Friday is the beginning of the month of Ramadan for Muslims around the world. Contrary to what many non-Muslims believe, Ramadan is not an Islamic holiday or a religious observance. It's actually the name of the ninth month on the Islamic calendar.
As amateur astronomers and other skygazers know, Friday is also the day of July's first crescent moon. This is not a coincidence. More on that in a moment.
The reason Ramadan is important to Muslims is that it's the month in which the Prophet Muhammad is said to have begun receiving revelations from Allah (the Arabic name for God, also used by many Arab Christians). Those revelations are believed to have been recorded in the Quran, the Muslim holy book.
During the month of Ramadan, as an expression of submission to God, Muslims abstain from food and drink between sunrise and sunset. Sunset marks the beginning of the next day on the Islamic calendar. Traditionally, Islamic families gather at the beginning of each day — shortly after sunset, that is — for a feast that is meant to carry them through that day's fast.
Muslims can't simply look on a Western calendar to figure out when Ramadan starts. To do that, they need to look to the sky. Muslims believe that Muhammad told them how to do this in this sacred hadith, a saying attributed to Muhammad himself:
"Fast when you see the new moon and break your fast when you see the new moon."
Once this "moonsighting" is achieved, the Ramadan fast begins the following morning.
In the event that overcast skies prevent the first crescent moon from being observed, Muslims are instructed to count 30 days from the beginning of the Islamic month of Sha'baan, which precedes Ramadan.
Among Muslims, the question of what constitutes a valid "moonsighting" has long been controversial. Debates continue about whether one must spot the moon in their local area or whether it's enough for an Islamic authority anywhere in the world to declare that the first crescent has been sighted. Another tradition states that it's sufficient for any two reliable witnesses to vouch for the appearance of the first crescent (which, it should be noted, is unrelated to the star and crescent logo often associated with Islam).
Given the spiritual importance of Ramadan, it shouldn't be surprising to learn that medieval Muslims expended a lot of effort to make sure that they didn't miss the very first dimly visible sliver of the waxing crescent moon.
During the early Middle Ages, a time when the scientific knowledge of what would later be known as Western civilization was mostly in a state of stagnation, the Islamic world was preserving and building upon scientific disciplines it had absorbed when it conquered most of the lands of Roman and Greek antiquity during the seventh century after Christ.
Volumes of ancient knowledge that were lost to Western Europe were pored over and translated by Islamic scholars. The works of Ptolemy, the great second-century mathematician and astronomer of Roman Egypt, were closely studied by Islamic astronomers, who built upon and challenged his observations while expanding their own understanding of the night sky. When, in the later Middle Ages, the scientific minds of Europe began to rediscover this ancient knowledge, it had acquired a distinctly Islamic flavor.
Astronomy today has thus inherited many stars bearing Arabic names, such as Deneb ("Tail of the fowl") and Altair ("The flying"), both which hang overhead on clear midsummer nights. We still use the Arabic term azimuth to help describe a star's location in the sky.
It seems fair to wonder how much ancient astronomical knowledge would have been lost forever if observations of the sky had not been so central to the practice of the Islamic faith. If, next month, you happen to see the next crescent moon, which marks the end of Ramadan and the celebration of Eid ul-Fitr ("the festival of breaking the fast,") remember the contributions of Islamic culture to modern science and take a moment to wish a Muslim friend "Eid Mubarak" ("Blessed Eid").