<p>The difference between the time around civil and nautical twilight and the middle of the night is the background brightness of the sky itself - and thus by proxy the brightness contrast between the illuminated part of the moon and the sky - and also the unilluminated part of the moon which is subject to Earth-shine. The amount of Earthshine the moon receives during a night does not vary (unless we have a lunar eclipse, of course).</p>
<p>Additionally, near the horizon, the attenuation of the moon through the atmosphere of the Earth is larger than when it is around zenith or high in the sky. So this further reduces the contrast of the moon brightness with respect to the sky brightness.</p>
<p>Both effects combined make it in the middle of the night for the human eye harder to see the earthshine (or perceive it as less strong) than during the twilight hours when the brightness contrast is not so large.</p>

CC BY-SA 4.0
2023-06-25T22:42:43.440Z
The amount of Earthshine that the Moon receives during the night varies a lot depending on what features on the Earth are illuminated by the Sun. Open (unclouded) ocean water has a rather low albedo of 0.06 while clouds can have an albedo as high as 0.9.
David Hammen
CC BY-SA 4.0
2023-06-26T22:00:07.327Z
@DavidHammen: True, but in general, a full hemisphere’s reflected light will be about the same. Clouds rarely cover (or are absent from) a full hemisphere at a time…
Pierre Paquette
CC BY-SA 4.0
2023-06-27T23:34:56.737Z
@PierrePaquette "rarely cover (or are absent from)" (i.e. albedo is rarely 0.9 or 0.05) These are just the two extremes, which is far from a valid argument that earthshine  can not vary a lot. However, poking around the internet I get the distinct impression the effect is not huge, though don't have the energy to go deep enough. Just for example Glenar et al. (2019) "Earthshine as an Illumination Source at the Moon" [1](https://arxiv.org/abs/1904.00236), [2](https://escholarship.org/content/qt36n888wz/qt36n888wz_noSplash_f49db76d8d95f31dfb6b2c14f7a0f123.pdf)
uhoh
CC BY-SA 4.0
2023-06-27T23:35:40.720Z
Some fun here https://www.bbso.njit.edu/Earthshine_webpage.html but they're looking for year-to-year variations for climate effects
uhoh
CC BY-SA 4.0
2023-06-28T00:37:47.017Z
@uhoh: Look at any weather-satellite map that shows a whole hemisphere or close to it, and the average cloud cover will be approximately the same. The link to njit.edu you give mentions that “The global albedo value is ∼0.3 in the visible,” without any mention of large deviations from that value, so I would put it between, say, 0.25 and 0.35. Not a huge effect indeed… There also is some influence from continents v. oceans, but I suppose that’s also counted in the “~0.3” value given above.
Pierre Paquette
CC BY-SA 4.0
2023-06-28T04:47:36.977Z
@PierrePaquette yes could be. Since infrared satellite images from geostationary (i.e. weather satellites) constantly capture cloud cover 24/7, somewhere there's going to be hard data on that.
uhoh
CC BY-SA 4.0
2023-06-28T04:47:49.260Z
Tangentially, I think this is cool/interesting: [In particular, we find a strong correlation between the evolution of the spectral intensity of the red edge and changes in the cloud-free vegetated area of the Amazon over the course of observations (Montañés-Rodríguez et al., ApJ, 629, 2005, and ApJ 651, 544, 2006). Thus, we “discovered” Amazonia measuring the earthshine. This matters in the search for terrestrial-like extra-solar planets.](https://www.bbso.njit.edu/Earthshine_webpage.html#:~:text=In%20particular%2C%20we,extra%2Dsolar%20planets.)
uhoh
CC BY-SA 4.0
2023-06-28T05:51:45.097Z
Whoa! Very interesting indeed!
Pierre Paquette
CC BY-SA 4.0
2023-06-25T22:03:41.047Z
2023-06-25T22:03:41.047Z
planetmaker
<p>So, as the other answer says, the earthshine is still there throughout the night. I was able to observe it for several nights including tonight, 2023-08-22 19:08 UTC+4.</p>
<p>There are some reasons why it appears invisible:</p>
<ol>
<li>At least for my post-LASIK eyes, spherical aberration when pupils are dilated leads to kind of streaks emanating from bright sources of light, which mask the glow of the dark side of the moon. Bringing a bright screen of the phone near the eyes, so that the light causes the pupils to contract, makes it easier to notice earthshine.</li>
<li>In some weather conditions (and especially in phases with wide lunar crescent) the lunar aureole masks the earthshine.</li>
</ol>
<p>To see the earthshine in such phases when even contracting eyes' pupils doesn't help, try hiding the bright crescent behind an object several meters away from the eyes or farther, while leaving the dark part of the lunar disk accessible to viewing. If the sky is a little dusty (e.g. long after the last rain), the lunar aureole still may not let you see earthshine.</p>
<p>In today's weather I was able to take a picture of earthshine of a 5.7-days old Moon:</p>
<p><a href="https://i.sstatic.net/qtAKD.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qtAKD.jpg" alt="" /></a></p>

