Ramadan, the holiest month of the Muslim calendar, starts on Monday in some countries but on Tuesday in others. How is that possible?

The exact timings depend on when local Islamic authorities around the world declare the sighting of the new moon, the astronomical event that marks the start of the observance.

Here’s why there’s so much variation from place to place.

The basics

Islamic countries, and Muslims around the world, use the traditional Hijri calendar to mark religious events. Each month of that calendar begins with the sighting of the early crescent moon, and the holy month of Ramadan begins at the start of the ninth month.

These customs go back centuries. But the exact start time of Ramadan varies from place to place because it depends on a range of factors, including who observes the moon and how, and whether the sky is clear or cloudy at the time.

That explains why Saudi Arabia declared that Ramadan would start on Monday after reporting a sighting of the crescent moon on Sunday, but also why neighboring Oman reported the same day that the moon was not yet visible. As a result, the two countries will begin their Ramadan celebrations about 24 hours apart. The authorities in Iran, like Oman, have also declared Tuesday to be the official start of the holy month.

In Southeast Asia, Islamic authorities in several countries said over the weekend that they, too, would observe Ramadan from Tuesday, not Monday, after unsuccessful moon sightings. That gave millions of people across the region, including in Indonesia, the nation with the world’s largest Muslim population, an extra day to prepare.

‘A challenging mission’

Astronomers have argued for centuries about what constitutes a crescent moon sighting, according to a recent study in the journal Scientific Reports, which explored how artificial intelligence and machine learning could help predict the moon’s visibility.

New technologies make moon readings more accurate, but they can also make the process more complex, the study said. For instance, does it count as an official sighting if a new crescent moon can be seen by an optical aid but not by the naked eye?

“Deciding on the start of Ramadan has always been a challenging mission, and, as a result, not all Muslims start Ramadan synchronously,” the study’s authors wrote.

In Islamic countries, national guidance helps ensure that Muslims are on the same Ramadan timetable. In other places, the timing can depend on which religious guidance residents follow. This year, for example, the Fiqh Council of North America chose Monday, while the Council of Shia Muslim Scholars of North America chose Tuesday.

Other variables

It isn’t just the start date of Ramadan that varies from place to place. The exact amount of time that people hold their dawn-to-dusk fasts depends on which year it is, and where they live in relation to the Equator.

Because the Hijri calendar is about 11 days shorter than the 365-day-ish Gregorian calendar, the month of Ramadan moves around every year in Gregorian terms. That means the fasting day in the Northern Hemisphere will get shorter between now and 2031, when Ramadan coincides with the winter solstice, Al Jazeera reported. In the Southern Hemisphere, it will grow incrementally longer over the same period.

As for location, latitude matters because it determines the timing of local sunrise and sunset. Muslims who live close to the Equator, where the length of a day changes less from season to season, can expect a relatively consistent fasting schedule each year. For those living at extreme northern or southern latitudes, there is a lot more variation.

That helps explain why, in some parts of Scandinavia, Muslims observe Ramadan fasts based on the time in Mecca, thousands of miles away. If they didn’t, depending on when the holiday fell in a given year, they would either be fasting for most of the day, or barely at all.