What level of percent-illumination of moon (waxing crescent), given by Stellarium, is enough to make it visible with naked eye, in clear sky?
thanks
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$\begingroup$ 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: en.wikipedia.org/wiki/Lunar_phase $\endgroup$ – Jonathan Jun 27 '14 at 10:27
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1$\begingroup$ 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. $\endgroup$ – barrycarter Dec 28 '15 at 7:11
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$\begingroup$ The answers I have read are nothing more than unknowing guesses based on what seems logical to the writers. Is there anyone out there that can give an answer based on essential illumination for actual observations. If you say, "That depends on the location of the observers", then Jerusalem would be an excellent location because of the regular watching for the first visible sliver of the new moon from Jerusalem and its surroundings. $\endgroup$ – user13692 Aug 6 '16 at 16:24
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$\begingroup$ FWIW, a couple of times while (naked eye) viewing Venus and the nearby very thin waning crescent Moon around sunrise, the Moon became invisible to me; Venus was still quite visible. $\endgroup$ – PM 2Ring Jun 5 '19 at 4:27
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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 - http://adsabs.harvard.edu/full/1943MNSSA...2...24J, 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 itself 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.
answered Jun 27 '14 at 19:27
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$\begingroup$ So, that means that we cant be sure that whether crescent moon be visible or not with naked eye, depending on percent-illumination? $\endgroup$ – kaka Jun 27 '14 at 21:06
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I have been watching for almost 30 years on rosh ha shanah. I have been able to see it when aprox. 3 percent is technically visible.
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2$\begingroup$ @peterh it's not your call to say whether a post will be deleted. You may not agree with this answer (although you haven't said why) but it's nonetheless a proper answer within the rules of our site, and without doubt addresses the question, i.e. what percentage is the smallest visible. You might consider whether your comment meets the kindness standard. $\endgroup$ – Chappo Hasn't Forgotten Monica Sep 10 '18 at 4:06
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$\begingroup$ @Chappo I think I tried to care the new user with my best intents. I actually have some votes in some queues, thus also my vote might count if I vote, but it is a community decision. I think, I have some experience, how the community will decide in a case, but of course I can't guarantee it. (ref) $\endgroup$ – peterh - Reinstate Monica Sep 10 '18 at 4:44
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$\begingroup$ @peterh I accept your intent :-) so if you have a concern about the post, the best welcome for a new user is to explain how it can be improved. What is it about this post you disagree with? More particularly, what is so egregious about it that it should be deleted (a much harsher step, usually indicating a serious problem with the post)? Providing guidance to a newcomer shows more care. $\endgroup$ – Chappo Hasn't Forgotten Monica Sep 10 '18 at 4:53
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$\begingroup$ @Chappo I think it is a comment, not an answer. Thus, I suggested the OP to collect 50 rep, and tried to ensure that he remains here. I could also flag the answer for comment conversion, but now I am not sure if it is the best to do. I did not vote the post, and if I find it in a review, I will skip it. $\endgroup$ – peterh - Reinstate Monica Sep 10 '18 at 4:57
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3$\begingroup$ @peterh the post directly answers the question, so I have to disagree with your assessment. If you still think it's not an answer, I encourage you to flag it and let the community then vote on the issue. $\endgroup$ – Chappo Hasn't Forgotten Monica Sep 10 '18 at 5:08
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According to this Wikipedia article on Lunar phase, 0.1% is the minimum percentage. However, at the very best I've observed 0.17% with my naked eye.
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$\begingroup$ The Wikipedia article does not say the Moon is visible at 0.1%, only that it can be called a crescent, with no justification for that threshold. Naked eye sightings below 1% illumination are rare if not impossible. $\endgroup$ – Mike G Jun 5 '19 at 1:12
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As already mentioned naked eye sightings are dependent on location and conditions.
Some have made a point of photographing the earliest new Moon with interesting results.
Here's a site that might add some additional information for those interested in the topic.
https://earthsky.org/astronomy-essentials/young-moon-visibility
Edit: Please keep in mind viewing or photographing anywhere near the sun is not safe for the viewers eyes or equipment. Be aware of the dangers and protect yourself appropriately.
