Around 25% of the Earth’s population is expected to participate in fasting.

This weekend, a quarter of the global population is expected to begin Ramadan, the Islamic holy month most commonly known for the fasting of food and liquids from dawn to sunset by its followers.

What many non-Muslims may not know is that, while Muslims generally know when Ramadan is approaching, they may only be notified a few hours before its official start.

Moreover, the event is not observed uniformly around the world – the start date can vary from country to country and may even differ from mosque to mosque within the same city. But what unites all Muslims is the observation of the crescent moon in the ninth month of the Islamic calendar.

In Saudi Arabia, for example, where Islam was first established by the Prophet Muhammad in the 7th century, the new moon is expected to be visible on the night of February 28. Therefore, Ramadan would begin on Saturday, March 1.

Thousands of kilometers east of Saudi Arabia, in Indonesia, the lunar visibility angles are different, and it is expected to begin on Sunday, March 2.

In 2024, Saudi Arabia and its smaller neighbor on the Arabian Peninsula, Oman, began one day apart.

This discrepancy occurs due to a complex mix of reasons, according to author and professor Scott Kugle from the Department of Middle Eastern and South Asian Studies at Emory University in Atlanta.

These reasons include, among others:

• Astronomical sightings and how they are made
• Global geography and time zones
• Various traditions among different Muslim groups
• Even current weather conditions

“One key point that is very important, I think, for Westerners to understand is that there really is no central authority among Muslims. It’s all very local, depending on which mosque you attend, what your family network is,” Kugle said in a phone interview.

This is why there are national variations and even differences within the same city regarding the start time, says Kugle. And given the different interpretations, it is possible that the actual start date falls one day earlier or later than the provisional date.

“That’s why people get so excited in the week leading up to it because they’re shopping, preparing,” he said. “There’s a lot of uncertainty: ‘What day is it going to start?’”

The answer lies in the sighting of a specific phase of the moon. It’s all about the crescent moon.

The Islamic calendar does not follow the sun. Instead, it follows the lunar cycle and the phases of the moon.

Probably the best-known phase is the “full moon,” when the largest surface of the moon is brightly illuminated from Earth’s perspective. The invisible phase of the moon, when its illuminated side faces the sun and its dark side faces Earth, is the “new moon.”

Ramadan begins when a small slice of the moon, known in astronomical terms as the “waxing crescent,” emerges and becomes visible. And this is true throughout the Islamic world.

But from there, things begin to get complicated and decentralized quickly, Kugle said. First, there are two ways to determine a crescent moon sighting.

“One is by seeing it, and the other is by calculating it astronomically,” Kugle said. “And the traditional, old-fashioned way, of course, is to see it.”

“It’s not that everyone has to see it, but the people responsible have to see it. So, some communities will have a moon-sighting committee, and they’ll go to a high spot or to a beach where there’s an unobstructed view of the horizon and wait to see if they can spot the crescent moon, which usually appears for a short time after sunset.”

And if they don’t see it? “Then they say, ‘Okay, the month hasn’t started.’”

Naked-eye observations can bring other complicating factors, such as cloud cover in certain places. A crescent might be seen in one place but not another nearby.

Other places rely on astronomical calculations, with Turkey being an example of this.

“Islamic scholars have declared that both forms of moon observation are allowed, and that people should act according to their local situations,” according to Turkish Radio and Television World.

The variables in crescent moon sightings are not the only things at play. “There’s no central authority for Muslims, and there are some basic divisions among them, just like in Christianity [with] Catholics, Protestants, and Orthodox,” Kugle said.

“So, in the Muslim world too, you have Sunnis, you have Shias, and you have others. And these groups can make their own determination of when the moon is sighted. And it might be on different days, right?”

Even human emotions may come into play.

“There’s a kind of pride in starting on the same day as the other group. Even in a Sunni-majority country, a Shiite minority might start on a different day,” Kugle said.

Despite regional differences and variations in interpretations, Ramadan remains one of the most significant periods in the Islamic calendar, promoting reflection, spirituality, and unity among Muslims worldwide. Fasting and prayers not only reinforce individual faith but also strengthen community and family bonds. Regardless of the exact start date, the spirit of Ramadan remains unchanged, celebrating devotion, self-discipline, and solidarity within the global Islamic community.