By
: A E Roy and D Clarce
From
The Book “ Astronomy Principles and Practice”
First
theories were necessarily simple. The Earth was a flat plane with
rivers, hills, seas and land, fixed, eternal. The heavenly bodies
revolved, passing from east to west. But if the land continued
indefinitely, how could the Sun that set in the west be the same Sun
that rose in the east the next morning? Perhaps, the Babylonians
reasoned, the Earth was flat but finite with a circle of ocean beyond
which a ring of mountains supported the heavens, the firmament. Then,
if doors were provided in the base of this great solid half-sphere on
the eastern and western sides, the celestial bodies would be able to
slip through the western doors on setting and be transported in some
miraculousway to the east to reappear as ordained.
The
Babylonians were skilled astronomers though their world-picture was
na¨ıve. They observed the positions of the Sun, Moon, planets and
stars for many centuries with great accuracy. They found that they
could predict eclipses. Their observations were motivated by their
belief that the future of human beings could be predicted from
celestial configurations and events such as eclipses or the
appearance of comets. Because of this, kings kept court astrologers
and the wealthy paid for horoscopes. This belief in astrology, found
in all nations, should have withered away with alchemy and the search
for the philosopher’s stone but even today there are many who set
great faith in this pseudo-science. It is perhaps needless to say
that modern astronomy demonstrates how ludicrous such beliefs are.
The
Egyptians, astronomers almost as skilled as the Babylonians, had
equally simple worldpictures. They noticed that the yearly inundation
of the Nile valley coincided with the days when the star Sirius could
be seen best in the morning twilight. This linking of celestial and
earthly events spurred on their development of astrology and brought
religion into the picture. The Sun-god descended at night, passing
beneath the Earth to visit the dead. Farming people were more
interested in the solar cycle since it was linked with seed time and
harvest. Seafaring peoples like the Phoenicians and the Minoans used
the rising and setting directions of the stars as navigational aids.
It may well have been as an aid to memory that the stars were grouped
in constellations, embodying myths current at that time.
As
is to be expected, the ancient Chinese civilizations produced schools
of astronomy and cosmological theories. Serious Chinese astronomy
probably began prior to 2000 BC although details of events in that
era are largely legendary. The story of the two Chinese astronomers,
Ho and Hi, executed for failing to predict an eclipse of the Sun in
2137 BC is possibly apocryphal and may refer to two astronomical
colleges of a much later date destroyed in civil strife. Reliable
historical details begin about 1000 BC. A farming people required a
calendar and so the lengths of month and year were quickly
ascertained. A year of 365 1/4 days was certainly used by 350 BC.
By
that date, the Chinese constellation figures, 122 in number and quite
different from those handed down to us by the Greeks, had been mapped
out, the Sun’s path—the ecliptic—being divided into 12 regions.
The size of a region was not only connected with the heavenly arc
inhabited by the Sun each month but also with the yearly journey of
the planet Jupiter. The other planetary motions were also studied. As
in the west, a pseudo-science of astrology developed from such
studies. China was the centre or hub of the flat Earth with heavenly
and human events in close harmony: not only did celestial events
guide and control men, in particular the Emperor and his court but
the decisions and actions of such powerful rulers influenced the
state of Heaven.
As
mathematical knowledge grew and more accurate astronomical
instruments for measuring altitudes and angles were developed in
succeeding centuries, the movements of the Sun, Moon and planets were
systematized in remarkably accurate tables for prediction purposes.
Cometary appearances were noted, among them several apparitions of
Halley’s comet, and by the 14th century AD the state of Chinese
astronomy compared favourably with that of the Arabs in the West.
In
various other places where a civilization had developed, astronomical
schools flourished. The ravages of time and barbarism have sadly
destroyed most of the works of such schools, though happily some
traces remain to tell us of the heights of thought their
practitioners achieved. For example, we shall see later how ingenious
were the steps megalithic man took to keep track of the Sun and Moon.
This remarkable civilization flourished in Western Europe in the
third and second millennia BC.
Observations
of eclipses were also recorded by early American Indians as, for
example, by Mayans. A sundial remaining in the ‘lost city’,
Macchu Piccu, provides us with evidence that the Incas of Peru used
solar observations to some purpose. The ‘Puerta del Sol’ at
Tiahuanaco, Bolivia, tells us of solar observations prior to the
Incas.
However,
very few of the ideas and notions of astronomy and cosmology from any
of these civilizations have had an influence on the development of
our understanding of the astronomical Universe. Our starting points
find their origins mainly in ancient Greece.
A
completely new departure in mankind’s contemplation and
interpretation of the heavens came with the flowering of Greek
civilization. Many of their thinkers had extraordinarily original
minds, were mentally courageous and devoted to rational thought. They
were not afraid of questioning cherished beliefs and of following
unsettling, disturbing trains of thought.
