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What level of percent-illumination of moon (waxing crescent), given by Stellarium, is enough to make it visible with naked eye, in clear sky?


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I would think even 1% would be visible, but am not completely sure. The problem is that the moon is very close to the sun at this point (in the sky, not actually close to the sun). For more information, see: – Jonathan Jun 27 '14 at 10:27
According to Stellarium, even the new moon (at least the one coming up on 10 Jan 2016) has a magnitude of -1.39, which would be visible if it weren't so close to the Sun. The definition of astronomical twilight is that 6th magnitude stars are visible at the zenith when the Sun is 18 degrees below the horizon (ie, 108 degrees away from the stars themselves), but I don't think there's a general formula mapping angular distance from sun to faintest visible magnitude. – barrycarter Dec 28 '15 at 7:11
up vote 3 down vote accepted

To add to Florin's great answer, even the dark part of the moon is visible due to illumination by the earth (has a magnitude of around -3, probably calculated for a new moon/solar eclipse configuration -, first paragraph of 'brightness of earthshine on moon'). So, the moon is always visible if not in the 'glow' of the Sun.

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Any percentage at all would be visible, if the sky is dark enough. The surface on the illumined part is as bright as the street in front of your house during the day. So even a very tiny sliver would be clearly visible.

But you're running into a different problem here. As the illumined portion decreases, the apparent position of the Moon in the sky gets closer to the Sun. At some point it will be lost in the glare. It's a contrast issue.

Your best bet is a coincidence: if the Moon's apparent position is very close to the Sun's, while the Sun is setting, and you have clear visibility to a flat western horizon. In that case, the Moon's crescent could become very thin indeed, and still remain visible.

I've seen it like that, once. The Moon was very close to the Sun, several lunar diameters away. The Sun had just descended under the horizon. The Moon was very low in the sky, extremely thin, almost too thin to estimate the width of the illumined portion. The expression "razor edge" comes to mind. The thin curved line made less than half a circle (maybe 1/3?), with the two ends gradually tapering off in terms of brightness.

Due to the unusual shape, it took me a second or two to realize that that was actually the Moon. I showed it to my kids and their reaction was: "That's the Moon? NO! Wait... Really?" It was the Moon, really, confirmed with SkySafari on my phone.

It was not drawing attention to it by any means, being almost lost in the sunset glare - not bright, just a thin, discreet, whitish curve in a yellow-red sky. But once you saw it, it was clearly there. It would have been invisible if the Sun was not below horizon, I think, or maybe not. Definitely invisible at mid-day, without any doubt.

I wish I had access to a telescope during that rare sight. :( Although, with the Moon at such low elevation, seeing must have been atrocious.

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So, that means that we cant be sure that whether crescent moon be visible or not with naked eye, depending on percent-illumination? – kaka Jun 27 '14 at 21:06

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