Sky & Telescope June 1986 [antikvár]

Ulugh Beg's Forgotten ObservatoryErnest W. Piini, San Jose Astronomical AssociationThe Persian Empire once stretched into what is now the southern Soviet Union, and there, at the ancient city of Samarltand, many crossroads met. Famous travelers once walked there, including Marco Polo's father Nicolo. Today Samarkand, nestled in the Zaravshan River valley some 140 miles from the Soviet-Afghanistan border, is the second largest city in the Soviet Socialist Republic of Uzbekistan. This thriving city is a wonderland of towering minarets, majestic domed mosques, and colorfully tiled madrasahs and mausoleums.On a windswept hilltop about two miles from the Registan (where the ancient trade routes met) stands a long, low brick building with an arched roof. Inside, two curved tracks of masonry descend into a narrow trench like some kind of subterranean roller coaster. A low, circular brick wall about 60 meters in diameter surrounds these strange constructs. These are the remains of an astronomical observatory built between 1424 and 1429 by Ulugh Beg, an astronomer who later became ruler of much of Persia.THE GRAND PRINCEUlugh Beg (the name means grand prince) was born Muhammad Taragay in the central Asian city of Sultaniyya in 1394. He grew up in the court of his grandfather, the infamous Tamerlane, Mongol conquerer of all Islam during the late fourteenth century. At age 15 Ulugh Beg became governor of Maverannakhr, the city now called Samarkand. Unlike his grandfather, he had no appetite for conquest and chose instead to become a scientist.In 1420 Ulugh Beg founded a madrasah, an Islamic institution of higher learning, with astronomy as its most important subject. This university, which bears his name, stood in the Registan. He personally interviewed and selected the scientists who taught there. His madrasah was only one of many in central Asia, but it became known for its unique subject matter and high standards. A letter written by one of the teachers at the school, Giyath al Din, says something of the character of Ulugh Beg:The truth is that, first of all, he knows most of the Holy Koran by heart. His knowl-542 Sky Telescope, June, 1986A bust of Ulugh Beg, the Persian regent who built the largest astronomical instrument of the I5th century."Unlike his grandfather,he had no appetite for conquest and chose to become a scientist."edge of grammar and syntax is very good, and he writes Arabic extremely well. His majesty has great skill in the branches of mathematics. He derived the longitude of the sun to a fraction of two minutes by mental calculation while riding on horseback.The Moslems had begun building observatories early in the ninth century to help determine the precise time of the start of Ramadan, a holy month in the Islamic calendar, and as surveying tools to accurately locate cities for the benefit of pilgrims journeying to Mecca. Ulugh Beg decided to build an observatory to support the astronomical studies at his madrasah. He also planned to create a new star catalogue only the second in the 17 centuries since that of Hipparchus.As a child, Ulugh Beg had visited theMaragha Observatory in Tabriz, now part of Iran. That observatory, established in the mid-13th century, boasted a great circular arc, aligned with the meridian, with a radius of 18.4 meters (61 feet). For his own observatory Ulugh Beg chose to follow much of Maragha's design, but his dream was to build a meridian arc with a 40-meter radius, making it the worid's largest astronomical instrument of that time.The ground-breaking ceremony on a spring day in 1424 must have attracted hundreds of students and followers. Work began on the trench for the meridian arc, 2.5 meters wide and 11 meters deep at its lower southern end. A total of more than 500 tons of rock and dirt were cleared in the digging.Other workers began the building's circular foundation. The walls, partitions, pillars, and floors ascended to three levels. Some 33 meters above ground level, just below the roof line, was the upper end of the meridian arc, supported by the rubble removed from the trench. As Ulugh Beg had planned, the arc measures 40 meters in radius. The observatory featured 28 portals on each level, signifying the visible days, or mansions, of the Moon.WHAT WAS THEMERIDIAN ARC?The meridian arc was the central instrument of the observatory. Although some scholars think it was a quadrant, it was most likely what is called a Fakhri sextant. This device can be used to help determine several basic astronomical constants, such as the inclination of the ecliptic to the celestial equator, the point of the vernal equinox, the precession of the equinoxes, and the length of the tropical year.Little is known of the precise configuration of the arc when it was in use. But it is reasonable to speculate that the astronomer sat on a curved stone staircase between the two rails of the meridian arc and sighted on the Sun, Moon, planets, and stars as shown in the illustration on page 543. The two polished marble rails that formed the arc were inscribed with grooves for determining the positions of celestial objects. The largest division between grooves was 70.2 centimeters, corre-