For the world's 1.8 billion Muslims, planning for the opening day of Ramadan, the holy month of prayer and fasting, isn't as easy as consulting a calendar.

Every year, the start date is determined by a scientific prediction of when the slender crescent of the new moon will appear in the evening sky. According to the astronomical calculation for 2018 — in Islam, the year 1439 — the observance was to begin Tuesday at sunset, with the first fasting day on Wednesday.

Many Muslims, however, insist on seeing it with their own eyes before they believe it.

In an annual ritual, a trio of area Muslim leaders — Brother Amin Abdul Aziz and Imams Muhammad Abdur-Razzaq Miller and Yahya A. Latif  — fan out across the region to spots affording the best view of the ascendant silver sliver. They've gazed at the sky from a parking lot in Upper Darby, the old Sears Tower in Northeast Philadelphia, a bridge in North Philadelphia, and a mosque in Coatesville.

If and when the moon appears, they spread the word to their  local community of 10,000 Muslims that Ramadan has arrived.

"The Prophet Muhammad said when you spot the moon, you start the month," said Aziz, 70, a member for more than 35 years of the moon-sighting committee for the Majlis Ash Shura of Philadelphia and the Delaware Valley, an advisory council of area imams and mosques. "So we are following the moon in our town, our time zone."

This year, though, science and the moon-sighters didn't see eye-to-eye. On Tuesday evening in Philadelphia, lightning and drenching rains kept the committee from venturing out at all. "There were no moons available," Aziz lamented.

The problem, however, spanned the globe, as Tuesday was a bust in most Muslim population centers. Storm clouds and dust shrouded the moon even in Saudi Arabia, home to Mecca, the faith's holiest site. There was no choice but to quickly reset the holiday start for Wednesday evening, with the fast beginning on Thursday.

It was the last possible moment for Ramadan to begin. On the Islamic calendar, months are either 29 or 30 days, Aziz explained. The last day of Sha'ban, the month prior to Ramadan, began either Monday or Tuesday evening. With no moon-sighting on Tuesday, Ramadan had to start on Wednesday evening.

But there were exceptions.

The Fiqh Council of North America, which interprets Islamic law, puts its faith in science. It stuck to the mathematical calculation that the holiday began Tuesday evening.

Still, Zulfiqar Ali Shah, its secretary-general, traveled to the big island of Hawaii to view the moon that night because optimal sighting conditions were expected to be in the Pacific. The moon was scheduled to set about 37 minutes after the sun went down, providing the longest viewing time, Shah said. But once he reached the top of Mauna Kea, a dormant volcano on the island, bad weather prevented him and his three-person team from glimpsing the moon.

"We tried our best," Shah said, adding that another scientist representing the Fiqh Council did spot the moon Tuesday in Southern California.

Ideally a unifying celebration in the Islamic world, Ramadan marks the month in which the Prophet Muhammad is believed to have received the first revelation of the Quran from the angel Gabriel.

When followers begin the observance on varying dates, there "is a reason for some consternation," said Zain Abdullah, an associate professor of religion at Temple University.  "You want to start on the same day and end on the same day so you can celebrate together."

Ramadan is the ninth of 12 months on the Islamic calendar, which has 354 days and is based on the lunar cycle. The start dates for the months fluctuate annually according to the moon.

"This is the [reason] that the tradition of actual moon-sighting with the naked eye became part and parcel with the Islamic faith," Shah said. "Muslims have been doing this for the last 14 centuries to make sure they worship in the proper sacred timings."

At the Gracious Center of Learning and Enrichment Activities mosque in Cherry Hill, the community relies on a combination of methods, Imam John Starling said.

Community leaders consider science to help predict potential Ramadan start dates, which are used to guide local moon-sighting committees. The mosque also considers international reports of moon sightings from Far Eastern countries that would typically see the new moon before the West does.

Immigrant Muslims often call relatives still living in their home countries for word on when the holiday begins. But Starling said he likes the local moon-sighting method because it democratizes the process.

"One of the things I find beautiful in our faith is that it's accessible to everyone," he said, "and the visual moon-sighting is something that everyone can do."

More than 30 years ago, Aziz began studying the phases of the moon because he wanted to bring expertise to the sighting process. He took an astronomy course at the University of Pennsylvania and boned up at the Franklin Institute. His committee now trains younger Muslims who want to carry on the tradition.

"Their eyesight," fellow member Latif said, "is better."