Purposes of Islamic Astronomy
Perhaps the most vital reason that the Muslims studied the sky
in so much detail was for the purpose of time-keeping. The Islamic religion
requires believers to pray five times a day at specified positions of the sun. Astronomical
time-keeping was the most accurate way to determine when to pray, and was
also used to pin-point religious festivals. The Muslim holy book, the Koran,
makes frequent reference to astronomical patterns visible in the sky, and
is a major source of the traditions associated with Islamic astronomy.
Another important religious use for astronomy was for the determination
of latitude and longitude. Using the stars, particularly the pole star, as
guides, several tables were compiled which calculated the latitude and longitude
of important cities in the Islamic world. Using this information, Muslims could
be assured that they were praying in the direction of Mecca, as specified
in the Koran.
Aside from religious uses, astronomy was used as a tool for
navigation. The astrolabe, an instrument which calculated the positions of
certain stars in order to determine direction, was invented by the Greeks
and adopted and perfected by the Arabs (see picture below).
The sextant was developed by the Arabs to be a more sophisticated
version of the astrolabe. This piece of technology ultimately became the cornerstone
of navigation for European exploration.
Great Islamic Astronomers
Science was considered the ultimate scholarly pursuit in the
Islamic world, and it was strongly supported by the nobility. Most scientists
worked in the courts of regional leaders, and were financially rewarded for their achievements.
In 830, the Khalifah, al-Ma'muun, founded Bayt-al-Hikman, the 'House of Wisdom',
as a central gathering place for scholars to translate texts from Greek and
Persian into Arabic. These texts formed the basis of Islamic scientific knowledge.
One of the greatest Islamic astronomers was al-Khwarizmi (Abu Ja'far
Muhammad ibn Musa Al-Khwarizmi), who lived in the 9th century and was the
inventor of algebra. He developed this mathematical device completely in words,
not mathematical expressions, but based the system on the Indian numbers borrowed
by the Arabs (what we today call Arabic numerals). His work was translated
into Latin hundreds of years later, and served as the European introduction
to the Indian number system, complete with its concept of zero. Al-Khwarizmi
performed detailed calculations of the positions of the Sun, Moon, and planets,
and did a number of eclipse calculations. He constructed a table of the latitudes
and longitudes of 2,402 cities and landmarks, forming the basis of an early
Another Islamic astronomer who later had an impact on Western science
was al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani). In
the late 9th century, he wrote extensively on the motion of celestial bodies.
Like most Islamic astronomers, he accepted the Ptolemaic model of the universe,
and was partially responsible for spreading Ptolemaic astronomy not only in
the Islamic world but also throughout Europe. In the 12th century, his works
were translated into Latin, and it is said that Dante got his astronomical
knowledge from al-Farghani's books.
In the late 10th century, a huge observatory was built near
Tehran, Iran by the astronomer al-Khujandi. He built a large sextant inside
the observatory, and was the first astronomer to be capable of measuring to
an accuracy of arcseconds. He observed a series of meridian transits of the
Sun, which allowed him to calculate the obliquity of the ecliptic, also known
as the tilt of the Earth's axis relative to the Sun. As we know today, the
Earth's tilt is approximately 23o34', and al-Khujandi measured it as being
23o32'19". Using this information, he also compiled a list of latitudes
and longitudes of major cities.
Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi
al-Khayyami) was a great Persian scientist, philosopher, and poet who lived
from 1048-1131. He compiled many astronomical tables and performed a reformation
of the calendar which was more accurate than the Julian and came close to
the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156
days long, which is accurate to the 6th decimal place!
Western science owes a large debt to Islamic and Arab scientists, whose
contributions range from the Arabic names of stars which we still use today
to the mathematical and astronomical treatices used by Europeans to enter
our modern world of science.