The calendar we use in civil society (the ‘Gregorian’ calendar) is a solar one – based on the time it takes for the Earth to orbit the Sun. Many religious calendars, however, are based on the phases of the Moon. These include the Catholic, Jewish and Islamic religious calendars. The dates of festivities, holidays and important events in the lunar calendar move by about 10 days every year within the Gregorian calendar.
The ninth month of the Islamic calendar, known as Ramadan, is the Islamic month of fasting. The Hilal, or crescent moon, marks the beginning of the fasting period. However, there are differences of opinion on how to define ‘crescent’. While some simply demand an unaided sighting by eye of the crescent moon, others are leaning towards using astronomical calculations to avoid confusion.
The following astronomical data concern the new and crescent moons in June and July of 2016.
The simplest useful criterion is the lagtime between sunset and moonset. If that time is greater than 47 minutes (at the latitude of Sydney) the crescent moon should be visible to the unaided eye after sunset and before the setting of the Moon.
The most common method of prediction is to use a scheme developed by Dr Bernard Yallop of HM Nautical Office and proposed in 1997. This scheme or algorithm involves the altitude difference between the Sun and the Moon; a calculated ‘best time’ to view the Moon; and the width of the crescent. The Yallop method is applicable to any location. More details of this method and maps displaying the Moon’s visibility are available here.
The new moon in June 2016 will occur at 1:00pm on Sunday, June 5 (all times are AEST, i.e. Sydney time). On June 5 the Sun will set at 4:53pm and the Moon at 5:16pm. The lagtime is only 23 minutes so the crescent moon will not be visible to the unaided eye at Sydney’s latitude, and the Yallop method concurs. On June 6 the Sun will again set at 4:53pm, but the Moon sets at 6:13pm. The lagtime is now 80 minutes so the crescent moon should be easily visible (at Sydney’s latitude) to the unaided eye if the western sky is clear of cloud, and the Yallop method concurs.
The following new moon (marking the end of Ramadan and thus the beginning of Eid-ul-Fitr) occurs on Monday, July 4th at 9:01pm, i.e. after sunset. On the following day, July 5, the Sun will set at 4:59pm and the Moon at 5:56pm. The lagtime is 57 minutes so the crescent moon should be visible to the unaided eye at Sydney’s latitude. By the Yallop method the crescent moon should be visible on July 5 if the sky is perfectly clear of cloud and dust and if there is no heat haze.