From a canyonside perch outside Riyadh--longitude 40 degrees, 27 minutes east, latitude 24 degrees, 36 minutes north, as reckoned by a global positioning system--Salih Saab trained his Meade LX 200 computer-driven telescope on the expansive desert horizon, wired it to his laptop and set the coordinates.

For 1,400 years, Muslims have scanned the late afternoon sky for the crescent moon that signals the start of the holy month of Ramadan. Saab, head of astronomy at Saudi Arabia's King Abdulaziz City for Science and Technology, wants to make sure it's done right.

"We must wait until the sun sets, and then . . . maybe," Saab said.

With the moon only emerging from its monthly conjunction with the sun and Earth by a few degrees, its appearance tonight would be tentative, perhaps not even long enough to overcome the sun's colorful afterglow or high enough to avoid a low-lying film of sand and dust rising from the Arabian desert.

Not to worry.

On this night, one of Islam's most expectant and joyous, the astronomers know the crescent is out there, and that someone among the world's body of Muslims, from the Arabian peninsula, westward to Morocco, north into Jordan and Iraq, or beyond, will spot the hovering sickle that the prophet Muhammad declared would start the month of fasting.

"There's a saying of the Prophet: 'Ramadan starts according to the sighting of the crescent, and finishes on the sighting of the crescent,' " at the start of the following lunar month, Saab said. To adhere as closely as possible to Mohammed's instructions, "we started to depend on this," he said of the carload of technology he toted into the desert as part of his work on the crescent-spotting committee in Riyadh.

Six such committees are in charge of supporting and verifying the work that citizens are encouraged to do: watching the skies and phoning local authorities if they see the new moon.

Although some purists contend that, to be valid, the sighting must be made by the naked eye, Saab said Saudi religious authorities settled that issue in favor of the telescope in 1982--a judgment with its own historic resonance, considering early Islamic advancements in astronomy and given that the religion, tuned to the lunar calender and using the position of the sun to time its five daily prayers, is tightly linked to the mechanics of the solar system.

The star watching triggers a month-long religious undertaking in which tens of millions of people literally synchronize their diet with the sun, obeying Mohammed's call for abstinence during the month when the revelation of the Koran began.

Even some of the most casual adherents to Islam follow the basic call to abstain from food, drink, tobacco and sex each day from sun-up to sundown, for the entire month. The more observant try to tame all the senses and focus on the Koran, spending whole days and nights in a mosque in contemplation.

Work crawls to a standstill, especially when the revolving lunar calendar places Ramadan among the long days of summer, while charity moves into high gear and life's rhythms shift to the night.

In more open and rambunctious cities like Cairo, that means a wild dash for food at sundown, late-night parties, elaborate hotel musicals and special tents stocked with coffee, sweets and aromatic water pipes. In more ascetic countries like Saudi Arabia, the emphasis is on family gatherings and quiet acts of charity--money or food donated from one family to another, or left at the mosque for those who need it.

"Muslims wait all year for Ramadan. It is the best month," said Mohammed Qudaibi, a spokesman for an Islamic research agency in Riyadh. "It is when you clean yourself and clean your mind. Think about the future. Think about everything."

By 5:20 p.m., the sun was gone but the sky remained light, with orange and blue streaks painting the desert vista. Saab's friends and colleagues--hobbyist astronomers, a computer programmer who helps maintain his system and a local sheik who also serves on the Ramadan crescent committee--had gathered at the impromptu observatory nicknamed "Laban 1" for the nearby wadi, or canyon.

The moon still was not visible as the group scanned the horizon with binoculars and Saab's telescope, and joked about the clouds of dust they feared would obscure the moon.

"It look likes Los Angeles," said one stargazer.

They sipped coffee, munched dates and pulled their red checked scarves and flowing robes tight around their heads and bodies as the desert wind turned chilly.

The crescent would stay hidden tonight, at least from this point on the Arabian peninsula. But the spirit remained.

"Allahu akbar, allahu akbar," one man chanted, "God is great." And the group knelt for their sunset prayer.

CAPTION: Astronomer Salih Saab sets up his telescope in the Saudi desert to search for the new crescent moon, following a 1,400-year-old tradition.