A young crescent Moon. Photo Nick Lomb
The first visibility of the crescent Moon after the astronomical instant of new Moon is not only of interest to astronomers, but is of particular significance in the Islamic calendar. The crescent Moon is always first visible after sunset. For it to be visible the Moon has to have moved away from the glare of the Sun and it has to be suitably high above the horizon at sunset so that there is time for the sky to become sufficiently dark before moonset.
A number of criteria for predicting the visibility of the crescent Moon have been developed over the years.
The simplest useful criterion is the lagtime 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. This value for the time lag at Sydney’s latitude comes from the work of Mohammad Ilyas of the University of Science Malaysia (Quarterly Journal of the Royal Astronomical Society, volume 35, pages 425 to 461, 1994).
In July 2014 the Islamic month of Shawwal begins after the new Moon of 27 July. Although this occurs in the morning at 8:42 am AEST, there is no possibility of the Moon being seen that evening as in Sydney it sets only 24 minutes after the Sun.
A global map of the visibility of the Moon for 27 July is shown here. The map clearly indicates that the new Moon is not visible that day from Australia, but under perfect conditions can be seen from most of South America.
From Sydney the crescent Moon will be visible on Monday 28 July. The best time to view will be 5:47 pm when it be at azimuth 289° and altitude 7°. Its age at the time will be 33 hours 5 minutes.
Finding chart for the crescent Moon from Sydney at 5:47 pm AEST on 28 July 2014. Chart made with Stellarium
Anyone interested in trying to view the crescent Moon needs to find a place with a good view towards the western horizon as it will be very low in the sky. A pair of binoculars could help to locate it first in the still bright sky and only expect to see the crescent if the sky towards the west is absolutely free from clouds. Please let us know in the comments if you do manage to see the crescent Moon.
The above information is an extract from a previous post titled, The visibility of the crescent Moon from Sydney in June and July 2014. It is reposted for the convenience of readers.