Moon and its effect on Milky Way?

Started 2 months ago | Discussions thread
Bill Ferris
Bill Ferris Veteran Member • Posts: 4,327
Re: Moon and its effect on Milky Way?

vegetaleb wrote:

Bill Ferris wrote:

A pristine night sky with no Moon has a surface brightness of about 22.0 magnitudes per square arcsecond. A full Moon in those same conditions increases the sky's surface brightness to roughly 18.0 magnitudes per square arcsecond. That's 4 magnitudes or 40x brighter.

So I guess the other night it was about 19, it was a 70% lighted moon raising above horizon.

What about a half moon that will rise 3 hours after taking the photos?

Astronomical twilight begins and ends when the Sun (-27 magnitude) is 18 degrees below the horizon. At that distance below the horizon, the Sun has no impact on sky brightnes above the horizon.

With the full Moon being quite a bit fainter (1/400,000th the brightness of the Sun), I would guess it needs to be within 5 degrees or less of the horizon to noticeably impact sky brightness. (Just a guess) A quarter Moon that's 3-hours from rising will not impact sky brightness.

If you have access to a site with western or eastern horizons not impacted by urban skyglow, you could gauge the Moon's impact by making 30-second exposures with a wide angle lens at intervals of 1, 5 ,10, 15, 20 and 30 minutes after moonset or before moonrise to see for yourself where the threshold lies.

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Flagstaff, AZ

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