<p>Deimos is, like our own moon, tidally locked to Mars, so one hemisphere of Deimos faces the planet, and the faces away. The Sub-Martian hemisphere faces Mars.</p>

<p>Since Deimos is locked, there is a point on its surface that always faces in the direction of orbital motion, that point is the leading apex. It is an apex in the sense of a "leading point"</p>

<p>A meridian is a line of longitude, running from the North to the South pole. One of Deimos's meridians goes through the centre of its hemisphere that faces away from Mars, this is the anti-Mars Meridian. As you say, the "meridian on the side of Deimos facing away from Mars.</p>

<p>The image here shows a map of Deimos:</p>

<p><a href="https://i.sstatic.net/XJdkj.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XJdkj.jpg" alt="The leading face is the left side of the image, and the trailing face is on the right. The central 50% is the sub-martian hemisphere and the anti mars meridian is the line on the left and right edge, where the 3D image of Deimos has been cut to make a 2D map"></a></p>

<p>The leading face is the left side of the image, and the trailing face is on the right. The central 50% is the sub-martian hemisphere and the anti mars meridian is the line on the left and right edge, where the 3D image of Deimos has been cut to make a 2D map</p>

CC BY-SA 3.0
2015-10-13T21:24:09.177Z
2015-10-14T17:11:42.733Z
2015-10-14T17:11:42.733Z
James K
<p>I am having difficulty with some terminology:</p>

<ol>
<li>sub-Martian hemisphere of Deimos ... what place on Deimos does it refer to?</li>
<li>leading/trailing apex of Phobos ... what place are those?</li>
<li>Apex ... in general it means a top of something, or an end of something pointed; but what does it mean on a relatively spherical body?</li>
<li>Anti-Mars meridian ... is this a meridian on the side of the moon facing away from Mars?</li>
</ol>

<p>Thank you.</p>

CC BY-SA 3.0
2015-10-14T10:29:42.257Z
@JanDoggen Maybe it is from this [mostly inaudible](http://sservi.nasa.gov/event/planetary-evolution-phobos-and-deimos/) online "course" by NASA SSERVI, where it looks as if a new astronomer talks to himself once a week. I've read those terms on their slideshows.
LocalFluff
<planet><fundamental-astronomy><terminology><observational-astronomy>
Questions about terminology used for Mars and its moons
atapaka
# Questions about terminology used for Mars and its moons

(Asked by: username_0 on 2015-10-13)

I am having difficulty with some terminology:

1. sub-Martian hemisphere of Deimos ... what place on Deimos does it refer to?
2. leading/trailing apex of Phobos ... what place are those?
3. Apex ... in general it means a top of something, or an end of something pointed; but what does it mean on a relatively spherical body?
4. Anti-Mars meridian ... is this a meridian on the side of the moon facing away from Mars?

Thank you.

**Tags:** planet, fundamental-astronomy, terminology, observational-astronomy

> **Comments:**
> **username_1**: @JanDoggen Maybe it is from this [mostly inaudible](http://sservi.nasa.gov/event/planetary-evolution-phobos-and-deimos/) online "course" by NASA SSERVI, where it looks as if a new astronomer talks to himself once a week. I've read those terms on their slideshows.  

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## 1 answers

### Answer by username_2 on 2015-10-13. Score: 3 | Accepted answer

Deimos is, like our own moon, tidally locked to Mars, so one hemisphere of Deimos faces the planet, and the faces away. The Sub-Martian hemisphere faces Mars.

Since Deimos is locked, there is a point on its surface that always faces in the direction of orbital motion, that point is the leading apex. It is an apex in the sense of a "leading point"

A meridian is a line of longitude, running from the North to the South pole. One of Deimos's meridians goes through the centre of its hemisphere that faces away from Mars, this is the anti-Mars Meridian. As you say, the "meridian on the side of Deimos facing away from Mars.

The image here shows a map of Deimos:

[![The leading face is the left side of the image, and the trailing face is on the right. The central 50% is the sub-martian hemisphere and the anti mars meridian is the line on the left and right edge, where the 3D image of Deimos has been cut to make a 2D map](https://i.sstatic.net/XJdkj.jpg)](https://i.sstatic.net/XJdkj.jpg)

The leading face is the left side of the image, and the trailing face is on the right. The central 50% is the sub-martian hemisphere and the anti mars meridian is the line on the left and right edge, where the 3D image of Deimos has been cut to make a 2D map


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