BOB ABERNETHY (anchor): Also on our calendar, this coming week, Muslims around the world will begin celebrating Ramadan, the holy month of prayer and fasting. During Ramadan, adult Muslims are expected to fast and abstain from sex during daylight hours. There's a specific -- and complicated -- process to determine when Ramadan begins. Muslims use a lunar calendar, and Ramadan starts with the new moon. But for devout Muslims, it's not enough to determine astronomically when the crescent moon rises. It has to be seen. Kim Lawton reports.

KIM LAWTON: At the headquarters of the Islamic Society of North America, the phones are ringing off the hook. Muslims are wondering whether anyone has seen the new moon.

UNIDENTIFIED CALLER #1: Is there any news for the moon tonight?

OPERATOR: No news until this point.

LAWTON: It's a process that combines science and technology with religious tradition. The Qur'an teaches Muslims to study the heavens -- and specifically the moon -- to mark time. Today, scientists can calculate where and when the new crescent moon may be most visible, but Islamic scholars say the actual sighting is still necessary.

Dr. AHMAD (lecturing): It's going to be challenging to see the new moon from this spot on the Earth.

LAWTON: Tonight Muslim astronomer Imad-Ad-Dean Ahmad is giving some moon-sighting advice to the Muslim Student Association at George Washington University. The accepted practice for moon sighting varies from country to country. Because of the Earth's size and curvature, Ahmad says the new moon may be seen on different days in different places. That means Ramadan and its month-long fasting may begin on different days as well.