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By themselves, the brightness difference between solar and lunar twilight, for a given depression angle, will be equal to the brightness differences of the sources. The Sun is -26.7 mag., and let's say a fat gibbous Moon is -10.7. That's a difference of 16 magnitudes.

When well above the horizon, and in the absence of any significant light pollution, that Moon will make the sky about as bright as 19 mag/arcsec^2.

If we take the dark sky brightness at a fairly good location as 21.5 mag/arcsec^2, we see that a bright Moon makes for a sky some 2.5 mag. brighter. Let's say that any residual moonlight that does not brighten the sky by more than 0.2 magnitudes (about 20%) is not overly significant. So our otherwise pristine 21.5 mag/arcsec^2 sky must be no brighter than 21.3. Note that this vale of 21.3 is the summation of both airglow *and* moonlight. To make the sky 20% brighter means that the moonglow by itself is 20% as bright as that of airglow which is a surface brightness 0.8 magnitudes fainter, i.e., 21.5 + 0.8 = 22.3.

We can now see that the contribution from moonglow must darken from 19 to 22.3 mag/arcsec^2, or 3.3 magnitudes. One needs only monitor the sky during sunset to see at what point it becomes 3.3 magnitudes darker, and then note the solar depression angle.

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Until that post, I used to think I was good at math!

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Celestron CPC 1100
Denkmeier S2 Power Filter Switch Diagonal (.66x Reducer, 2x Barlow)
Ethos 17mm, Baader Scopos 30mm
Lunt 60mm Hydrogen-Alpha Solar Scope, Lunt Zoom

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Going on memory here, but that little chestnut in my sig is attributed to Admiral Hyman Rickover, USN.

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"Great minds discuss ideas;
Average minds discuss events;
Small minds discuss people."
Hyman Rickover

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Alex
12" GSO DOB
Paracorr
20mmNT5; GSO 2" 2X ED Barlow
15mm Panoptic; Tele Vue 1 1/4" 2x Barlow
10, 9, and 7.5 mm Plossls
Tectron Collimation Tools
Hotech Crosshair Laser

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