<p>The moon is lit by the sun, so when the sun is below the horizon, the tips of the crescent moon will point &quot;up&quot; and away from the sun, while the outer bowl will point towards the sun.</p>
<p>This is sometimes hard to recognise because of an optical illusion created by the horizon.  We tend to see curves that are parallel to the horizon as &quot;straight&quot;, but when I write &quot;point towards the sun&quot; I mean &quot;along a great circle&quot;.</p>
<p>The axis of the Earth is consistently pointing towards the North. As I mention in an answer to <a href="https://astronomy.stackexchange.com/questions/48014/tips-of-crescent/48016">Tips of Crescent</a>, the tips are not exactly aligned with North or East.
The Earth's orbit means it is moving in a direction towards a point that is about 90 degrees behind the sun (in April that means roughly towards Saggitarius). Again, that's not really related to the angle of the moon.</p>

CC BY-SA 4.0
2022-04-04T20:33:13.233Z
There's nothing unusual about the angle of the moon in the picture.  It does vary from month to month, but not by a lot.  Take a look at a simulation in stellarium of similar.  That is just how the moon looks when it is a crescent and the sun is below and to the right.
James K
CC BY-SA 4.0
2022-04-04T20:56:27.337Z
`+1` although *if I say it* [David Hammen downvotes](https://astronomy.stackexchange.com/questions/48651/how-is-this-picture-of-the-moon-possible/48653#comment106621_48653) :-)
uhoh
CC BY-SA 4.0
2022-04-04T22:21:24.310Z
_"because of an optical illusion created by the horizon"_ — not only by the horizon. There's a specific configuration when the Sun may be at (or a bit under) the horizon, while the Moon high in the sky, and in this case the lit part of the Moon will seem to point _up_ instead of down or back. This is explained in my answer [here](https://physics.stackexchange.com/a/331058/21441)
Ruslan
CC BY-SA 4.0
2022-04-05T21:10:56.073Z
"not by a lot". I wrote a script, and was surprised to see the moon tilt vary between -10° and -70° at my location (49°N), for conditions similar to OP's picture.
Eric Duminil
CC BY-SA 4.0
2022-04-05T21:35:33.523Z
@uhoh that's the question I was looking for lmao
htmlcoderexe
CC BY-SA 4.0
2022-04-04T09:24:41.980Z
2022-04-04T09:38:22.313Z
2022-04-04T09:38:22.313Z
James K
<p>I took these cellphone snapshots on the previous two evenings, living nearer the equator than I used to I see slightly unfamiliar things from time to time, like the crescent Moon as a horizontal smiley face and my shadow completely disappearing at local noon in June.</p>
<p>In <a href="https://astronomy.stackexchange.com/a/48653/7982">this answer</a> I link to @DavidHammen's recommended source <a href="https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf" rel="noreferrer">The Moon Tilt Illusion</a> by Andrea K. Myers-Beaghton and Alan L. Myers which goes into great detail about how the orientation of the Moon's lit side varies.</p>
<p>See also (though they may confuse more than help)</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/27747/7982">How does the Moon move in the &quot;night&quot; sky as seen from the poles?</a></li>
<li><a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon" rel="noreferrer">Orbit of the Moon</a></li>
</ul>
<hr />
<p>Snapshots of the crescent Moon almost a horizontal smiley face from about 25° N</p>
<p><a href="https://i.sstatic.net/7NxLL.jpg" rel="noreferrer"><img src="https://i.sstatic.net/7NxLL.jpg" alt="snapshot of the crescent Moon almost a horizontal smiley face from about 25° N" /></a></p>
<p><a href="https://i.sstatic.net/wRvxt.jpg" rel="noreferrer"><img src="https://i.sstatic.net/wRvxt.jpg" alt="snapshot of the crescent Moon almost a horizontal smiley face from about 25° N" /></a></p>

