<p>You are confusing two different motions:</p>
<ul>
<li>Celestial objects appear to move 15 degrees per hour from east to west due to the Earth's rotation.</li>
<li>The Moon appears to move approximately 0.5 degrees (its diameter) per hour west to east relative to the stars due to the Moon's orbit around the Earth.</li>
</ul>
<p>The motion you were &quot;measuring&quot; with the cloud was the 15 degrees per hour. A few minutes to cover the Moon's diameter is reasonable.</p>
<p>Note: The rate is 15 degrees per hour in motion in right ascension, and is only accurate for how far an object moves in the sky when the object is near the celestial equator. The actual motion also depends on the cosine of the declination. (Polaris and sigma Octans, the pole stars, also move at 15 degrees per hour in right ascension, but being near the poles, they hardly move in the sky; that is, cosine(90)=0.)</p>
<p>In the case of the Moon rise, the Moon is generally rising at some angle instead of &quot;straight&quot; up from the horizon or cloud line. In other words, the Moon is moving &quot;up and right&quot; for example. This also adds to the time it would take to move its apparent diameter.</p>

CC BY-SA 4.0
2024-02-01T07:26:09.950Z
There is also a difference due to the parallax. The Moon passing the local meridian of the observer travels (apparently) faster than at the horizon. However, this might be a slight difference.
Jacob Miller
CC BY-SA 4.0
2023-08-01T00:47:20.157Z
2023-08-01T00:47:20.157Z
JohnHoltz
<p>I read that the apparent motion of the moon is approximately its diameter every hour.
However, we just observed the moon appearing to rise through a stationary cloud in about 10 minutes, it definitely appeared that the moon moved through its diameter as it rose far faster.</p>
<p>My son thought this was some kind of lensing effect at the horizon, but that makes the moon appear larger at the horizon, and this seemed far too fast.</p>
<p>If anything what we read is that the moon should appear to move slower near the horizon.
As far as we could tell, the cloud was stationary, and we watched the moon steadily get covered until it was completely hidden behind the cloud.</p>
<p>Obviously the moon moves at different rates depending on its distance to the earth, but that is a tiny figure. This must have been some optical illusion.</p>

CC BY-SA 4.0
2024-02-01T08:52:25.853Z
@Ruslan One of the things that is why the Moon looks bigger near the horizon can be, how human cognition negotiates the elevation-distance combination. More frequent terrain differences humans encounter in day-to-day experience are way less than their typical distance to the horizon.  So the brain may use an additional logic parameter that the same angular size + more slope from the horizontal plane means a nearby location. So smaller is the impression. That may be why even the astronauts at ISS notice this effect although there are almost no background objects other than the earth.
Jacob Miller
CC BY-SA 4.0
2024-02-01T15:47:47.270Z
@JacobMiller given that the astronauts aren't always aligned with their heads to zenith, I doubt your explanation makes much sense, especially without any actual citations of the astronauts' evidence.
Ruslan
CC BY-SA 4.0
2024-02-03T06:15:20.400Z
@Ruslan It is just one of the "maybe" factors. There are likely several factors at play here. Also, note that ISS is under microgravity. So Astronaut's sense of their orientation is also impeded. So it might be usual for the brain to consider a much bigger earth as terrain.
Jacob Miller
CC BY-SA 4.0
2024-02-03T06:21:02.063Z
@Ruslan It is important to consider the key issue here. Moon against earth + sky/space at the horizon vs Moon against only the sky/space at zenith. Also although there are no background objects there is still a background to compare against. When the moon is the only object in the observer's view field, the brain compares the size of the moon with the sky/space field and logs it. Then at the horizon, the brain compares the moon "also" against two smaller objects, the Earth and the sky/space. ...
Jacob Miller
CC BY-SA 4.0
2024-02-03T06:21:58.170Z
@Ruslan ... Both occupy the same view field hence each is smaller. So the second logging gives the moon a relatively bigger size than the first. So is the impression when the brain appraises the two memory logs against each other.
Jacob Miller
CC BY-SA 4.0
2023-08-01T14:46:11.403Z
The sky is about 360 moon-diameters across, but the moon certainly doesn't take 15 days to cross the sky.
Nuclear Hoagie
CC BY-SA 4.0
2023-08-01T18:58:04.060Z
*but that makes the moon appear larger at the horizon* — no it doesn't: to the contrary, it makes the moon smaller in vertical direction, so that a disk becomes flattened to an ellipse. The fact that it seems larger (but doesn't "appear", since e.g. if you take a photo, you'll notice it's the same angular width as in the zenith) is known as the [Moon illusion](https://solarsystem.nasa.gov/news/1191/the-moon-illusion-why-does-the-moon-look-so-big-sometimes/).
Ruslan
<the-moon><apparent-motion>
Moon moving near horizon much faster than at zenith, reason? error?
Dov
# Moon moving near horizon much faster than at zenith, reason? error?

