Biruni is one of the greatest muslim astronomers. He was born in today's Uzbekistan, which was part of the great Persia at that time. The most important astronomical writing of Biruni is Canon (not to be confused with a similar title from Avecina which is written in medicine). His Astrolabe is also considered as one of the most valuable books which describes the construction and function of this ancient astronomical instrument. The Astrolabe is considered as a Greek invention. However, muslims undoubtedly developed and elaborated it.Persian astrolabe is a circular piece of brass with a projection of the celestial sphere engraved on it. An overlying root-like matrix called rete is cut according to the position of bright and important stars like Vega and Rigel. By rotating this matrix around the holding axis which goes through the center of the brass disk, the daily positions of the stars can be indicated against a horizontal altitude-azimuth coordinate system.

There is also a straight bar or index passing through the projected north pole showing the projection of an hour circle. It is also used to measure the apparent altitude of celestial bodies.    Biruni is also famous for measuring the diameter of the Earth most accurately in his times. In Canon he writes:      I found a mountain in India which overlooked a flat region like the sea surface. I measured the angle of the horizon at the mountain top there. The angle was a little less than a third and a quarter of a degree and I considered it to be about 34 minutes of arc. Then I measured the height of the  mountain's top by observing it from two different points coplanar with it. This was equal to 652 "zera's" resulting to about 58 miles ( the mile scale used by muslim astronomers was different from the current mile, as pointed  by the Italian expert C.A. Nallino ). In this way the results of al-Mamun's astronomers were confirmed.   Biruni used this previously known trigonometric method in order to obtain a more reliable estimate of the diameter of the Earth. He is also known for his observations of luner eclipses in Kharazm in late 10th century.