CC BY-SA 4.0
2022-04-04T21:15:04.587Z
2022-04-04T21:15:04.587Z
uhoh
<h2>The moon tilt was indeed steeper than usual, and the steepest for 2022</h2>
<p><a href="https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf" rel="nofollow noreferrer"><img src="https://i.sstatic.net/tra9g.png" alt="enter image description here" /></a></p>
<p>I wrote a small script to calculate the <a href="https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf" rel="nofollow noreferrer"><em>moon tilt</em></a> <span class="math-container">$\alpha$</span> for every minute of 2022, when those conditions are fulfilled:</p>
<ul>
<li>after sunset, during nautical and astronomical <a href="https://en.wikipedia.org/wiki/Twilight" rel="nofollow noreferrer">twilights</a>, i.e. when the sky is dark enough to clearly see the moon.</li>
<li>the moon is above the horizon</li>
<li>its <a href="https://en.wikipedia.org/wiki/Phase_angle_(astronomy)" rel="nofollow noreferrer">phase angle</a> is between 120° and 140°. This corresponds to a thin crescent, just like in your picture.</li>
</ul>
<p>I was surprised to see the moon tilt vary between -10° (almost flat) and -60° (clearly pointing towards the ground), even at my latitude (49°N).</p>
<p>The steepest angle in 2022 was indeed a few days ago and a month ago, at -59°:</p>
<p><a href="https://i.sstatic.net/Gd2p9.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Gd2p9.png" alt="moon pointing down at sunset in march" /></a></p>
<p>The flattest angle will be in September, at -12°:</p>
<p><a href="https://i.sstatic.net/xPqMQ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xPqMQ.png" alt="moon pointing to the right at sunset in september" /></a></p>
<h2>Variations over the years</h2>
<p>I modified the script and let it run for every sunsets between 2000 and 2030, at my location:</p>
<p><a href="https://i.sstatic.net/l0Lvx.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/l0Lvx.png" alt="range of sunset moon tilts" /></a></p>
<p>The steepest moon tilt happened on 2010-02-17 (at -70°):</p>
<p><a href="https://i.sstatic.net/8GksS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8GksS.png" alt="enter image description here" /></a></p>
<p>The flattest moon tilt happened on 2009-08-23 (at -9°):</p>
<p><a href="https://i.sstatic.net/g4x11.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/g4x11.png" alt="enter image description here" /></a></p>
<h2>Code</h2>
<p>If you're interested, here's the &quot;code&quot; I used to calculate the moon tilt. The program was written in <a href="https://insel.eu/en/home_en.html" rel="nofollow noreferrer">INSEL</a>, which is similar to Simulink:</p>
<p><a href="https://i.sstatic.net/Q7vgy.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Q7vgy.png" alt="INSEL model for moon tilt" /></a></p>
<p>Another, possibly easier way would be to use <a href="https://www.astropy.org/" rel="nofollow noreferrer">Astropy</a> or <a href="https://rhodesmill.org/pyephem/quick.html" rel="nofollow noreferrer">PyEphem</a>.</p>

CC BY-SA 4.0
2022-04-06T02:04:07.710Z
Would you share the script, or at least the library you used for it? I've been wanting to write similar scripts and am looking for a nice API for it (even posted this question on reddit a couple months ago: https://www.reddit.com/szsirc )
Christopher Shroba
CC BY-SA 4.0
2022-04-06T04:02:19.670Z
Excellent answer, it's great to know that my feeling that the Moon "looked funny" and [taking snapshots](https://astronomy.stackexchange.com/a/49005/7982) of it was somehow warranted. :-)
uhoh
CC BY-SA 4.0
2022-04-06T07:56:32.713Z
@ChristopherShroba: I've edited the answer. I've used INSEL, a graphical programming language, which only knows the position of the moon and the sun. For your reddit question, I'm pretty sure both astropy and pyephem would work fine, and they can give you the position of millions of objects.
Eric Duminil
CC BY-SA 4.0
2022-04-06T10:50:55.753Z
**(1/2)** Nice answer. some interesting point: one can literally see the asymmetry of the contribution of the Moon 5 degrees inclination with respect to the ecliptic, to the tilt angle: it is stronger in increasing the tilt than flattening it. It makes sense: if, following the other answer of James K, the tilt is always parallel to the ecliptic than to have no tilt, the Moon most be on the nonagesimal: the highest point on the ecliptic to have no tilt.
d_e
CC BY-SA 4.0
2022-04-06T10:52:07.903Z
**(2/2)** even if the Moon has some latitude, it is not going to change because, again, at the nonagesimal the ecliptic is parallel to the horizon, so the Moon earns no bonus from his 5 degrees latitude. 