CC BY-SA 4.0
2023-08-22T15:44:29.047Z
2023-08-22T15:44:29.047Z
Ruslan
<p>While watching the Moon in the thin-crescent phase (a few days after new Moon), I often noticed that the earthshine becomes noticeable near the end of civil twilight, and very noticeable during nautical twilight, after which, as the sky becomes dimmer, the earthshine finally fades out.</p>
<p>But on the next evening, when the crescent is still thin, the earthshine returns. This must mean that somehow its presence depends on time of day. But this behavior must be the same for any time zone (I observed it from several longitudes). So somehow the earthshine appears to exist for observers on one part of the Earth (who observe the correct phase of twilight), and not exist for others (the night part of the Earth).</p>
<p>How can this be explained? Isn't earthshine produced due to the light from the Earth, which doesn't depend too much on time of day from the lunar perspective?</p>

CC BY-SA 4.0
2023-06-25T17:55:56.333Z
For my understanding / clarification: you ask why the dark not-illuminated part of the moon is better visible near dusk or dawn than in the middle of the night?
planetmaker
CC BY-SA 4.0
2023-06-25T18:04:44.943Z
It might just be a horizon effect:  when the moon is a crescent and beyond nautical twilight, the moon must be very low in the sky and so Earthshine might just be lost in the thickness of the atmosphere.
James K
CC BY-SA 4.0
2023-06-25T19:31:00.643Z
You should also notice that even the lit side of the moon gets substantially dimmer near the horizon.
Greg Miller
CC BY-SA 4.0
2023-06-25T19:32:55.193Z
@planetmaker no, I'm asking about roughtly end of civil twilight vs end of nautical twilight.
Ruslan
CC BY-SA 4.0
2023-06-25T19:36:54.727Z
@JamesK What I observed on June 22 recently was with the Moon being at 32° altitude (the crescent was not so thin already, but the earthshine still visible), where atmospheric optical depth won't change radically after going 6° lower — but the earthshine will change from being very noticeable to invisible.
Ruslan
CC BY-SA 4.0
2023-06-25T19:38:14.497Z
Location please?
James K
CC BY-SA 4.0
2023-06-25T19:38:30.040Z
@JamesK Mauritius
Ruslan
CC BY-SA 4.0
2023-06-25T19:49:01.213Z
The actual amount of light being reflected off the Earth onto the moon isn't changing after all, your "civil twilight" is sunset for some to your west, and nautical twilgith for someone to your east.  So this has to be a combination of atmospheric effects, and the optical illusion of increasing contrast between the lit part of the moon and the night sky,.
James K
CC BY-SA 4.0
2023-06-25T21:36:29.553Z
@JamesK that comment is what I believe is the answer to the question - and it's why I asked for clarification - which I kinda got.
planetmaker
<the-moon><earth><light>
Why does earthshine fade out when night comes and reappear on the next evening?
Ruslan
# Why does earthshine fade out when night comes and reappear on the next evening?

(Asked by: username_0 on 2023-06-25)

While watching the Moon in the thin-crescent phase (a few days after new Moon), I often noticed that the earthshine becomes noticeable near the end of civil twilight, and very noticeable during nautical twilight, after which, as the sky becomes dimmer, the earthshine finally fades out.

But on the next evening, when the crescent is still thin, the earthshine returns. This must mean that somehow its presence depends on time of day. But this behavior must be the same for any time zone (I observed it from several longitudes). So somehow the earthshine appears to exist for observers on one part of the Earth (who observe the correct phase of twilight), and not exist for others (the night part of the Earth).

How can this be explained? Isn't earthshine produced due to the light from the Earth, which doesn't depend too much on time of day from the lunar perspective?

**Tags:** the-moon, earth, light
> **Comments:**
> **username_1**: For my understanding / clarification: you ask why the dark not-illuminated part of the moon is better visible near dusk or dawn than in the middle of the night?  
> **username_2**: It might just be a horizon effect:  when the moon is a crescent and beyond nautical twilight, the moon must be very low in the sky and so Earthshine might just be lost in the thickness of the atmosphere.  
> **username_3**: You should also notice that even the lit side of the moon gets substantially dimmer near the horizon.  
> **username_0**: @planetmaker no, I'm asking about roughtly end of civil twilight vs end of nautical twilight.  
> **username_0**: @JamesK What I observed on June 22 recently was with the Moon being at 32° altitude (the crescent was not so thin already, but the earthshine still visible), where atmospheric optical depth won't change radically after going 6° lower — but the earthshine will change from being very noticeable to invisible.  
> **username_2**: Location please?  
> **username_0**: @JamesK Mauritius  
> **username_2**: The actual amount of light being reflected off the Earth onto the moon isn't changing after all, your "civil twilight" is sunset for some to your west, and nautical twilgith for someone to your east.  So this has to be a combination of atmospheric effects, and the optical illusion of increasing contrast between the lit part of the moon and the night sky,.  
> **username_1**: @JamesK that comment is what I believe is the answer to the question - and it's why I asked for clarification - which I kinda got.  

