Does the Moon still brighten the sky before moonrise or after moonset?
Asked 1/8/2015
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If the Moon is fairly bright (for example, around half illuminated or more), but it is still below the horizon for a few hours before moonrise or after moonset, will it still wash out stars? In other words, does the Moon affect sky darkness even when it is not yet visible, and if so, how does that compare with when it is above the horizon?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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The strict answer to your question is no. I say strict because your question says "the same as". The degree of wash out is related to proximity to the moon and, of course, the brightness of the moon. When the moon is visible in the sky the area of wash out is greater than when it's below the horizon. The further away you look from the moon the less the wash out. You'll get some wash out at the horizon where the moon is even when the moon is below the horizon. This will decrease as you look further away across the sky from the moon's position. The spread of the wash out will also depend on the atmosphere. If you are in a murky, polluted environment there'll be more Rayleigh scattering of the moon light and the wash out effect will be greater.
I had exactly this experience last night when I was making a time lapse of Orion hoping to catch comet Lovejoy. Even though the moon was below the horizon for the whole sequence I could see the wash out area increasing with time.
Originally by user31209. Source · Licensed CC BY-SA 4.0
user31209
11y ago
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Not to the same extent. A bright Moon can still brighten part of the sky when it is just below the horizon, but the effect is weaker than when the Moon is actually above the horizon.
The glow is strongest near the horizon in the direction where the Moon is located, and it decreases as you look farther away across the sky. When the Moon is visible, the washed-out area is larger and brighter. When it is below the horizon, you may still see some sky glow, especially if the atmosphere is hazy or polluted, because scattered moonlight can spread through the air.
So if you have a few hours before moonrise or after moonset, conditions are generally better for seeing faint stars than when the Moon is up, but the sky may not be equally dark in every direction—particularly low toward the Moon’s position.
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AI11y ago
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