It follows that in the night sky (after the Sun already set), in order to see the Moon without tilt, it must be 90 degrees from the Sun at least: for it if is less than 90 degress it means the Moon has already passed the nonagesimal point.

All this means that we can't see the Moon less than half full after sunset without tilt.
d_e
CC BY-SA 4.0
2022-04-06T11:22:29.250Z
@d_e: Thanks for the interesting comment. I've never heard "nonagesimal" before, I'll take a look at the definition. The ecliptic does seem to play a large role in the moon tilt. Do you have any idea why the range of tilts look like this over the years? It doesn't seem to fit the lunar standstill curve, for example. For what it's worth, I've found that on 2020-08-25 at 19:20 in Stuttgart, the sun was just below the horizon, the half moon was at 20° elevation, and pretty much perfectly without tilt. atan function makes it hard to find values higher than 0°. I'll try with atan2.
Eric Duminil
CC BY-SA 4.0
2022-04-06T12:15:28.037Z
@EricDuminil, the "nonagesimal" is the highest point on the ecliptic: exactly 90 degree from each of the ecliptic intersection with the horizon. Re: The example with Stuttgart: the tilt of the half moon is always expected to be very low when the Sun sets. we are talking about really small numbers. maybe parallax and/or refraction issues might render my analyses very-little wrong in practice. but I still think it works in theory. Also I can see the relation to the standstill curve. peaks are in the middle between Major and Minor standstill. I can see why it so but can't explain in comment.
d_e
CC BY-SA 4.0
2022-04-05T20:35:42.950Z
2022-04-06T07:54:10.110Z
2022-04-06T07:54:10.110Z
Eric Duminil
<p>I never noticed it before that moon I see in the sky is tilted. Does the tilt of the moon in the sky indicate the direction of the Earth's axis and direction of Earth's rotation around the sun?</p>
<p><a href="https://i.sstatic.net/E3p2P.jpg" rel="noreferrer"><img src="https://i.sstatic.net/E3p2P.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2022-04-04T14:45:09.560Z
This is called the Parallactic angle: https://en.m.wikipedia.org/wiki/Parallactic_angle
Greg Miller
CC BY-SA 4.0
2022-04-04T18:21:14.033Z
@GregMiller: Are you sure about that? Parallactic angle seems to describe the orientation of the features of the moon (say, the Sea of Tranquillity), not which part of it is illuminated by the sun. The angle described by OP indeed seems to be related to the parallactic angle, but I don't think they're the same.
Eric Duminil
CC BY-SA 4.0
2022-04-05T14:19:32.153Z
Try this; Look at the tilt of the moon when it rises, then look at it's tilt when it sets.  At most latitudes at most times of the year, these will be significantly different.
RBarryYoung
CC BY-SA 4.0
2022-04-05T14:39:54.220Z
@EricDuminil, yes I'm sure.  The parallactic angle affects everything, the entire moon appears to rotate based on the angle from due south.
Greg Miller
CC BY-SA 4.0
2022-04-05T15:08:04.440Z
@GregMiller: We agree that the parallactic angle affects the angle of the crescent, because it affects the entire moon. I don't think the angle of the crescent *is* the parallactic angle, though.
Eric Duminil
<the-moon>
The moon in the first phase visible in the sky is tilted
Wojciech Domalewski
# The moon in the first phase visible in the sky is tilted

(Asked by: username_0 on 2022-04-04)

I never noticed it before that moon I see in the sky is tilted. Does the tilt of the moon in the sky indicate the direction of the Earth's axis and direction of Earth's rotation around the sun?

[![enter image description here](https://i.sstatic.net/E3p2P.jpg)](https://i.sstatic.net/E3p2P.jpg)

**Tags:** the-moon

> **Comments:**
> **username_1**: This is called the Parallactic angle: https://en.m.wikipedia.org/wiki/Parallactic_angle  
> **username_2**: @username_1: Are you sure about that? Parallactic angle seems to describe the orientation of the features of the moon (say, the Sea of Tranquillity), not which part of it is illuminated by the sun. The angle described by OP indeed seems to be related to the parallactic angle, but I don't think they're the same.  
> **username_3**: Try this; Look at the tilt of the moon when it rises, then look at it's tilt when it sets.  At most latitudes at most times of the year, these will be significantly different.  
> **username_1**: @username_2, yes I'm sure.  The parallactic angle affects everything, the entire moon appears to rotate based on the angle from due south.  
> **username_2**: @username_1: We agree that the parallactic angle affects the angle of the crescent, because it affects the entire moon. I don't think the angle of the crescent *is* the parallactic angle, though.  

