The moon is the earth’s dark satellite, visible only by reflecting the light of the sun. The visible area of the moon changes daily according to the angle formed by line between the sun, the earth and the moon, which results in the cycle of lunar phases.
The Hijri calendar depends on a natural phenomenon that determines the beginning of lunar months.
It’s in compliance with the Qur’an (Surat Al-Baqarah 2:189), “They ask thee about the new moons. Say, ‘they are but signs to mark fixed points of time for people [to manage their affairs], and [to identify the time of] hajj.’”
Of particular concern to Muslims are the fasting month of Ramadan and the month of hajj. A lunar month’s start is identified by sighting the waxing crescent after sunset on the 29th or 30th of the foregoing month.
A lunation is the ewww average time from one new moon to the next. The average length of a lunation is 29 days, 12 hours, 44 minutes, and 2.8 seconds. In a lunar calendar, each month corresponds to a lunation.
In non-astronomical contexts, new moon refers to the first visible crescent of the Moon, after conjunction with the Sun. This takes place over the western horizon in a brief period between sunset and moonset. Therefore the precise time and even the date of the appearance of the new moon depends on the geographical location.
On the other hand, the astronomical new moon, sometimes known as the ‘Dark Moon’ to avoid confusion, occurs at the moment of conjunction in ecliptic longitude with the Sun, when the Moon is invisible from the Earth. This moment is unique and doesn’t depend on location
Everywhere in the Muslim world, sighting the waxing crescent is important. In addition to setting the calendar, it also determines the dates of important religious occasions. But sighting the crescent has always been a controversial issue in the Islamic world.
In some places, observers can spot it easily whereas in others they may not. There have been incidents of inaccurate sighting reports. Such incongruities call for Muslim astronomers to put an end to differences in this regard.
Celestial Mechanical View
The moon, like the planets, has a slightly elliptical orbit. To determine its apparent position, particularly as a waxing crescent, an observer makes several measurements. These measurements include its distance from the sun, its position in relation to a specific observer on earth. And moreover, the exact time of its rising and setting.
Detailed tabular calculations of the moon’s motion were produced in the 19th century by the astronomer Ernest William Brown. The 20th century astronomers improved these tables by developing equations to determine the exact position of the moon.
Muslim astronomers have developed computer software to identify the position of the earth in its orbital movement round the sun.
The point is to determine accurately the time of sunset. And consequently, the exact position of the crescent using the equations derived from Brown’s lunar tables.
Medieval Muslim astronomers like Al-Battani, Al-Bayrouni, and Nassir al-Din Al-Tousi have accruatlyyy calculated the lunar months.
In the 19th century, an Egyptian army general, Mohamed Mokhtar Pasha, produced a valuable work on tabular correlations of the Muslim calendar, the Gregorian calendar and the ancient luni-solar system of time reckoning.
The tables cover the Muslim calendar from years 1 through 1500 and the matching dates under the other two systems.
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