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Referencing the following data for December 11th 2017.

https://www.timeanddate.com/moon/@4799274?month=12&year=2017

To get a dark sky for stargazing does it matter if the moon is at 37.8% so long as it is below the horizon when I'm observing?

Since the moon will be set at 1:32PM on the 11th and not rise again until 1:59AM on the 12th I would assume the moon would not affect viewing between those times.

Furthermore is there a guideline as to how close to moonrise the moon will start to have an affect?

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    $\begingroup$ Do you, perhaps, mean the full moon? $\endgroup$ – Mick Nov 28 '17 at 8:21
  • $\begingroup$ physics.stackexchange.com/questions/1968/… may or may not be helpful. Also, calling the 37.8% moon "new" is a bit of a stretch... it's at least crescent if not quarter. $\endgroup$ – user21 Nov 28 '17 at 15:52
  • $\begingroup$ To some extent, a dirty atmosphere (particulates) could scatter OTH (over the horizon) moonlight back into your field of view. I do not know what the estimated values of this effect would be. $\endgroup$ – Carl Witthoft Nov 28 '17 at 16:01
  • $\begingroup$ I do not have any references at the moment, but I think a 30%-37% illuminated Moon will not create any interference until it rises. So your statement about the sky being dark until 2 am on the 12th is correct. Depending on what you are observing, the 30'something % Moon may not be much of a hindrance after moonrise. The Full Moon is approximately 400 000 times fainter than the sun, so lunar "twilight" before moonrise will be that much fainter and solar twilight. $\endgroup$ – JohnHoltz Nov 29 '17 at 17:31
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For most people the Moon does not significantly impact on stargazing, even when it is full, unless they are trying to view something in a direction close to the Moon.

For people doing astrophotography, the Moon has a bigger impact since the sky is noticeably brighter and results in fainter objects not being able to be photographed.

And for astronomical observatories, some operations are impossible at worst and severely restricted at best since the telescopes are usually larger than "home" varieties and cameras much more sensitive but most noticeably a few days either side of the full moon.

So, depending on what you are actually doing, a ~38% Moon will not be a problem and if you are looking more than about 30° away from the Moon you will not even notice it.

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