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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?

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    $\begingroup$ 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? $\endgroup$ Jun 25, 2023 at 17:55
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    $\begingroup$ 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. $\endgroup$
    – James K
    Jun 25, 2023 at 18:04
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    $\begingroup$ You should also notice that even the lit side of the moon gets substantially dimmer near the horizon. $\endgroup$ Jun 25, 2023 at 19:31
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    $\begingroup$ @planetmaker no, I'm asking about roughtly end of civil twilight vs end of nautical twilight. $\endgroup$
    – Ruslan
    Jun 25, 2023 at 19:32
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    $\begingroup$ @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. $\endgroup$
    – Ruslan
    Jun 25, 2023 at 19:36

2 Answers 2

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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.

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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:

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