Moon sighting is a key part of Muslim life – how the lunar cycle determines the start of Ramadan and Eid

Each year as Ramadan approaches, more than 2 billion Muslims around the world prepare for a month of fasting, prayer and communality. But Ramadan does not begin on the same day everywhere in the world.

This year, some countries will commence the month of fasting on Thursday, February 19, whereas other countries may begin it a day earlier. Similarly, at the end of Ramadan, different communities will celebrate Eid on different dates.

The reason for this lies in the nature of the lunar visibility calendar – the new crescent Moon is not always visible everywhere on the same date. The prophet Muhammad is reported to have said: “Observe fast on sighting it [the crescent] and break fast on sighting it [the next crescent] – but if the sky is cloudy for you, then complete the number [30 days].”

In modern times, some countries such as Turkey have implemented calendar reforms, removing the act of monthly visual sightings of the crescent. They rely instead on pre-determined calculations.

Nevertheless, the visual sighting of the Moon to determine the start of the month remains the majority practice of Muslims across the world. Most believe this has to be done with the naked eye.

Cloud cover can therefore affect when the month begins in different locations, making it an unpredictable calendar. This imbues Moon sighting occasions with a sense of communal wonder – but it can also make the issue surprisingly contentious.

A very British problem

When Muslim migrants arriving in the UK in the middle of the 20th century tried to sight the new crescent Moon, they would often struggle – in part due to a very British problem: the cloudy weather.

As a result, various mosques and communities would outsource their Moon sightings to different countries. Some followed Morocco, others Turkey or Saudi Arabia. As each country might confirm a first sighting on different days, it meant UK mosques could end up with divided dates for Ramadan and Eid.

This has been a source of pain for some people in the British Muslim community. For me (Imad) growing up in London, it meant my school friends might start celebrating Eid on a different date to me and my family. This felt pretty sad – but I just assumed it had to be like this.

That changed when I witnessed the communal practice of Moon sighting during a family holiday to Cape Town in South Africa. When I saw thousands of Muslims gathering on the beach to celebrate seeing the new crescent Moon, I asked myself: “Why can’t we do this in the UK?”

When I returned from Cape Town, I founded a Muslim calendar lunar observation astronomy club called the New Crescent Society. Our aim is to find a way of celebrating Moon sighting communally throughout the UK – and to develop a viable lunar Islamic calendar here, like they have in other parts of the world.

Sometimes you can see the Moon in Cardiff but not in Cambridge. Sometimes the sky is clear in London but cloudy in Manchester. Our UK-wide astronomy education programme, Moonsighters Academy, now supports Muslims to lead their own lunar observation groups in their communities.

The astronomy of lunar visibility

Every month, the Moon goes from a thin crescent, waxing each night to become gibbous (more than half full) and then full, before waning back down to a crescent and disappearing again. This cycle occurs due to the Moon’s orbit around the Earth, and takes 29 and a half days.

The Moon does not create its own light. What we see is reflected sunlight – and the same side of the Moon is always facing towards us. It rotates on its axis at the same rate it orbits the Earth, a phenomenon called tidal locking.

The precise moment at which some amount of lunar illumination is first visible from Earth each month depends on geometrical physics. At this point, the crescent is so thin that even cameras struggle to determine it.

But as the Moon moves further away from the Sun in the sky, the crescent slowly becomes thicker as the angle of separation increases. There is now a longer “lag” between sunset and moonset, which also makes the new crescent more visible. The best time to view a young crescent is approximately halfway between sunset and moonset, balancing sky brightness with lunar altitude.

Astronomers relish the challenge of spotting a very thin crescent Moon when it is less than 24 hours old. But just how young a Moon can people see with the naked eye? One established landmark of 15 hours 32 minutes was set by the astronomer Stephen James O’Meara, who is also known for first spotting Halley’s comet on its return in 1985.

When you introduce optical aids like binoculars, even younger and thinner crescents can be seen. With the right conditions and technology, you can even image the Moon at the moment of conjunction, with an age of exactly zero hours. This was first achieved by astrophotographer Thierry Legault in July 2013, using an infrared filter on a telescope that had been “baffled” to block out the precariously nearby Sun.

When will Ramadan and Eid start?

This depends on where you are in the world. On February 17, the Moon will only be around three hours old at the time of sunset in Saudi Arabia, and moonset is only a few minutes after sunset at this point in the lunar cycle. So the Moon will be too close to the Sun in the sky for it to be astronomically visible.

Nonetheless, if the Saudi Supreme Court receives a report that someone has seen the Moon, it is likely to accept this, leading to a first day of fasting in Saudi Arabia (and all mosques around the world that follow its calendar) on Wednesday, February 18.

In the UK, Europe and North Africa, we are likely to have positive sightings the day after and commence fasting on Thursday, February 19. Countries further east, such as Australia, will probably see the Moon a day later still, and thus have their first fast on Friday, 20 February.

In March, on Thursday, 19, the Moon will be between 17 and 18 hours old at sunset, so difficult – but not impossible – to see in the UK, Europe and North Africa. So we expect communities following these sightings to start celebrating Eid on Saturday, March 21. Mosques following Saudi Arabia are likely to celebrate Eid a day earlier.

However, this is not just a story about calendars. When people gather to search the horizon for the new crescent Moon, they are participating in a practice that links them to the most ancient of human practices: observing and connecting with the natural world around them. In Britain, we hope our work can help make this an even more unified celebration.

The Moonsighters Academy was funded by a Science and Technology Facilities Council Spark award.

Emma L Alexander receives funding from the Science and Technology Facilities Counci for her research at the University of Leeds.