HELWAN, Egypt — The observatory director, Salah M. Mahmoud, squinted at the smog gathering over the distant Nile.

“It looks like trouble,” he said.

His deputy, Ahmed Fathy, concurred with a sigh, “I’m afraid we’re not going to see anything tonight.”

The two Egyptian physicists on Tuesday night had a delicate mission: They were charged with providing the scientific imprimatur to the start of the holiest time for Muslims worldwide, the lunar month of Ramadan. Egypt plays a major role in this ritual because it is the seat of Al Azhar, the world’s most prominent Sunni Muslim institution.

According to the Koran, Ramadan, a month of fasting and prayer, begins on the first night that the crescent moon is visible to the naked eye. For centuries, clerics and laymen jostled to spot the Ramadan moon first and often differed. An area with cloudy skies or a different longitude and latitude might declare Ramadan a day or even two later than the rest of the Islamic world.

Modern astronomy long ago took the mystery out of the lunar calendar, whose year lasts some 11 to 12 days less than the Gregorian year used by most non-Islamic countries.

Mr. Mahmoud, the president of Egypt’s National Research Institute of Astronomy and Geophysics, publishes a hefty book of tables that lists the precise time and location that the Ramadan moon will appear in various cities throughout the Islamic world.

But science, it seems, can go only so far.

“We know it’s there, but Shariah requires us to see it with our eyes,” Mr. Mahmoud explained. The grand mufti, Egypt’s highest religious authority, awaits a report from Mr. Mahmoud’s team and from a secondary group of spotters organized by the Egyptian National Survey Authority.

The lunar month literally turns at the moment of conjunction, the astronomical term for the moment at which Sun, Earth and Moon lie on the same plane. This year, according to Mr. Mahmoud, conjunction occurred at 03:08 Greenwich Mean Time or 6:08 a.m. Cairo time. But the crescent moon will be visible only at sunset, and then for only a few minutes.

There is a narrow window of opportunity to lay eyes on the sliver of moon. In Cairo this year, the Ramadan crescent would be on the horizon for 11 minutes after sunset. In Tehran, by comparison, it would be on the horizon for just one minute, giving astronomers scant chance to spot it. Mauritania was in the best position for a sighting, with 20 minutes of Ramadan moon visibility.

Add to that atmospheric conditions — dust and smog in Cairo, humidity and haze along coastlines and in the sun-baked inland deserts — and you have the makings of confusion.

“We don’t expect to see it, but we must in order to follow the Shariah,” or Islamic law, Mr. Mahmoud said.

On Tuesday, Mr. Mahmoud and his deputy nervously assembled their staff at the Helwan observatory in the late afternoon. British colonialists built the observatory here in 1903, when the desert about 20 miles south of downtown Cairo afforded a clear view of the night sky.

As long as 50 years ago, Mr. Mahmoud said, smoke from the nearby brick factories and light pollution from Cairo had rendered the Helwan telescope useless for serious astronomy.

Now, he and Mr. Fathy organize scientific teams at locations across Egypt on the eve of Ramadan. Astronomers set up telescopes on the Mediterranean coast, near the Aswan Dam, in the Sinai, along the Nile, and in the desert west of Cairo.

“We hope that at least one of them will see the moon tonight,” Mr. Mahmoud said 30 minutes before sunset. Despite the nearly 100-degree temperatures, he was wearing a suit, and on it a lapel pin with the observatory logo.

The observatory’s principal telescope is too powerful for this sort of thing, so the staff lugged two reflective telescopes to the parking lot facing west. Below a sharp dun-colored bluff stretched the brick and cement factories of Helwan.

Mr. Fathy pointed the larger of the two telescopes, a white double-barreled apparatus about the size of a car axle, eight degrees south of the sun’s trajectory, toward the spot where he knew the Ramadan moon would be lurking.

Several families materialized. “We’re here to see the moon,” Tarek Ghazi, a factory supervisor, said brightly, his daughter and son in tow. He was wearing a loud pink plaid shirt. “I’ve never done this before,” he added.

“I want to see the crescent!” his 18-year-old daughter, Hiba, said.

It was 7:20 p.m., and the sun had sunk low enough to turn a dull orange. The correspondent for Al Jazeera carefully selected a position for his live shot that would feature the telescopes in the background and, he hoped, that anxiously awaited crescent moon above his right shoulder.

A thick gray miasma obscured the horizon. “You can’t even see the sun!” Mr. Mahmoud said.

A jetliner rumbled directly overhead, on the approach to Cairo International Airport, before it, too, entered the fog of pollution.

“It’s like a black cloud,” Mr. Fathy said.

At 7:36 p.m., Mr. Fathy was still peering into the telescope, but he was just going through the motions. “We’re not going to see anything here,” he said glumly. He clutched his cellphone. “Maybe the other teams will see something.”

A few of the sky watchers phoned in: nothing.

“What happens if none of you see it?” an onlooker asked.

“We will inform the mufti, and he will decide, from his point of view,” Mr. Mahmoud replied.

“We are only providing the scientific opinion,” Mr. Fathy added.

Another cellphone rang. “You didn’t see anything either?” Mr. Mahmoud shouted into his receiver, exasperation creeping into his voice.

He hung up.

“All negative,” he announced to the crowd that had swelled to a few dozen.

Mr. Ghazi, the factory supervisor who had brought his children to the hilltop, looked concerned. “Is it Ramadan then?” he asked his daughter.

“Yes, father,” she said.

Mr. Mahmoud made a few more phone calls. Apparently other teams — technically amateurs — reported a moon sighting in Abu Simbel, far south of Aswan, and in Sohag, in central Egypt.

“It is the mufti who will make the final announcement,” the observatory director said.

Beneath the plateau and to the north, in the city, the official Ramadan eve fete was under way, moon or no moon. In a vast banquet hall at Al Azhar, notables had assembled from the Islamic institution, the government, and the Dar al-Ifta, or House of Fatwa, which issues rulings on Islamic behavior.

There the grand mufti was waiting to certify the sighting of the moon and the commencement of the 30 days of Ramadan.

By the time Mr. Mahmoud had tabulated the results of his scientific observation teams, it was too late to make it to the celebration. Even though none of the astronomers had seen the crescent moon, the mufti already had declared Ramadan anyway on live television at 8:30 p.m.