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Picture Archive: Cairo Mosque, Circa 1920

As Ramadan 2013 comes to a close, a look inside a mosque from a century ago.

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A Muslim observes prayer in Cairo's Al-Mardani Mosque.

Ramadan, the ninth month of the Islamic calendar—and the holiest, because it's when Allah revealed the Koran to Mohammed—ends on August 7, closing 30 days of sunrise-to-sunset fasting, reading of the Koran, and additional daily prayer.

It's a moving month, shifting forward about ten days a year because it's tied to lunar cycles. Its beginning and end are marked by the appearance of the new moon.

Today there are about 1.6 billion Muslims worldwide, nearly one billion of whom live in the Asian Pacific region, according to the Pew Research Center. During Ramadan all of them—except for those who are very young, very old, pregnant, menstruating, ill, or traveling—must fast until sunset, often breaking their fast after sundown with a bite of a date. Since this year Ramadan falls in summer, the season's longer days require longer daily fasts.

National Geographic photo editor Franklin L. Fisher bought this photo of an observant Muslim in Cairo, Egypt, in the summer of 1920.

"Moslem at prayer in the Mardani Mosque," reads Fisher's handwritten note on the photo's border. "Built 1338-40, one of the largest in Cairo. Like many other mosques in the city it has been built largely of material from ancient temples. The four central columns are of the Ptolemai[c] period."