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 2017.
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 2017 will occur at 5:44am on Friday, May 26 (all dates & times are for Sydney and in AEST, i.e. Sydney time). On May 26 the Sun will set at 4:56pm and the Moon at 5:35pm. The lagtime is only 39 minutes so the crescent moon will not be visible to the unaided eye at Sydney’s latitude, and the Yallop method concurs. On May 27 the Sun will again set at 4:56pm, but the Moon sets at 6:33pm. The lagtime is now 97 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 Saturday June 24 at 12:31pm, i.e. just after midday. On June 24 the Sun will set at 4:55pm and the Moon will set at 5:13pm. The lagtime is only 18 minutes so the crescent moon will not be visible to the unaided eye at Sydney’s latitude, and the Yallop method concurs. On June 25 the Sun will again set at 4:55pm, but the Moon sets at 6:17pm. The lagtime is now 82 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.
What about Australian locations other than Sydney?
If your latitude is within about a few degrees 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. In 2017 the above conclusions for the dates of the beginning and end of Ramadan should hold. 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 2017) for the unaided visibility of the crescent Moon on the above dates (May 26 & 27 and June 24 & 25) for all locations in Australia. However, on May 26, if you are west of a line that joins the Gulf of Carpentaria to the Great Australian Bight then the crescent Moon may be visible to your unaided eye after first being found with binoculars/telescopes. Please wait until after the Sun has set before using binoculars/telescopes to avoid the risk of eye damage.