Most calendars in human history started by watching the Moon, not the Sun. The reason is simple: the Moon changes face every day, and anyone can track it without instruments. The Sun rises and sets in roughly the same place all year, so detecting its annual movement takes careful measurement. The Moon hands you the rhythm for free, in the sky, in plain sight.
The Hijri is purely lunar. Twelve months of 29 or 30 days each, totaling about 354 days a year. There's no adjustment to keep it in step with the seasons, so Ramadan, for example, falls in different parts of the Gregorian year: sometimes winter, sometimes summer.
Traditionally, each month begins when the thin crescent is sighted with the naked eye after the new moon. In some countries this is still decided by committees that look at the sky on the appointed night. In others, astronomical calculation is used to fix the dates in advance. The difference produces a periodic dispute over the exact start of Ramadan and Eid.
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