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 April and May of 2020 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 April 2020 will occur at 12:26pm (just after midday) on Thursday, April 23 (all dates & times here are for Sydney and in AEST, i.e. Sydney time). On April 23 the Sun will set at 5:23pm and the Moon will set at 5:42pm. The lagtime is only 19 minutes so the crescent moon will not be visible to the unaided eye at Sydney’s latitude, and the Yallop method concurs. Further, the Yallop method also shows that the crescent moon will not be visible from any location in Australia on April 23.
On Friday April 24 the Sun sets at 5:22pm and the Moon sets at 6:12pm. The lagtime is now 50 minutes so the crescent moon should be visible (at Sydney’s latitude) to the unaided eye if the western sky is clear of cloud. The Yallop method concurs. The Yallop method provides additional information for locations beyond Sydney: If you are south of a line joining (approximately) Port Augusta to Brisbane the crescent Moon should be visible to the unaided eye under perfect atmospheric conditions, i.e. no cloud, no dust and a very clear western horizon – fortunately, this time of the year is Autumn and the atmospheric conditions are often nearly perfect; If you are north and west of the line joining Port Augusta to Brisbane then the crescent Moon should be easily visible to the unaided eye on April 24.
In summary, the crescent Moon will not be visible to the unaided eye on April 23. But on April 24 it should be easily visible from most parts of Australia, and visible if the western sky is very clear from the south-eastern parts of Australia.
The following new Moon occurs on Saturday May 23 at 3:39am. On the evening of May 23 the Sun will set at 4:58pm and the Moon will set at 5:24pm. The lagtime is just 26 minutes so the crescent moon will not be visible (at Sydney’s latitude), and the Yallop method concurs. From other Australian locations the crescent moon will also not be visible.
On Sunday May 24 the Sun will set at 4:57pm and the Moon will set at 6:06pm. The lagtime is now 69 minutes so the crescent Moon should be visible (at Sydney’s latitude), and the Yallop method concurs. It will also be readily visible from other Australian locations.
If you are not in Sydney but 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.
For Melbourne we can provide the following additional information: At the moment of sunset on April 23 the Moon will be at an altitude above the horizon of just 3-degrees and it will be directly above the Sun. On April 24, again at the moment of sunset, the Moon will now be at an altitude above the horizon of slightly over 8-degrees and about 10-degrees to the right of where the Sun set.