(Asked by: username_0 on 2023-08-01)

I read that the apparent motion of the moon is approximately its diameter every hour.
However, we just observed the moon appearing to rise through a stationary cloud in about 10 minutes, it definitely appeared that the moon moved through its diameter as it rose far faster.

My son thought this was some kind of lensing effect at the horizon, but that makes the moon appear larger at the horizon, and this seemed far too fast.

If anything what we read is that the moon should appear to move slower near the horizon.
As far as we could tell, the cloud was stationary, and we watched the moon steadily get covered until it was completely hidden behind the cloud.

Obviously the moon moves at different rates depending on its distance to the earth, but that is a tiny figure. This must have been some optical illusion.

**Tags:** the-moon, apparent-motion

> **Comments:**
> **username_3**: The sky is about 360 moon-diameters across, but the moon certainly doesn't take 15 days to cross the sky.  
> **username_2**: *but that makes the moon appear larger at the horizon* — no it doesn't: to the contrary, it makes the moon smaller in vertical direction, so that a disk becomes flattened to an ellipse. The fact that it seems larger (but doesn't "appear", since e.g. if you take a photo, you'll notice it's the same angular width as in the zenith) is known as the [Moon illusion](https://solarsystem.nasa.gov/news/1191/the-moon-illusion-why-does-the-moon-look-so-big-sometimes/).  
> **username_1**: @username_2 One of the things that is why the Moon looks bigger near the horizon can be, how human cognition negotiates the elevation-distance combination. More frequent terrain differences humans encounter in day-to-day experience are way less than their typical distance to the horizon.  So the brain may use an additional logic parameter that the same angular size + more slope from the horizontal plane means a nearby location. So smaller is the impression. That may be why even the astronauts at ISS notice this effect although there are almost no background objects other than the earth.  
> **username_2**: @username_1 given that the astronauts aren't always aligned with their heads to zenith, I doubt your explanation makes much sense, especially without any actual citations of the astronauts' evidence.  
> **username_1**: @username_2 It is just one of the "maybe" factors. There are likely several factors at play here. Also, note that ISS is under microgravity. So Astronaut's sense of their orientation is also impeded. So it might be usual for the brain to consider a much bigger earth as terrain.  
> **username_1**: @username_2 It is important to consider the key issue here. Moon against earth + sky/space at the horizon vs Moon against only the sky/space at zenith. Also although there are no background objects there is still a background to compare against. When the moon is the only object in the observer's view field, the brain compares the size of the moon with the sky/space field and logs it. Then at the horizon, the brain compares the moon "also" against two smaller objects, the Earth and the sky/space. ...  
> **username_1**: @username_2 ... Both occupy the same view field hence each is smaller. So the second logging gives the moon a relatively bigger size than the first. So is the impression when the brain appraises the two memory logs against each other.  

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## 1 answers

### Answer by username_4 on 2023-08-01. Score: 30 | Accepted answer

You are confusing two different motions:

* Celestial objects appear to move 15 degrees per hour from east to west due to the Earth's rotation.
* The Moon appears to move approximately 0.5 degrees (its diameter) per hour west to east relative to the stars due to the Moon's orbit around the Earth.

The motion you were "measuring" with the cloud was the 15 degrees per hour. A few minutes to cover the Moon's diameter is reasonable.

Note: The rate is 15 degrees per hour in motion in right ascension, and is only accurate for how far an object moves in the sky when the object is near the celestial equator. The actual motion also depends on the cosine of the declination. (Polaris and sigma Octans, the pole stars, also move at 15 degrees per hour in right ascension, but being near the poles, they hardly move in the sky; that is, cosine(90)=0.)

In the case of the Moon rise, the Moon is generally rising at some angle instead of "straight" up from the horizon or cloud line. In other words, the Moon is moving "up and right" for example. This also adds to the time it would take to move its apparent diameter.

> **Comments:**
> **username_1**: There is also a difference due to the parallax. The Moon passing the local meridian of the observer travels (apparently) faster than at the horizon. However, this might be a slight difference.  

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