Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī was a Zanji slave who was taken to Persia in the 9th century.

Together with [[ʿAlī ibn ʿĪsā al-Asṭurlābī]] in 827, he measured at 35 degrees north

35th parallel north

The 35th parallel north is a circle of latitude that is 35 degrees north of the Earth's equatorial plane. It crosses Africa, the Mediterranean Sea, Asia, the Pacific Ocean, North America and the Atlantic Ocean....

 latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

, in the valley of the Tigris

Tigris

The Tigris River is the eastern member of the two great rivers that define Mesopotamia, the other being the Euphrates. The river flows south from the mountains of southeastern Turkey through Iraq.-Geography:...

, the length

Length

In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

 of a meridian arc

Meridian arc

In geodesy, a meridian arc measurement is a highly accurate determination of the distance between two points with the same longitude. Two or more such determinations at different locations then specify the shape of the reference ellipsoid which best approximates the shape of the geoid. This...

 and thus the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's circumference

Circumference

The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....

, getting a result of 40,248 km (or, according to other sources, 41,436 km). The two researchers measured in Arabian ell

Ell

An ell , is a unit of measurement, approximating the length of a man's arm.Several national forms existed, with different lengths, includingthe Scottish ell ,the Flemish ell ,the French ell...

, and determined the geographical latitudes of the end points they used from the star altitudes

Altitude

Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 in a celestial horizontal coordinate system

Horizontal coordinate system

The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane. This coordinate system divides the sky into the upper hemisphere where objects are visible, and the lower hemisphere where objects cannot be seen since the earth is in...

. We believe that 1 Arabian ell was 49 1/3 cm. Thus, they found the length of 1° of meridian to be 111.8 km (115.1 km), which differs from the actual value by 850 metres.