LONDON (RNS) — The Greenwich Royal Observatory is an ancient observatory of the stars and night sky, and home to Greenwich Mean Time, historically the worldwide basis for timekeeping, based on its prime meridian.

This week, its astronomers will be looking to the heavens not to study the stars, but to help people get closer to God. On Wednesday (Feb. 18), the Royal Observatory will join forces with the New Crescent Society, a Muslim moon-sighting organization, to look for the crescent moon that marks the start of Ramadan — the Muslim period of fasting running up to the festival of Eid, expected this year to be on March 20.

Although Ramadan is set to start on Wednesday, it officially begins only when the first sighting of the crescent moon takes place. The Royal Observatory will be streaming online, live from Greenwich, discussing Islam and astronomy, while astronomers on its roof keep their telescope trained on the night sky for the silvery sliver of the crescent moon. Members of the public will also be able to call in during the online broadcast with their own sightings of the crescent moon.

Moon sighting for Ramadan has become an increasingly popular activity in the U.K. in the last few years, as the number of Muslims living in the country has grown.

The U.K.’s considerable amount of rain this time of year means moon sighting can be difficult. Most mosques still depend on sightings of the crescent moon from desert countries such as Morocco and Saudi Arabia to declare Ramadan has begun. But the New Crescent Society, which is organizing the Greenwich event with the Royal Observatory, is hoping moon sighting will become fully established in Britain and be used by mosques.

“I say that you can see the crescent moon in the U.K. despite the cloud and we work together as a community, sharing what we can see,” said Imad Ahmed, the New Crescent Society’s director. “We can establish sighting of the crescent moon, rather than rely on outsourcing it to other countries, including Morocco and Saudi Arabia. We are bringing the moon back home.”

Islam shares its use of the lunar calendar with Judaism and Christianity, all ancient religions that emerged out of desert lands with clear skies. The phases of the moon became not only important for survival for desert peoples — marking growing times for food and the harvest, while its light helped navigation at night — but it also became integral to the practice of their faiths. This understanding of the moon’s purpose, as measuring time and seasons, is expressed repeatedly in both the Quran and the Hebrew Bible. The Quran speaks of the moon’s phases “that you may know the number of years and the calculation,” and the Psalmist writes that God “made the moon to mark the seasons.”

For both Judaism and Christianity, it is the full moon that determines their religious calendar, while for Muslims, it is the crescent moon, seen in the sky when less than half the moon is illuminated by the sun. This year, major moments in all three religions’ calendars abut and overlap: Ramadan, Lent and Passover. Ash Wednesday, which starts the Lenten season leading up to Easter, falls on Feb. 18 this year – the same day as the likely start of Ramadan. For many Christians, there is an obligation to abstain from meat on Ash Wednesday and Good Friday, while some voluntarily fast throughout Lent. For Muslims, Ramadan is a monthlong season of fasting from dawn to dusk each day.

Passover is calculated to begin on the night of a full moon after the vernal, or spring, equinox in the Northern Hemisphere — the day when the sun appears to rise due east and to set due west, making night and day of equal length. This year the spring equinox is on March 20, the same day as Eid al-Fitr, the celebration at the end of Ramadan. The post-equinox full moon — known as the pink moon — appears on April 2, meaning Passover will begin at sundown on April 1 and end on the evening of April 9. Easter Sunday will fall in the middle of Passover, on April 5.

The Royal Observatory, founded in 1675 by King Charles II and with a long history of stargazing and perfecting the use of longitude for navigation, has become increasingly committed to public engagement in recent years. This has led to growing cooperation with the Muslim community and to developing interest in Islam’s connections with astronomy, particularly the moon.

Jake Foster, a Royal Observatory astronomer who will host Wednesday’s event with Ahmed, said: “When you study astronomy at university, you don’t learn about the contribution of Islam to astronomy. Since engaging with the Muslim community about this, I have learnt so much about the ingenuity of the Islamic calendar.”

Arabic astronomers created some of the earliest images of the constellations of the stars, laying the foundations of astronomy, which were later built upon by the Greeks, Romans and Western science. Ahmed pointed out that hilal, the Arabic word for the crescent moon, also means shouting for joy and has an etymological link with the word alleluia.

“The moon is a very easily observed celestial body if it’s not blocked by cloud,” said Ahmed, “and you don’t need a telescope. We can train people to see it with the naked eye. Moon sighting can be fun. I don’t see a contradiction between reverence for God and your faith and joy.”

The most important requirement for sighting the moon is a clear view of the western horizon, because the new crescent moon always emerges near the setting sun. With the sun below the horizon, the sky is dark enough to spot the crescent moon above the horizon. The moon and the sun also need to be about 5 degrees apart in order to see the new crescent moon — this can be measured by a moon gazer holding out their first three fingers at arms length.

The Royal Observatory began its involvement in Ramadan moon sighting after Ahmed attended an astronomy course that started there seven years ago and suggested an astronomy and Islam planetarium show, which sold out within minutes. According to Foster, the Royal Observatory has seen Ramadan moon sighting take off in the last five years. “We now have thousands of people joining in online. Interest is growing all the time.”