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 May and June of 2019 for Australia.
The simplest useful criterion is the lagtime, or difference, 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, however, 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 May 2019 will occur at 8:45am on Sunday, May 05 (all dates & times are for Sydney and in AEST, i.e. Sydney time). On May 05 the Sun will set at 5:11pm and the Moon at 5:38pm. The lagtime is only 27 minutes so the crescent moon will not be visible to the unaided eye at Sydney’s latitude, and the Yallop method concurs. On May 06 the Sun sets at 5:10pm and the Moon sets at 6:16pm. The lagtime is now 66 minutes so the crescent moon should be 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 occurs on Monday June 03 at 8:02pm, but this is already after sunset so the Moon will not be seen on this night. On June 04 the Sun will set at 4:54pm and the Moon will set at 5:43pm. The lagtime is 49 minutes so the crescent moon should be visible (at Sydney’s latitude) if the western sky is clear of cloud, and the Yallop method concurs.
What about Australian locations other than Sydney?
If your latitude is within a degree or so of Sydney’s latitude then the lagtime method of 47 minutes should work sufficiently well for you – but you will need to find the time of sunset and moonset for your particular location. Nevertheless, in 2019 the above conclusions for the dates of the beginning and end of Ramadan should hold for your location. For other latitudes different lagtimes may be required but these are beyond the scope of this article.
The Yallop method also draws the same conclusions (in 2019) for the unaided visibility of the crescent Moon on May 05 & 06 for all locations in Australia.
However, using the Yallop method the situation on June 04 is more complicated. If you are north of a line that (roughly) joins Wollongong to Mount Gambier (in South Australia) then the crescent moon should be visible to the unaided eye if the western sky is clear of cloud. If you are south of this line (i.e. in most of Victoria and all of Tasmania) then the crescent Moon may be visible to the unaided eye, but only after you have found it with binoculars or a telescope. Please wait until after the Sun has set before using binoculars or telescopes to avoid the risk of eye damage. By June 05 the crescent Moon should be readily visible from all locations in Australia.