<p>If you take a line from the middle of the terminator, but at 90 degrees to the termination, it more or less points towards the sun.  Now consider the moon at first quarter: as the moon rises, the sun is due south, so imagine a line from the moon to the sun, and the terminator will be at 90 degrees to this, so pointing from bottom right to top left.  At mid-day, the sun is in the west, and the terminator will be sloping bottom left to top right.</p>

CC BY-SA 3.0
2017-04-03T21:47:35.670Z
2017-04-03T21:47:35.670Z
Dr Chuck
<p>A relatively simple way to understand why the moon axis seems to rotate is to observe that the stars do the same thing. Watch over several hours as <a href="https://en.wikipedia.org/wiki/Cassiopeia_(constellation)" rel="nofollow noreferrer">Cassiopeia</a> - located near the North Star moves and you'll notice the orientation changes. Cassiopeia, for example, is shaped like a "W" when lowest in the sky, a "3" as it rises, an "M" as it reaches its highest point overhead, a Σ (Sigma) as it is setting. Below is the constellation as it appears lowest in the sky.</p>

<p>[<img src="https://i.sstatic.net/3PvCQ.png" alt="Cassiopea at its lowest point"></p>

<p>Image from IAU and <a href="http://www.iau.org/public/themes/constellations/#cas" rel="nofollow noreferrer">Sky &amp; Telescope magazine (Roger Sinnott &amp; Rick Fienberg)</a></p>

<p>Because Cassiopeia is close to the north celestial pole observers in the Northern  hemisphere can watch the constellation rotate through all 360 degrees through the year.  Orion, however is near the celestial <em>equator</em> and the amount of apparent rotation relative to the ground is always less than 180 degrees and as little as 45 degrees - depending on your latitude.</p>

<p>The <strong>moon</strong> can be found anywhere from +28.5° to −28.5 (above or below) the celestial equator - its position relative to your observing latitude dictates the maximum amount of apparent rotation.</p>

CC BY-SA 3.0
2017-05-25T17:09:43.350Z
2017-05-25T17:09:43.350Z
Steven the Easily Amused
<p>I have noticed that the inclination of the moon at a given location varies throughout the night. From the UK, the terminator in a half-moon is clearly sloping from top left to bottom right early in the evening, but later on, as the moon is setting, it is clearly sloping from top right to bottom left. I know that the shape of the moon changes from night to night, but I just can't see why the inclination would change during a single night. I know that the appearance of the moon is due to a combination of the tilt of the earth's axis and the latitude of my position (the moon slopes oppositely in the southern hemisphere), but I just can't visualise in three dimensions exactly what is going on. Please can you help me to do this.</p>

<moon-phases>
Changing Appearance of the Moon During a Single Night
Melvin Hurst
# Changing Appearance of the Moon During a Single Night

(Asked by: username_0 on 2017-04-03)

I have noticed that the inclination of the moon at a given location varies throughout the night. From the UK, the terminator in a half-moon is clearly sloping from top left to bottom right early in the evening, but later on, as the moon is setting, it is clearly sloping from top right to bottom left. I know that the shape of the moon changes from night to night, but I just can't see why the inclination would change during a single night. I know that the appearance of the moon is due to a combination of the tilt of the earth's axis and the latitude of my position (the moon slopes oppositely in the southern hemisphere), but I just can't visualise in three dimensions exactly what is going on. Please can you help me to do this.

**Tags:** moon-phases


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## 2 answers

### Answer by username_1 on 2017-04-03. Score: 5

If you take a line from the middle of the terminator, but at 90 degrees to the termination, it more or less points towards the sun. Now consider the moon at first quarter: as the moon rises, the sun is due south, so imagine a line from the moon to the sun, and the terminator will be at 90 degrees to this, so pointing from bottom right to top left. At mid-day, the sun is in the west, and the terminator will be sloping bottom left to top right.


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### Answer by username_2 on 2017-05-25. Score: 3

A relatively simple way to understand why the moon axis seems to rotate is to observe that the stars do the same thing. Watch over several hours as [Cassiopeia](https://en.wikipedia.org/wiki/Cassiopeia_(constellation)) - located near the North Star moves and you'll notice the orientation changes. Cassiopeia, for example, is shaped like a "W" when lowest in the sky, a "3" as it rises, an "M" as it reaches its highest point overhead, a Σ (Sigma) as it is setting. Below is the constellation as it appears lowest in the sky.

[![Cassiopea at its lowest point](https://i.sstatic.net/3PvCQ.png)

Image from IAU and [Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)](http://www.iau.org/public/themes/constellations/#cas)

Because Cassiopeia is close to the north celestial pole observers in the Northern hemisphere can watch the constellation rotate through all 360 degrees through the year. Orion, however is near the celestial *equator* and the amount of apparent rotation relative to the ground is always less than 180 degrees and as little as 45 degrees - depending on your latitude.

The **moon** can be found anywhere from +28.5° to −28.5 (above or below) the celestial equator - its position relative to your observing latitude dictates the maximum amount of apparent rotation.


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