), so the easiest to find.

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

> **Comments:**
> **username_0**: 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.  
> **username_1**: 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,...).  
> **username_1**: 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)  
> **username_1**: 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."""  
> **username_0**: 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.  
> **username_1**: Haha, Good catch @PM2Ring  

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