the greatest of the Arabian astronomers was Mohammed ben Jabir Albategnius,
or El-batani, who was born at Batan, in Mesopotamia, about the year 850
A.D., and died in 929. Albategnius was a student of the Ptolemaic astronomy,
but he was also a practical observer. He made the important discovery of
the motion of the solar apogee. That is to say, he found that the position
of the sun among the stars, at the time of its greatest distance from the
earth, was not what it had been in the time of Ptolemy. The Greek astronomer
placed the sun in longitude 65 degrees, but Albategnius found it in longitude
82 degrees, a distance too great to be accounted for by inaccuracy of measurement.
The modern inference from this observation is that the solar system is
moving through space; but of course this inference could not well be drawn
while the earth was regarded as the fixed centre of the universe.
In the eleventh century
another Arabian discoverer, Arzachel, observing the sun to be less advanced
than Albategnius had found it, inferred incorrectly that the sun had receded
in the mean time. The modern explanation of this observation is that the
measurement of Albategnius was somewhat in error, since we know that the
sun's motion is steadily progressive. Arzachel, however, accepting the
measurement of his predecessor, drew the false inference of an oscillatory
motion of the stars, the idea of the motion of the solar system not being
permissible. This assumed phenomenon, which really has no existence in
point of fact, was named the "trepidation of the fixed stars," and was
for centuries accepted as an actual phenomenon. Arzachel explained this
supposed phenomenon by assuming that the equinoctial points, or the points
of intersection of the equator and the ecliptic, revolve in circles of
eight degrees' radius. The first points of Aries and Libra were supposed
to describe the circumference of these circles in about eight hundred years.
All of which illustrates how a difficult and false explanation may take
the place of a simple and correct one. The observations of later generations
have shown conclusively that the sun's shift of position is regularly progressive,
hence that there is no "trepidation" of the stars and no revolution of
the equinoctial points.
If the Arabs were
wrong as regards this supposed motion of the fixed stars, they made at
least one correct observation as to the inequality of motion of the moon.
Two inequalities of the motion of this body were already known. A third,
called the moon's variation, was discovered by an Arabian astronomer who
lived at Cairo and observed at Bagdad in 975, and who bore the formidable
name of Mohammed Aboul Wefaal-Bouzdjani. The inequality of motion in question,
in virtue of which the moon moves quickest when she is at new or full,
and slowest at the first and third quarter, was rediscovered by Tycho Brahe
six centuries later; a fact which in itself evidences the neglect of the
Arabian astronomer's discovery by his immediate successors.
In the ninth and
tenth centuries the Arabian city of Cordova, in Spain, was another important
centre of scientific influence. There was a library of several hundred
thousand volumes here, and a college where mathematics and astronomy were
taught. Granada, Toledo, and Salamanca were also important centres, to
which students flocked from western Europe. It was the proximity of these
Arabian centres that stimulated the scientific interests of Alfonso X.
of Castile, at whose instance the celebrated Alfonsine tables were constructed.
A familiar story records that Alfonso, pondering the complications of the
Ptolemaic cycles and epicycles, was led to remark that, had he been consulted
at the time of creation, he could have suggested a much better and simpler
plan for the universe. Some centuries were to elapse before Copernicus
was to show that it was not the plan of the universe, but man's interpretation
of it, that was at fault.
Another royal personage
who came under Arabian influence was Frederick II. of Sicily - the "Wonder
of the World," as he was called by his contemporaries. The Almagest of
Ptolemy was translated into Latin at his instance, being introduced to
the Western world through this curious channel. At this time it became
quite usual for the Italian and Spanish scholars to understand Arabic although
they were totally ignorant of Greek.
