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 moons in June and July of 2015.
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 at a calculated ‘best time’ to view the Moon plus the width of the crescent. More details of this method and maps displaying the Moon’s visibility are available here.
The new Moon in June will occur at 12:05am on Wednesday, June 17, i.e. just after midnight (all times are AEST, i.e. Sydney time). On June 17 the Sun will set at 4:53pm, and the Moon at 5:43pm. The lagtime is 50 minutes so the crescent Moon may be visible to the unaided eye. However, by the Yallop method, optical aid (binoculars for instance – but only use your binoculars after the sun has set) may be required to sight the Moon before it is visible to the unaided eye. On Thursday, June 18th, the Yallop method tells us that the Moon will be “easily visible to the unaided eye”.
The following new Moon (marking the end of Ramadan and thus the beginning of Eid-ul-Fitr) occurs on Thursday, July 16th at 11:24am. On July 16 the Sun will set at 5:05pm, and the Moon at 5:24pm. The lagtime is only 19 minutes so the crescent Moon will not be visible to the unaided eye, and the Yallop method concurs. On July 17, however, sunset is at 5:05pm and moonset is at 6:20pm so the lagtime is 75 minutes. The crescent Moon will be easily visible if the western sky is clear of cloud.