Abd al-Rahman al-Sufi, Astronomer

MS. Marsh 144, fol. 135v, Bodleian

The contributions of the Muslim world to astronomy are many, and I have only briefly touched on some of them (such as here). There were nine Muslim astronomers in particular who made major contributions. One of them was the Iranian Abd al-Rahman al-Sufi (7 December 903 - 25 May 986). His name indicates that he was a Sufi Muslim, like Rumi.

al-Sufi translated and expanded on the work of the Greeks, especially attempting to reconcile the Greek and Arabic star charts and constellations. In 964 he published Suwar al-Kawakib al-Thabitah, the "Book of Fixed Stars."* In it he gave the latitude and longitude of hundreds of stars for the year 964 from two views: from both the exterior and interior of a celestial globe. The oldest surviving manuscript known is in the Bodleian Library and was created about 1009 by al-Sufi's son. There was no English translation of this book until 2013.

Among the "firsts" that can be credited to al-Sufi's work are the following identifications:

Ursa Major

• "the little cloud" that we call the Andromeda Galaxy.
• the Large Magellanic Cloud*
• the Omicron Velorum star cluster
• a "nebulous object" in Vulpecula, now called "Al Sufi's Cluster"

He also describes the astrolabe and lists a thousand uses of it.

The significance of al-Sufi's work led the astronomical community to name other objects after him, such as a a lunar crater (Azophi) and 12621 Alsufi, a minor plant in the asteroid belt with a period of 2000 days.

*There is an argument that he could not have known of the Magellanic Cloud until the same time as Western European astronomers in the 15th century because of its position in the Southern Hemisphere.

The Jalāli Calendar

Syrian Astrolabe

Yesterday I mentioned that Omar Khayyam spent some of his time working on calendar reform. This was not the same calendar reform being done in the Christian world, however. The Persian calendar was—and still is—far more accurate than the Gregorian calendar.

Originally, the Persian calendar was lunar, following the 28-day cycle of the Moon. Since the year does not fit into an equal number of lunar cycles, however, the lunar calendar creates "seasonal drift" without a lot of alterations. This calendar was begun over 1000 years BCE. Khayyam was one of several scholars using astronomical observations to create a revised version. It was approved on 15 March 1079 by the Seljuk Sultan, Malik Shah I.

Khayyam and his team calculated the length of the year to be 365.24219858156 days; modern science puts it at 365.2422464 days. Some aspects of the new calendar:

• The year started within a day of March 21st, the vernal equinox
• Months were based on when the sun transited to a new sign of the zodiac, not 12:00AM
• Months could last from 29-32 days, and
• Months could change their length from year to year

That 4th point is because of the 2nd point. Months weren't given arbitrary numbers of days as in the West. The Jalāli calendar depended on strict astronomical data, not cultural numerical choices. Therefore, the 6th month of the year might have 30 days one year and 31 days the next, depending on when the sun passed across the line in the sky that separated the zodiacal signs. It also means that seasonal drift—the tendency of seasons to start and end on widely varying dates over time—never exceeded one day. Leap years were unnecessary.

Eventually, the varying length of the months was considered a liability. The calendar—still used in Iran and Afghanistan—was changed in 1925 in order to have a more regular look and to save the hassle of applying the results of constant astronomical observation.