ASTRONOMY OF THE MIDDLE AGES
With the fall of Alexandria and the victory of Mohammed throughout the
West, and a consequent decline in learning, supremacy in science passed
to the East and centered round the caliphs of Bagdad in the seventh and
eighth centuries. They were interested in astronomy only as a practical,
and to them useful, science, in adjusting the complicated lunar calendar
of the Mohammedans, in ascertaining the true direction of Mecca which
every Mohammedan must know, and in the revival of astrology, to which
the Greeks had not attached any particular significance.
Harun al-Rashid ordered the Almagest and many other Greek works
translated, of which the modern world would otherwise no doubt never
have heard, as the Greek originals are not extant.
Splendid observatories were built at Damascus and Bagdad, and fine
instruments patterned after Greek models were continuously used in
observing. The Arab astronomers, although they had no clocks, were
nevertheless so fully impressed with the importance of time that they
added extreme value to their observations of eclipses, for example, by
setting down the altitudes of sun or stars at the same time. On very
important occasions the records were certified on oath by a body of
barristers and astronomers conjointly--a precedent which fortunately has
never been followed.
About the middle of the ninth century, the Caliph Al-Mamun directed his
astronomers to revise the Greek measures of the earth's dimensions, and
they had less reverence for the Almagest than existed in later
centuries: indeed, Tabit ben Korra invented and applied to the tables of
the Almagest a theoretical fluctuation in the position of the ecliptic
which he called "trepidation," which brought sad confusion into
astronomical tables for many succeeding centuries.
Albategnius was another Arab prince whose record in astronomy in the
ninth and tenth centuries was perhaps the best: the Ptolemaic values of
the precession of the equinoxes and of the obliquity of the ecliptic
were improved by new observations, and his excellence as mathematician
enabled him to make permanent improvements in the astronomical
application of trigonometry.
Abul Wefa was the last of the Bagdad astronomers in the latter half of
the tenth century, and his great treatise on astronomy known as the
Almagest is sometimes confused with Ptolemy's work. Following him was
Ibn Yunos of Cairo, whose labors culminated in the famous Hakemite
Tables, which became the standard in mathematical and astronomical
computations for several centuries.
Mohammedan astronomy thrived, too, in Spain and northern Africa.
Arzachel of Toledo published the Toledan Tables, and his pupils made
improvements in instruments and the methods of calculation. The Giralda
was built by the Moors in Seville in 1196, the first astronomical
observatory on the continent of Europe; but within the next half century
both Seville and Cordova became Christian again, and Arab astronomy was
at an end.
Through many centuries, however, the science had been kept alive, even
if no great original advances had been achieved; and Arab activities
have modified our language very materially, adding many such words as
almanac, zenith, and radii, and a wealth of star names, as Aldebaran,
Rigel, Betelgeuse, Vega, and so on.
Meanwhile, other schools of astronomy had developed in the East, one at
Meraga near the modern Persia, where Nassir Eddin, the astronomer of
Hulagu Khan, grandson of the Mongol emperor Genghis Khan, built and used
large and carefully constructed instruments, translated all the Greek
treatises on astronomy, and published a laborious work known as the
Ilkhanic Tables, based on the Hakemite Tables of Ibn Yunos.
More important still was the Tartar school of astronomy under Ulugh Beg,
a grandson of Tamerlane, who built an observatory at Samarcand in 1420,
published new tables of the planets, and made with his excellent
instruments the observations for a new catalogue of stars, the first
since Hipparchus, the star places being recorded with great precision.
The European astronomy of the Middle Ages amounted to very little
besides translation from the Arabic authors into Latin, with
commentaries. Astronomers under the patronage of Alfonso X of Leon and
Castile published in 1252 the Alfonsine Tables, which superseded the
Toledan tables and were accepted everywhere throughout Europe. Alfonso
published also the "Libros del Saber," perhaps the first of all
astronomical cyclopedias, in which is said to occur the earliest diagram
representing a planetary orbit as an ellipse: Mercury's supposed path
round the earth as a center.
Purbach of Vienna about the middle of the 15th century began his
"Epitome of Astronomy" based on the "Almagest" of Ptolemy, which was
finished by his collaborator Regiomontanus, who was an expert in
mathematics and published a treatise on trigonometry with the first
table of sines calculated for every minute from 0 deg. to 90 deg., a most
helpful contribution to theoretical astronomy.
Regiomontanus had a very picturesque career, finally taking up his
residence in Nuremberg, where a wealthy citizen named Walther became his
patron, pupil, and collaborator. The artisans of the city were set at
work on astronomical instruments of the greatest accuracy, and the comet
of 1472 was the first to be observed and studied in true scientific
fashion. Regiomontanus was very progressive and the invention of the new
art of printing gave him an opportunity to publish Purbach's treatise,
which went through several editions and doubtless had much to do in
promoting dissatisfaction with the ancient Ptolemaic system, and was
thus most significant in preparing a background for the coming of the
new Copernican order.
The Nuremberg presses popularized astronomy in other important ways,
issuing almanacs, the first precursors of our astronomical Ephemerides.
Regiomontanus was practical as well, and invented a new method of
getting a ship's position at sea, with tables so accurate that they
superseded all others in the great voyages of discovery, and it is
probable that they were employed by Columbus in his discovery of the
American continent. Regiomontanus had died several years earlier, in
1475 at Rome, where he had gone by invitation of the Pope to effect a
reformation in the calendar. He was only forty, and his patron Walther
kept on with excellent observations, the first probably to be corrected
for the effect of atmospheric refraction, although its influence had
been known since Ptolemy. The Nuremberg School lasted for nearly two
centuries.
Nearly contemporary with Regiomontanus were Fracastoro and Peter Apian,
whose original observations on comets are worthy of mention because they
first noticed that the tails of these bodies always point away from the
sun. Leonardo da Vinci was the first to give the true explanation of
earth-shine on the moon, and similarly the moon-illumination of the
earth; and this no doubt had great weight in disposing of the popular
notion of an essential difference of nature between the earth and
celestial bodies--all of which helped to prepare the way for Copernicus
and the great revolution in astronomical thought.