---

## 3 answers

### Answer by username_4 on 2022-04-04. Score: 10

The moon is lit by the sun, so when the sun is below the horizon, the tips of the crescent moon will point "up" and away from the sun, while the outer bowl will point towards the sun.

This is sometimes hard to recognise because of an optical illusion created by the horizon. We tend to see curves that are parallel to the horizon as "straight", but when I write "point towards the sun" I mean "along a great circle".

The axis of the Earth is consistently pointing towards the North. As I mention in an answer to [Tips of Crescent](https://astronomy.stackexchange.com/questions/48014/tips-of-crescent/48016), the tips are not exactly aligned with North or East.
The Earth's orbit means it is moving in a direction towards a point that is about 90 degrees behind the sun (in April that means roughly towards Saggitarius). Again, that's not really related to the angle of the moon.

> **Comments:**
> **username_4**: There's nothing unusual about the angle of the moon in the picture.  It does vary from month to month, but not by a lot.  Take a look at a simulation in stellarium of similar.  That is just how the moon looks when it is a crescent and the sun is below and to the right.  
> **username_5**: `+1` although *if I say it* [David Hammen downvotes](https://astronomy.stackexchange.com/questions/48651/how-is-this-picture-of-the-moon-possible/48653#comment106621_48653) :-)  
> **username_6**: _"because of an optical illusion created by the horizon"_ — not only by the horizon. There's a specific configuration when the Sun may be at (or a bit under) the horizon, while the Moon high in the sky, and in this case the lit part of the Moon will seem to point _up_ instead of down or back. This is explained in my answer [here](https://physics.stackexchange.com/a/331058/21441)  
> **username_2**: "not by a lot". I wrote a script, and was surprised to see the moon tilt vary between -10° and -70° at my location (49°N), for conditions similar to OP's picture.  
> **username_7**: @username_5 that's the question I was looking for lmao  

---

### Answer by username_5 on 2022-04-04. Score: 9

I took these cellphone snapshots on the previous two evenings, living nearer the equator than I used to I see slightly unfamiliar things from time to time, like the crescent Moon as a horizontal smiley face and my shadow completely disappearing at local noon in June.

In [this answer](https://astronomy.stackexchange.com/a/48653/7982) I link to @DavidHammen's recommended source [The Moon Tilt Illusion](https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf) by Andrea K. Myers-Beaghton and Alan L. Myers which goes into great detail about how the orientation of the Moon's lit side varies.

See also (though they may confuse more than help)

* [How does the Moon move in the "night" sky as seen from the poles?](https://astronomy.stackexchange.com/q/27747/7982)
* [Orbit of the Moon](https://en.wikipedia.org/wiki/Orbit_of_the_Moon)

---

Snapshots of the crescent Moon almost a horizontal smiley face from about 25° N

[![snapshot of the crescent Moon almost a horizontal smiley face from about 25° N](https://i.sstatic.net/7NxLL.jpg)](https://i.sstatic.net/7NxLL.jpg)

[![snapshot of the crescent Moon almost a horizontal smiley face from about 25° N](https://i.sstatic.net/wRvxt.jpg)](https://i.sstatic.net/wRvxt.jpg)


---

### Answer by username_2 on 2022-04-05. Score: 7

## The moon tilt was indeed steeper than usual, and the steepest for 2022

[![enter image description here](https://i.sstatic.net/tra9g.png)](https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf)

I wrote a small script to calculate the [*moon tilt*](https://www.seas.upenn.edu/%7Eamyers/MoonPaperOnline.pdf) $\alpha$ for every minute of 2022, when those conditions are fulfilled:

* after sunset, during nautical and astronomical [twilights](https://en.wikipedia.org/wiki/Twilight), i.e. when the sky is dark enough to clearly see the moon.
* the moon is above the horizon
* its [phase angle](https://en.wikipedia.org/wiki/Phase_angle_(astronomy)) is between 120° and 140°. This corresponds to a thin crescent, just like in your picture.

