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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Sign up to join this communityWhat level of percent-illumination of moon (waxing crescent), given by Stellarium, is enough to make it visible with naked eye, in clear sky?
thanks
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.
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.
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.
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.
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.