Predicting the visibility of the crescent Moon in May and June 2017

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.

31 responses to “Predicting the visibility of the crescent Moon in May and June 2017

    • Ahmad Ali, We are pleased you find this useful. We provide this information due to the many requests we receive and for its particular importance in these couple of months. Unfortunately, we don’t have the resources to provide the same for the whole year! You can, however, consult the websites noted above to find the visibility of the Moon for any month.

    • Kul, For Sydney the August new moon (waxing crescent to astronomers) was first visible on Wed Aug 23rd by the Yallop method and the lagtime method agrees on this date. On Tue Aug 22 the lagtime, at Sydney, was only 29 minutes and the Moon was at New phase at 04:30am on Aug 22.

  • Thankyou Andrew for your article, knowledge and insight. Despite differences in opinions within faiths and between faiths, respectful open discussion can only build bridges, increase understanding and reinforce our common humanity. Peace and love to all!

  • Islamic injunctions are NOT based on the birth of new moon, rather these are based on the visibility of the new moon called Crescent english or Hilal in Arabic and this is unanimously agreed upon by all the schools of thought in Islam in the past. Jewish practice is probably based on the birth of the moon (conjunction) and other phases of the moon. Hence, the importance of the first sightability of the new moon is paramount for determining the 1st day of Ramadan and Shawal ins Islam.

  • How come we rely on technology for the prayer times and also for the start and end of fasting but can’t use the same technology for months?

    • The technology indicates the crescent will NOT be visible tonight, but how many Muslims understands the intricacies between Technology and Islamic Jurisprudence? If there is a contradiction between technology and Islamic Rules, the islamic rules prevail for the validity of our worship. The basic difference between the timing for 5 daily prayers and fasting is that for prayers – entering the time for a particular prayer is the condition for the validity of that prayer and how you determine (or the method of determining) the time is NOt part of Islamic rulings. But for fastings and Eids – the method of determining the first day of the Islamic month is the validity for fasting and Eid.
      Further more, the technology is there to guide us. The technology can tell us today the exact times of sunset and sun rise and mid day etc. based the location of the sun relative to planet earth’s position and therefore, the scholars accepted it. But technology cannot predict even today the exact timing of the visibility of the new moon because of various factors like pollution in air, twilight, etc.

      • If the technology tells us that moon will be visible on a particular day , then why can’t we trust it even if that day is cloudy. We are still following the formula given by Prophet Mohammed SAW but making it easier for everyone . This is just as we were given a rule for prayers start and finish time. On a cloudy day when we can’t see the sun we still believe it is there and rely on the technology without breakig any islamic rule. There is no difference between the movements of sun or moon , they both are calculated and can’t change.

    • Because for the salat, start and breaking the fast it depends on the position of the sun and for the arabic month it depends on the sighting of the moon, specially when we were told by the prohet mohammad (sw), there are big diffrent between borning, positioning and sighting, as we can understand from above discussion moon can be born but it requirs some times before it can be seen, jajakhalla khair

  • Hi.. Interesting insight. I am from South Africa , and I read the information on the planetariums website we need a lag time of 46 minutes to view it.. its 42 mins here, whats the chances of it being sighted in South Africa?

    • Tasneem, The lag time criteria of 47 minutes quoted above is for Sydney. Given most of South Africa covers about the same latitudes as NSW your lag time would be similar. If that is the case the Moon will not be visible. But check the maps for the Yallop predictions at the link above to see if that criteria gives a different result.

    • Yusuf, No, on 24 June the crescent Moon will be too close to the Sun to be visible. It will be within just a few degrees of the Sun, it will be a very thin crescent and the sky at that point will be very bright. Combined, this means the Moon won’t be visible, even with telescopes, on June 24. In fact it would be dangerous to point a telescope that close to the Sun. But on June 25 it should be easily visible by the naked eye if the western sky is clear.

      • Thank you for your article Andrew Jacobs.
        At approximately what hour on the 25th will the crescent moon be visible using ‘aided technology’ (telescopes) in Sydney.

        • Sarah, It’s hard to say. Try looking at any time *after* the Sun has set. This ensures you don’t see the Sun through the telescope by accident, which could lead to irreversible eye damage.

    • If via Science and technology we have opportunity to know the moon is born then tat should be good for everyone. You dont have to site the moon with naked eye as the in centuries gone by. Eid should be Sunday.

  • Hello,
    It will be interesting to find out whether the moon can be seen with optical aid on Saturday the 24th at or after sunset. As the world progresses and technology takes over, sometimes it’s important to be more physical and enjoy the hands on experience.
    Your thoughts!

  • Andrew ,
    In Perth sunset is 5:21 pm
    Moonset is 6:05pm
    Which gives it lagtime of 44 minutes
    So the moon also wont be seen in perth because it needs 47 minutes to be seen by the naked eye
    Are my calculations correct ?

    • Rad, Yes, I think you’re right. Perth is on almost the same latitude as Sydney (only 2-degrees further north) so the 47-minute criterion should still be valid. However, given tat the lag-time of 44-minutes is so close to the 47-minute criterion you might want to try looking for the moon in case it is visible. I would recommend heading to the Perth hills or a high spot near the coast.

  • “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.”

    Is our Society really that civil? I’d actually prefer saying: “The civil calendar we use in society (the ‘Gregorian’ calendar) is a solar one – based on the time it takes for the Earth to orbit the Sun.”

  • Ummm… “Many religious calendars, however, are based on the phases of the Moon. These include the Catholic, Jewish and Islamic religious calendars.”

    The earliest visibility of the first phase of the moon actually dates well back into ancient history, and mostly +2000 years before the origins of the religions. I remember a great talk on this by the now late- I. Debono and it appeared in the old BAA NSW Branch “Bulletin’ in the mid-1970s. It derives for both the Mesopotamia and Babylonian empires. It was adopted by the Jewish people to the cycle of 19 years and 233 lunation – and persist even today. Babylonians began in the month of Nisanna, but the later Jewish calendar required the visual conformation when a month began. Their start date is 7th October 3761 BC – the Creation but the earliest evidence of the lunar phase observation is around 800 BC.
    Even the Greeks were doing this c.480 BC., using the Metonic cycle, which the oldest record is in 6th Century B.C. They did formally change to about 380BC when they used a start dating date – scholars claim this was stolen from the Babylonians.

    All ecclesiastical calendars really hold sway after about 325AD. when the lunar calendar was dumped (excepting Easter.)

    It worries me when we discard the ancient people and their struggles finding a useful and regular calendar. Like those of faith today, their rituals ultimately focus on the repeated endless cycles of birth, life and death; in which the cycles of the Moon exhibit in the heavens. They ignored the Sun as a astronomical tool, not because it wasn’t accurate to tell the time during a years, but because it did not reflect our human existence. Neoclassical religions really adopted both lunar and solar calendars, whose practice is still used today. (This is neither bad or good, but a reflection of many ancient practices that date back into prehistory. Had they not burnt and destroyed the Great Library at Alexandria, no doubt we would be better aware of these nuances of why the crescent is deemed so importantly in human’s lives and their beliefs.)

    Whilst the challenge of being first to see the Moon departing from the solar glare, the spiritual connection to the event to some is much more important. Pity our modern world is slowly consuming these old traditions and replacing them with personal challenges or records without understanding their origins. Astronomy to me just becomes slightly weaker by rapidly killing the wonder of the enduring celestial cycle. A little pause for thought.

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