I was surprised to see the moon tilt vary between -10° (almost flat) and -60° (clearly pointing towards the ground), even at my latitude (49°N).

The steepest angle in 2022 was indeed a few days ago and a month ago, at -59°:

[![moon pointing down at sunset in march](https://i.sstatic.net/Gd2p9.png)](https://i.sstatic.net/Gd2p9.png)

The flattest angle will be in September, at -12°:

[![moon pointing to the right at sunset in september](https://i.sstatic.net/xPqMQ.png)](https://i.sstatic.net/xPqMQ.png)

## Variations over the years

I modified the script and let it run for every sunsets between 2000 and 2030, at my location:

[![range of sunset moon tilts](https://i.sstatic.net/l0Lvx.png)](https://i.sstatic.net/l0Lvx.png)

The steepest moon tilt happened on 2010-02-17 (at -70°):

[![enter image description here](https://i.sstatic.net/8GksS.png)](https://i.sstatic.net/8GksS.png)

The flattest moon tilt happened on 2009-08-23 (at -9°):

[![enter image description here](https://i.sstatic.net/g4x11.png)](https://i.sstatic.net/g4x11.png)

## Code

If you're interested, here's the "code" I used to calculate the moon tilt. The program was written in [INSEL](https://insel.eu/en/home_en.html), which is similar to Simulink:

[![INSEL model for moon tilt](https://i.sstatic.net/Q7vgy.png)](https://i.sstatic.net/Q7vgy.png)

Another, possibly easier way would be to use [Astropy](https://www.astropy.org/) or [PyEphem](https://rhodesmill.org/pyephem/quick.html).

> **Comments:**
> **username_8**: Would you share the script, or at least the library you used for it? I've been wanting to write similar scripts and am looking for a nice API for it (even posted this question on reddit a couple months ago: https://www.reddit.com/szsirc )  
> **username_5**: Excellent answer, it's great to know that my feeling that the Moon "looked funny" and [taking snapshots](https://astronomy.stackexchange.com/a/49005/7982) of it was somehow warranted. :-)  
> **username_2**: @username_8: I've edited the answer. I've used INSEL, a graphical programming language, which only knows the position of the moon and the sun. For your reddit question, I'm pretty sure both astropy and pyephem would work fine, and they can give you the position of millions of objects.  
> **username_9**: **(1/2)** Nice answer. some interesting point: one can literally see the asymmetry of the contribution of the Moon 5 degrees inclination with respect to the ecliptic, to the tilt angle: it is stronger in increasing the tilt than flattening it. It makes sense: if, following the other answer of username_4, the tilt is always parallel to the ecliptic than to have no tilt, the Moon most be on the nonagesimal: the highest point on the ecliptic to have no tilt.  
> **username_9**: **(2/2)** even if the Moon has some latitude, it is not going to change because, again, at the nonagesimal the ecliptic is parallel to the horizon, so the Moon earns no bonus from his 5 degrees latitude.   It follows that in the night sky (after the Sun already set), in order to see the Moon without tilt, it must be 90 degrees from the Sun at least: for it if is less than 90 degress it means the Moon has already passed the nonagesimal point.  All this means that we can't see the Moon less than half full after sunset without tilt.  
> **username_2**: @username_9: Thanks for the interesting comment. I've never heard "nonagesimal" before, I'll take a look at the definition. The ecliptic does seem to play a large role in the moon tilt. Do you have any idea why the range of tilts look like this over the years? It doesn't seem to fit the lunar standstill curve, for example. For what it's worth, I've found that on 2020-08-25 at 19:20 in Stuttgart, the sun was just below the horizon, the half moon was at 20° elevation, and pretty much perfectly without tilt. atan function makes it hard to find values higher than 0°. I'll try with atan2.  
> **username_9**: @username_2, the "nonagesimal" is the highest point on the ecliptic: exactly 90 degree from each of the ecliptic intersection with the horizon. Re: The example with Stuttgart: the tilt of the half moon is always expected to be very low when the Sun sets. we are talking about really small numbers. maybe parallax and/or refraction issues might render my analyses very-little wrong in practice. but I still think it works in theory. Also I can see the relation to the standstill curve. peaks are in the middle between Major and Minor standstill. I can see why it so but can't explain in comment.  

---

