The Astronomy Book

31

See also: Shifting stars 22 ■ Consolidating knowledge 24–25 ■ Mapping the galaxies 27 ■
The Copernican model 32–39 ■ The Tychonic model 44–47

FROM MYTH TO SCIENCE

completed in 1429. It was there,
with his team of astronomers and
mathematicians, that he set about
compiling a new star catalog.

Giant instruments
Ptolemy’s catalog had largely
been derived from the work of
Hipparchus, and many of its star
positions were not based on
fresh observations.
To measure accurately, Ulugh
Beg built the observatory on an
immense scale. Its most impressive
instrument was the so-called
Fakhri sextant. In fact, more like
a quadrant (a quarter-circle rather

than a sixth), it is estimated to

have had a radius of more than

130 ft (40 m) and would have been

three stories high. The instrument

was kept underground to protect

it from earthquakes and rested in

a curved trench along the north–

south meridian. As the sun and

the moon passed overhead, their

light focused into the dark trench,

and their positions could be

All that remains of the Fakhri

sextant is a 6½-ft (2-m) wide trench

gouged in a hillside. The observatory

was destroyed after Ulugh Beg’s death

in 1449 and not discovered until 1908.

The religions disperse,

kingdoms fall apart,

but works of science

remain for all ages.

Ulugh Beg

A precisely built sextant

in a protected location

gives more accurate

measurements.

The understanding of astronomy is based

on the study of the work of past scholars.

With better instruments,

the work of past

astronomers is

often found to

contain errors.

measured to within a few

hundredths of a degree, as

could the positions of the stars.

In 1437, Zij-i Sultani (“The

Sultan’s Catalog of Stars”)

was published. Of the 1,022

stars included in the Almagest,

Ulugh Beg corrected the positions

of 922. Zij-i Sultani also contained

new measurements for the solar

year, planetary motion, and the

axial tilt of Earth. These data

became very important, enabling

the prediction of eclipses, the time

of sunrise and sunset, and the

altitude of celestial bodies, which

were needed to navigate. Ulugh

Beg’s work remained the definitive

star catalog until Tycho Brahe’s,

nearly 200 years later. ■