Observations

Predicting the visibility of the crescent Moon in April and May 2021

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 2021 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.

Please note that all dates & times in this article are for Sydney and are in Australian Eastern Standard Time (AEST), i.e. Sydney time.

The New Moon in April 2021

The new moon in April 2021 will occur at 12:31pm (just after midday) on Monday, April 12. On April 12 the Sun will set at 5:36pm and the Moon will set at 5:53pm. The lagtime is only 17 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 12.

On Tuesday April 13 the Sun sets at 5:35pm and the Moon sets at 6:21pm. The lagtime is now 46 minutes so once again the crescent moon will probably not be visible to the unaided eye at Sydney’s latitude. The Yallop method provides a more detailed picture for Australia: If you are north of a line joining (approximately) Perth to Fraser Island (on the Queensland coast) the crescent Moon should be easily visible to the unaided eye. For the remainder of the mainland the crescent Moon may 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. For Tasmania the crescent Moon may be visible to the unaided eye but only after first being found with binoculars or a telescope. To avoid irreversible eye damage please ensure the Sun has fully set before searching the western horizon for the crescent moon with your binoculars or telescope.

Finally, on Wednesday April 14 the Sun sets at 5:34pm and the Moon sets at 6:50pm. The lagtime is now 76 minutes and 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. Further, the Yallop method shows that the crescent moon should be visible to the unaided eye from all locations in Australia.

In summary, the crescent Moon will not be visible on April 12. It may be visible on April 13 depending on your location and the atmospheric conditions, and from Tasmania it may be visible after being found with binoculars or a telescope. On April 14 the crescent moon should be easily visible from all parts of Australia.

The New Moon in May 2021

The following new Moon occurs on Wednesday May 12 at 5:00am. On the evening of May 12 the Sun will set at 5:05pm and the Moon will set at 5:24pm. The lagtime is just 19 minutes so the crescent moon will not be visible to the unaided eye (at Sydney’s latitude), and the Yallop method concurs. From all other Australian locations the crescent moon will also not be visible.

On Thursday May 13 the Sun will set at 5:04pm and the Moon will set at 5:59pm. The lagtime is now 55 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. Further, the Yallop method shows that the crescent moon should be easily visible with the unaided eye from most other Australian locations. However, for locations south of a line joining (approximately) Mt Gambier (SA) to Bega (NSW) – this includes Melbourne and all of Tasmania – perfect atmospheric conditions may be required, i.e. no cloud, no dust and a very clear western horizon.

On Friday May 14 the crescent moon should be easily visible to the unaided eye from all parts of Australia.

Other 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 12 the Moon will be at an altitude above the horizon of just 3-degrees and it will be directly above the Sun. On April 13, again at the moment of sunset, the Moon will now be at an altitude above the horizon of slightly under 8-degrees and about 10-degrees to the right of where the Sun set.

 

12 responses to “Predicting the visibility of the crescent Moon in April and May 2021

  • Greetings,
    ON Wednesday 12th May 2021 can the moon be seen at the observatory.
    Do we have to make arrangement ?

  • Thankyou for your insightful and informative articles. It is extremely appreciated having someone so knowledgeable in his field, giving factual, scientific advice.
    We look forward to your indispensable articles each year, and thank you for your time and effort.

  • Hi Andrew,

    Thank you for your great service. Your article is very important to use, every Ramadan. Please continue the initiative.

  • Hi MAAS,

    Just trying to understand the new moon cycle, In your moon phase calendar, It show new moon will occur on 11th and 12th of May, 2021. What is the correct date for new moon ? I can see in your above comments that it will not be visible on 12th.

    • Burhan, Thank you for highlighting this for us. The New Moon, astronomically speaking, occcurs on May 12 at 05:00am AEST, as this post correctly states. The Moon Phase Calendar incorrectly shows the New Moon on both May 11 & 12 – the Calendar has some issues with times and rounding of small numbers which we are in the process of correcting. We apologise for the confusion this has caused.

  • Hi Andrew

    Your article is a go to every year.. quick question. You mentioned that the new moon will not be visible to the unaided eye on Mon 12 April from anywhere in Australia. How about with the ‘aided’ eye?? Would it be possible from Sydney or anywhere in Australia on Monday 12th April?

    Thanks Andrew

    • Hassan Alhasni, Glad to help. The Yallop maps show that new moon will not be visible on April 12 from anywhere in Australia, not with aided eye, not with any telescope either – it is just too close to the Sun that evening. In fact, at the moment of sunset the Moon is just 3-degrees, or barely 6 moon-widths above the horizon. Its crescent is too thin and the sky there too bright for it to be visible.

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