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Suppose somebody went on an island and don't have sense about time, dates, months. The only thing he has is seeing the sky. Is it possible to recognise which lunar month it is by just looking the sky? I ask this because I know how to see beginning of the lunar month, but how to recognise which month is it? So we can look at the Sun, Moon, the stars. Is there a way to figure that out?

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  • $\begingroup$ By "lunar month" do you mean "Muharram", "Safar" etc? Ie the month of the Islamic calendar? $\endgroup$
    – James K
    Jul 30 '18 at 16:12
  • $\begingroup$ As @JamesK notes, it depends on what kind of lunar month you mean. If you mean something like "Frost Moon" or "Harvest Moon", yes, someone could, provided they could observe the Sun as well, and were in a region where seasons exist (ie, outside the tropical zone). $\endgroup$
    – user21
    Jul 30 '18 at 16:27
  • $\begingroup$ The very-not-easy approach: observe the relative position of some "fast-moving" stars compared with a reference star chart from, say 1000 CE. Various software apps can generate star positions for any date you desire; you could run a match against that. $\endgroup$ Jul 30 '18 at 19:50
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Not easily.

The Lunar Islamic calendar is based on a strict succession of months, with each month based only on the position of the moon, and no intercalated months or months extended to 31 days in order to keep the cycle of months aligned with the solar year. The start of the months are determined by the moon, the names of the months are not defined by any astronomical feature.

So to determine which Islamic month it is you need to know the actual amount of time that has passed. Now the positions of the planets change day by day, and by careful observation of the planets, and given a detailed ephemeris of their positions, one could work out the exact date, and therefore the Islamic month.

This is similar to determining the day of the week by astronomical observations, since the start of the day is based on the motion of the sun, but the name of the day is not defined on any astronomical feature.

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  • $\begingroup$ This answer assumes that "lunar month" means "islamic month", as this is the lunar calendar most widely used. $\endgroup$
    – James K
    Jul 30 '18 at 17:53
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    $\begingroup$ OP does not indicate a preference between this and lunisolar calendars which add a 13th month every 2 or 3 years. $\endgroup$
    – Mike G
    Jul 30 '18 at 18:31
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    $\begingroup$ Yes, I decided I wouldn't wait to see what he meant. Since he mentions "I know how to see the start of the month by looking at the sky" I assume he means a month that begins when the crescent is first visible, not astronomical new moon. By far the most common such calendar is the Islamic calendar. If OP clarifies their question, I can edit or delete. $\endgroup$
    – James K
    Jul 30 '18 at 18:50

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