Many
of them dismissed the ‘common-sense’ picture of solid, flat Earth
and god-controlled Heaven. They saw that a spherical Earth poised in
space solved a lot of problems. Those stars and planets not seen
during the night were simply on the other side of the Earth. Stars
were not seen during the day because the dazzling bright Sun blotted
out their feeble light. The Moon caused solar eclipses. Pythagoras,
in the 6th century BC, taught that the movements of all the heavenly
bodies were compounded of one or more circular movements.
In
the next century, Philolaus, a follower of Pythagoras, suggested the
bold idea that the Earth was not the centre of the Universe and,
indeed, that it moved. At the centre of the Universe there was a
gigantic fire. Around this fire revolved the Earth, Moon, Sun and
planets in that order, in circles of various sizes. He also
postulated a body called the Anti-Earth to bring the total of moving
bodies up to the sacred number of ten. This Anti-Earth revolved about
the central fire within the Earth’s orbit and was never seen from
the Earth because the Earth faced outwards towards the home of the
gods— Olympus—situated beyond the sphere of the fixed stars.
Philolaus also believed that the Sun was not self-luminous but shone
by the light it absorbed from Olympus and the central fire.
In
contrast to this, Anaxagoras taught that the Sun was a mass of
glowing metal comparable in size with Greece itself. Aristarchus, in
the 3rd century BC, agreed with Philolaus that the Earth moved and
taught that it rotated on its axis, thus explaining the diurnal
motion of the heavens. Moreover, he said, the Sun is a star and the
Earth revolves round it, all other stars being very much farther
away.
Aristarchus,
like Anaxagoras, had ideas about the relative sizes of Sun, Moon and
Earth. The Sun’s diameter had to be about seven times the diameter
of the Earth, a figure far removed from the modern one but embodying
the right idea, namely that the Earth is much smaller than the Sun.
Eratosthenes
of Alexandria, living about 230 BC, used solar observations and a
knowledge of geometry and geography to calculate the circumference of
the Earth, obtaining a value within a few per cent of today’s
accepted figure.
He
knew that at the summer solstice the Sun passed through the zenith at
Syene in Upper Egypt, being reflected at the bottom of a well. At
Alexandria, at the same longitude as Syene, the obelisk at the same
solar solstice, cast a shadow at noon, showing by its length that the
Sun’s altitude was 82 1/2 degrees (figure 2.1). He also knew the
distance between Syene and Alexandria. Eratosthenes then made the
assumptions that the Sun was very far away and that the Earth was
spherical. The Sun’s rays arriving at Syene and Alexandria could
then be taken to be parallel and the angle the Sun’s direction made
with the vertical at Alexandria (7 1/2◦) would, therefore, be the
angle subtended at the Earth’s centre C by the arc from Syene to
Alexandria (figure 2.2). It was then a simple calculation to find the
length of the Earth’s circumference by asking what distance would
subtend an angle of 360◦ if the distance from Alexandria to Syene
subtended an angle of 7 1/2◦ at the Earth’s centre.
Other
outstanding Greek astronomers and mathematicians such as Hipparchus,
Thales, Apollonius, Aristotle and Ptolemy also put forward
world-pictures, or cosmologies, that arouse admiration for the way
their minds managed to successfully break free from their environment
and catch glimpses of the truth. For example, Hipparchus discovered
the precession of the equinoxes, noted by the secular change in
position of the solar crossing point of its ecliptic path over the
celestial equator at the times of the spring and autumnal equinox. He
measured the Sun’s distance and went a considerable way towards
providing theories to account for the motions of Sun and Moon.
Finally,
as Greek civilization decayed, the last and perhaps the most
influential thinker of them all embodied the work of many of the
predecessors in the Almagest. Ptolemy, who lived during the second
century AD, not only collected and discussed the work of Greek
astronomers but carried out original researches himself in astronomy,
geography, mathematics, music, optics and other fields of study. His
great astronomical work, the Almagest, survived the Dark Ages of
Western civilization, influencing astronomical thought right up to
and beyond the invention of the telescope in the early years of the
seventeenth century. The Ptolemaic System describing the apparent
motions of the Sun, Moon and planets is discussed in section 12.2.
During
the Dark Ages astronomy flourished within the Islamic Empire, once
the latter had been stabilized. Ptolemy’s Almagest was translated
into Arabic in 820 AD and thereafter guided the researches of Muslim
scientists. They measured astronomical phenomena more precisely than
ever before, amassing a wealth of information that proved of
inestimable value to Western astronomers when Europe emerged from the
Dark Ages. Many of the terms used in modern astronomy come from the
Arabic, for example ‘zenith’, ‘nadir’, ‘almanac’, while
the names of well-known stars such as Algol, Aldebaran, Altair and
Betelgeuse are also of Arabic origin. In addition, the Muslim
mathematicians introduced spherical trigonometry and Arabic numerals,
including a sign for zero—‘algebra’ is another Arabic word.
They
do not seem, however, to have left us new cosmologies. They were
content to accept the world-pictures of the Greeks into their custody
until theWestern world awoke intellectually once again and began anew
the study of natural science, including astronomy.
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