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I am new here, and hope to state the question well.

In Islam, it's known that the Lunar Month starts if the Moon is sighted after the sunset of the 29th day of that month.

If someone claimed that he sighted the moon before the conjunction or if the moon sets before the sun, then this claim is certainly false.

My question is, what is the least conditions on the possible visibility of the moon?

There are many criteria that confirm the impossibility of sighting the Moon, but many people still claim the sighting. Which one is wrong?

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Whether the moon or any other body is visible depends on the angular separation, the angle of the ecliptic, the atmospheric conditions (particularly cloud) and the observer's eyesight, or visual aids used.

It is not possible to prove that someone couldn't see the moon at a particular time and at a particular date, so any calendar system based on observation will be subjective and dependent in part on the skills of the observer. So it is not possible to say "The moon can't be observed at x hours after conjunction".

This would appear to the purpose of the Islamic calendar: that it can't be calculated and must be observed. In contrast to most other calendars, it is impossible to know how long each calendar month will be in advance, and it may vary from place to place and even from person to person.

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  • $\begingroup$ I'm not the downvoter, but it's always possible to prove that, at any given moment, half the world can't see the Moon. You are trying to say something about whether or not the Moon is visible to human eyes at some place and time when/where; (a) the Sun is arbitrarily close to, but entirely below the horizon, and (b) the Moon is arbitrarily close to, but at least partially above the horizon. $\endgroup$ – besmirched May 21 at 16:57
  • $\begingroup$ Well you know that's not what I mean.. I mean that "I can see the crescent" is not an objective fact. It is a subjective fact. Different subjects will have different and equally valid opinions. $\endgroup$ – James K May 21 at 20:04
  • $\begingroup$ @JamesK Thank you. Just to be sure, before the conjunction it's certainly not possible to see the moon? $\endgroup$ – Muath Karaki May 22 at 0:09
  • $\begingroup$ I don't really know what you mean by this. One week before the conjunction you can see the moon. Of course you can't see the moon after moonset, but its possible (at high latitudes) for sunset to happen before moonset even before new moon. You won't practically be able to see the moon then, but that is due to the atmosphere and the limits of human vision, ie it is a biological and meteorological question more than astronomical. $\endgroup$ – James K May 22 at 6:46
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    $\begingroup$ Not in Palestine. But What about Tromso in Norway? or antartica $\endgroup$ – James K May 22 at 7:12
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Here is 13 hours 48 minutes. That's a mighty early sighying: https://www.universetoday.com/107700/ultra-thin-young-crescent-moon-sighted-from-u-s-southwest/ 11 hours 40 minutes was the world record as of 2014.

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