In the field of
physical science one of the most important of the Arabian scientists was
Alhazen. His work, published about the year 1100 A.D., had great celebrity
throughout the mediaeval period. The original investigations of Alhazen
had to do largely with optics. He made particular studies of the eye itself,
and the names given by him to various parts of the eye, as the vitreous
humor, the cornea, and the retina, are still retained by anatomists. It
is known that Ptolemy had studied the refraction of light, and that he,
in common with his immediate predecessors, was aware that atmospheric refraction
affects the apparent position of stars near the horizon. Alhazen carried
forward these studies, and was led through them to make the first recorded
scientific estimate of the phenomena of twilight and of the height of the
atmosphere. The persistence of a glow in the atmosphere after the sun has
disappeared beneath the horizon is so familiar a phenomenon that the ancient
philosophers seem not to have thought of it as requiring an explanation.
Yet a moment's consideration makes it clear that, if light travels in straight
lines and the rays of the sun were in no wise deflected, the complete darkness
of night should instantly succeed to day when the sun passes below the
horizon. That this sudden change does not occur, Alhazen explained as due
to the reflection of light by the earth's atmosphere.
Alhazen appears to
have conceived the atmosphere as a sharply defined layer, and, assuming
that twilight continues only so long as rays of the sun reflected from
the outer surface of this layer can reach the spectator at any given point,
he hit upon a means of measurement that seemed to solve the hitherto inscrutable
problem as to the atmospheric depth. Like the measurements of Aristarchus
and Eratosthenes, this calculation of Alhazen is simple enough in theory.
Its defect consists largely in the difficulty of fixing its terms with
precision, combined with the further fact that the rays of the sun, in
taking the slanting course through the earth's atmosphere, are really deflected
from a straight line in virtue of the constantly increasing density of
the air near the earth's surface. Alhazen must have been aware of this
latter fact, since it was known to the later Alexandrian astronomers, but
he takes no account of it in the present measurement. The diagram will
make the method of Alhazen clear.
His important premises
are two: first, the well-recognized fact that, when light is reflected
from any surface, the angle of incidence is equal to the angle of reflection;
and, second, the much more doubtful observation that twilight continues
until such time as the sun, according to a simple calculation, is nineteen
degrees below the horizon. Referring to the diagram, let the inner circle
represent the earth's surface, the outer circle the limits of the atmosphere,
C being the earth's centre, and RR radii of the earth. Then the observer
at the point A will continue to receive the reflected rays of the sun until
that body reaches the point S, which is, according to the hypothesis, nineteen
degrees below the horizon line of the observer at A. This horizon line,
being represented by AH, and the sun's ray by SM, the angle HMS is an angle
of nineteen degrees. The complementary angle SMA is, obviously, an angle
of (180-19) one hundred and sixty-one degrees. But since M is the reflecting
surface and the angle of incidence equals the angle of reflection, the
angle AMC is an angle of one-half of one hundred and sixty-one degrees,
or eighty degrees and thirty minutes. Now this angle AMC, being known,
the right-angled triangle MAC is easily resolved, since the side AC of
that triangle, being the radius of the earth, is a known dimension. Resolution
of this triangle gives us the length of the hypotenuse MC, and the difference
between this and the radius (AC), or CD, is obviously the height of the
atmosphere (h), which was the measurement desired. According to the calculation
of Alhazen, this h, or the height of the atmosphere, represents from twenty
to thirty miles. The modern computation extends this to about fifty miles.
But, considering the various ambiguities that necessarily attended the
experiment, the result was a remarkably close approximation to the truth.
Turning from physics
to chemistry, we find as perhaps the greatest Arabian name that of Geber,
who taught in the College of Seville in the first half of the eighth century.
The most important researches of this really remarkable experimenter had
to do with the acids. The ancient world had had no knowledge of any acid
more powerful than acetic. Geber, however, vastly increased the possibilities
of chemical experiment by the discovery of sulphuric, nitric, and nitromuriatic
acids. He made use also of the processes of sublimation and filtration,
and his works describe the water bath and the chemical oven. Among the
important chemicals which he first differentiated is oxide of mercury,
and his studies of sulphur in its various compounds have peculiar interest.
In particular is this true of his observation that, tinder certain conditions
of oxidation, the weight of a metal was lessened.