The Islamic calendar operates in line with God's creation -
the Sun and the Moon.
The Sun determines the daily prayers
- Fajr: Between dawn (sun 18 degrees below horison) and sunrise
- Zhur: Between noon and when the shadow of an object is one or twice the length of the object
- Asr: From end Zuhr to sunset
- Magrib: From sunset to dusk (sun 18 degrees below horizon)
- Isha: From dusk till dawn
The Moon determines the Islamic months and so the Islamic year is a lunar year.
Each lunar month is determined by visibility confirmed by calculations. Each month is either 29 or 30 days (the moon takes approximately 29.5 days to go round the earth).
Wifaqululama has put together a paper on visibility curves and a few presentations covering the most important aspects of the topic. www.wifaqululama.co.uk/images/fatwa/fiqhmoonsighting.pdf
There's a five part series on YouTube
1. Introduction and importance of moonsighting
2. Basics of Moonsighting - Scripture and understanding of Hadith
3. Astronomical data
4. UK situation and the role of Astronomy