The Moon has held great importance to cultures worldwide for thousands of years.

Its regular phases and cycle in the sky makes the Moon ideal for marking time, from determining the number of months in a year to marking the start of religious festivals and national holidays.

The Islamic calendar for example is a 'lunar visibility' calendar. That means that the length of each month depends on the phases of the Moon. A month begins with the sighting of the new crescent Moon, making astronomy a key part of Islamic events including Ramadan and Eid.

Find out more about the Moon's phases and how to sight the new crescent Moon below.

Eid Moonsighting LIVE

Celebrate Eid al-Fitr and join us in sighting the new crescent Moon. Get the best tips for moonsighting from Royal Observatory astronomers and hear from guest amateur astronomer Imad Ahmed from the New Crescent Society

What are the Moon's phases?

The Moon does not produce any of its own light. The only way we can see it here on Earth is because it reflects light from the Sun.

As it orbits our planet, different portions of the Moon's surface are lit up by sunlight. Depending on where the Moon is around the Earth, we see a different shape in the sky. These shapes are known as the Moon's 'phases'.

There are eight phases of the Moon beginning with the 'new Moon'. This is where the side of the Moon facing the Earth is not being lit up by the Sun at all, and so the Moon is invisible. 

The next phase of the Moon is called a 'waxing crescent', where a small portion of the Moon's surface is lit and can be seen from Earth.

The phases continue to 'first quarter', 'waxing gibbous' and then 'full Moon', where the entire face of the Moon seen from the Earth is fully lit by the Sun. 

Then the Moon begins to get smaller, going through 'waning gibbous', 'last quarter' and 'waning crescent' phases, before the cycle begins once more with a new Moon. It takes 29.5 days for us to see the same phase of the Moon in the sky again.

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OM-67172-1_39_ Crescent Moon © Richard Addis.jpg

What is a new crescent Moon?

A new crescent Moon is the first time the Moon can be seen following a new Moon. According to the list of phases above therefore it is known as a waxing crescent Moon.

At this stage, the Moon is only the slimmest curve in the sky, with just a fraction of its surface illuminated by the Sun.

In the Northern hemisphere in places like the UK, the new crescent will appear as a backwards 'C' shape. In the Southern hemisphere however it will appear the other way around as a regular 'C' shape.

Why is the new crescent Moon important in Islam?

In the Islamic calendar, the new crescent Moon marks the start of a new month.

Although it takes 29.5 days for the Moon to go through all of its phases, it isn't practical for a month to have half a day. An Islamic month therefore can have either 29 days or 30 days. How many days in each month depends on when the new crescent Moon is first visible.

"On the 29th of each Islamic calendar month, Muslims go out after sunset looking for the Moon," explains Imad Ahmed, director of the New Crescent Society.

"If you can see the crescent Moon on the 29th, that month has 29 days. If you cannot, it means that month has 30 days. That’s why, for example, in some years Ramadan has 29 days and in others years it has 30."

A Moon visibility map showing different shaded areas of the globe explaining where the new crescent Moon may be visible
Moon visibility map (courtesy of HM Nautical Almanac)

Predicted Moon visibility maps like the one above can provide a useful scientific guide to when the new crescent Moon may be visible, with the likelihood varying depending on where in the world you are.

The new crescent visibility varies worldwide, just as sunset and sunrise times do, so not everyone will be able to see it at the same time or even on the same day. This variation across the world can lead to some countries marking religious holidays on different dates.

"Some Muslim communities in the UK follow the Islamic calendar of other countries," explains Imad Ahmed. "When Muslims in the UK follow the calendars of different countries around the world, it’s natural we will have some differences. Different countries might see the moon on different dates, and different countries might have alternative methods of determining the Islamic calendar, which might not correspond to the visibility of the moon."

However, basing the months on the sighting of the new crescent Moon means that anyone can participate. All you need to do is look up!

When is Eid 2021? How to sight the new crescent Moon that marks the end of Ramadan

As Ahmed explains, the actual date for Eid al-Fitr may differ depending on when the new crescent Moon, or 'Shawwal Moon', is first sighted.

This year it was expected to be visible on either 12 or 13 May 2021, although the likelihood of seeing it in the UK on the 12th was low.

This year, the Royal Observatory and the New Crescent Society joined forces to watch for the new crescent Moon. Watch the live broadcast back below.

If you want to try and sight the Moon, it's easy. All you need is a clear view of the Western horizon where you can see the sunset; this is because the new crescent Moon always emerges near the sunset. Depending on the date, you may want pair of binoculars or even a telescope to hand, as getting that very first glimpse can be tricky.

But even if you don't manage it one night, it should be visible with the naked eye the next. Watch the video above for more great moonsighting tips.

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A person holds three fingers up to the sky, illustrating the 'Danjon Limit' - the amount of distance between the Sun and the Moon required to see the Moon

Moonsighting top tips

There are certain criteria needed for the new crescent Moon to be visible.  

  1. The Sun must be below the horizon. This is because it needs to be dark enough to spot the small slither of the new crescent.
  2. The Moon needs to be above the horizon.
  3. The Moon and the Sun need to be far enough apart in the sky. This is known as the 'Danjon Limit', which states that the Sun and Moon need to be separated by around 5-7 degrees: that’s about the same width as your first three fingers held out at arms length.

Find out more about astronomy and Islam

Want to discover more about the links between Islam, astronomy and timekeeping? Keep an eye out for upcoming dates for our Astronomy and Islam planetarium show, just one of our fantastic community astronomy events.

See all our planetarium shows

Tycho Crater Region with Colours © Alain Paillou.jpg

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