Observing before the moon comes up?

[SingleMalt]

Hey guys,

Maybe a stupid question, but I was wondering how long before the moon rises will the sky be dark enough for decent observing? I'm aware that it will depend on a few factors, but using next Saturday as an example, there's an expected moonrise of around 2215h and illumination of around 75%. If end of astronomical twilight is around 1840h, would it be dark enough then (at a dark sky site) for deep sky observing, or will the glow from the moon already be washing things out from below the horizon?

Cheers!

[Wavytone]

Depends... on how clear the sky, and the phase of the moon, and where you are looking in the sky.

A few days after full moon, you will see the sky deteriorate about 30 mins before moonrise, in the general vicinity but if you're looking well away from that direction it's not going to matter much till 10 mins before.

Get out and try...

[naskies (Dave)]

I believe the end/start astronomical twilight is defined as the point when the Sun crosses an altitude of 18 degrees below the horizon.

Going by the same principle, if the moon is at least 18 degrees below horizon (i.e. from 6:40 pm until 8:40 pm on that Saturday) then there's effectively no moon.

[SingleMalt]

That's an excellent point Dave, I hadn't thought of it like that but if the sun doesn't create any skyglow from 18 degree below the horizon then the moon sure shouldn't! I'll have a go on Saturday night and see what time the sky starts brightening up.

[AstralTraveller (David)]

The Sun is 400,000 times brighter than the Moon and so its effect is seen much further below the horizon. The Moon has little effect until pretty close to rise. You will easily observe until 10 and perhaps more. Do Neptune last, or perhaps a final look at Saturn. As the sky brightens nebulosity is more affected than stars, so do some clusters at the end - 47 Tuc is rising.