Observations

Predicting the visibility of the crescent Moon in April and May 2020

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.

9 responses to “Predicting the visibility of the crescent Moon in April and May 2020

    • Syed, We leave that interpretation up to you. As noted at the beginning of this post we are aware there are differing opinions about just this. We are very happy to assist in providing the astronomical information but we leave its interpretation to those more knowledgeable in such matters.

  • Hi Andrew.
    I need your expert advice and clarification on the following:
    1)If the moon sets before the last light of the sun, does that make it impossible to be sighted to the unaided eye or by telescope or at the Observatory.
    2) If in Sydney on May 23rd the moon sets at 5.24pm and last light from the sun is also at 5.24pm is there any chance of sighting the moon by naked eye, telescope or at the observatory. I understand of course that 47 minutes is the optimal time.
    3) In Perth on May 23rd the moon sets at 5.52pm and the last light from the sun is at 5.49pm which means that the moon sets 3 minutes after last light of the sun. Is there any chance of sighting the moon by naked eye, telescope or at an observatory, whether Perth or Sydney because the moon is setting after the last light from the sun. I understand of course that 47 minutes is the optimal time.

  • Thank you, Andrew, for your annual Ramadan moon data.
    Like many others, I greatly appreciate the information you continue to provide for the benefit of our community.
    Regards,
    Siddiq Buckley.

  • Thanks Andrew,
    You’re amazing and most importantly you have provided the information with plenty of time to allow Muslims to concur and to plan ahead.

  • Thanks Jacob for useful information and support as always. We undestand new moon information are generally accurate having scientific evidence and widely available from a number of sources. However, crescent moon (i.e. visible moon) has been always challenging with accurate predictions. You know cresent moon is still significant to a number of communities. I’d like to make a suggestion (if possible) you write your blog every quarter (i.e. for 3 months say April to June) covering 3 moons sightings. This would help make predictions of other months as well. We muslims have a number of other events that lie in months other than Ramadhan and Shawal (i.e. 9th, 10th months respectively). Thanks.

    • AkhtarAbbas, Yes, you are correct. While it is easy (these days) to predict the position and phase of the Moon it is very challenging to predict when it will be visible to the eye. Unfortunately, we don’t have the staffing resources to write a quarterly visibility post. However, I encourage you to use the methods described (the lag-time method and the links to the Yallop method above) to make your own predictions. Rise and set times are available in the Australasian Sky Guide, or you could look them up on a site like Time&Date. You would, in time, be able to modify the predictions to account for the sensitivity of your own eyes and your local atmospheric conditions.

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