---

## 2 answers

### Answer by username_1 on 2023-06-25. Score: 2

The difference between the time around civil and nautical twilight and the middle of the night is the background brightness of the sky itself - and thus by proxy the brightness contrast between the illuminated part of the moon and the sky - and also the unilluminated part of the moon which is subject to Earth-shine. The amount of Earthshine the moon receives during a night does not vary (unless we have a lunar eclipse, of course).

Additionally, near the horizon, the attenuation of the moon through the atmosphere of the Earth is larger than when it is around zenith or high in the sky. So this further reduces the contrast of the moon brightness with respect to the sky brightness.

Both effects combined make it in the middle of the night for the human eye harder to see the earthshine (or perceive it as less strong) than during the twilight hours when the brightness contrast is not so large.

> **Comments:**
> **username_4**: The amount of Earthshine that the Moon receives during the night varies a lot depending on what features on the Earth are illuminated by the Sun. Open (unclouded) ocean water has a rather low albedo of 0.06 while clouds can have an albedo as high as 0.9.  
> **username_5**: @DavidHammen: True, but in general, a full hemisphere’s reflected light will be about the same. Clouds rarely cover (or are absent from) a full hemisphere at a time…  
> **username_6**: @PierrePaquette "rarely cover (or are absent from)" (i.e. albedo is rarely 0.9 or 0.05) These are just the two extremes, which is far from a valid argument that earthshine  can not vary a lot. However, poking around the internet I get the distinct impression the effect is not huge, though don't have the energy to go deep enough. Just for example Glenar et al. (2019) "Earthshine as an Illumination Source at the Moon" [1](https://arxiv.org/abs/1904.00236), [2](https://escholarship.org/content/qt36n888wz/qt36n888wz_noSplash_f49db76d8d95f31dfb6b2c14f7a0f123.pdf)  
> **username_6**: Some fun here https://www.bbso.njit.edu/Earthshine_webpage.html but they're looking for year-to-year variations for climate effects  
> **username_5**: @uhoh: Look at any weather-satellite map that shows a whole hemisphere or close to it, and the average cloud cover will be approximately the same. The link to njit.edu you give mentions that “The global albedo value is ∼0.3 in the visible,” without any mention of large deviations from that value, so I would put it between, say, 0.25 and 0.35. Not a huge effect indeed… There also is some influence from continents v. oceans, but I suppose that’s also counted in the “~0.3” value given above.  
> **username_6**: @PierrePaquette yes could be. Since infrared satellite images from geostationary (i.e. weather satellites) constantly capture cloud cover 24/7, somewhere there's going to be hard data on that.  
> **username_6**: Tangentially, I think this is cool/interesting: [In particular, we find a strong correlation between the evolution of the spectral intensity of the red edge and changes in the cloud-free vegetated area of the Amazon over the course of observations (Montañés-Rodríguez et al., ApJ, 629, 2005, and ApJ 651, 544, 2006). Thus, we “discovered” Amazonia measuring the earthshine. This matters in the search for terrestrial-like extra-solar planets.](https://www.bbso.njit.edu/Earthshine_webpage.html#:~:text=In%20particular%2C%20we,extra%2Dsolar%20planets.)  
> **username_5**: Whoa! Very interesting indeed!  

---

### Answer by username_0 on 2023-08-22. Score: 1

So, as the other answer says, the earthshine is still there throughout the night. I was able to observe it for several nights including tonight, 2023-08-22 19:08 UTC+4.

There are some reasons why it appears invisible:

1. At least for my post-LASIK eyes, spherical aberration when pupils are dilated leads to kind of streaks emanating from bright sources of light, which mask the glow of the dark side of the moon. Bringing a bright screen of the phone near the eyes, so that the light causes the pupils to contract, makes it easier to notice earthshine.
2. In some weather conditions (and especially in phases with wide lunar crescent) the lunar aureole masks the earthshine.

To see the earthshine in such phases when even contracting eyes' pupils doesn't help, try hiding the bright crescent behind an object several meters away from the eyes or farther, while leaving the dark part of the lunar disk accessible to viewing. If the sky is a little dusty (e.g. long after the last rain), the lunar aureole still may not let you see earthshine.

In today's weather I was able to take a picture of earthshine of a 5.7-days old Moon:

[![](https://i.sstatic.net/qtAKD.jpg)](https://i.sstatic.net/qtAKD.jpg)


---

