history_astronomyAstral themes

The sky is the most mysterious part of our everyday experience. Familiarity may make the amazing events going on at ground level seem almost ordinary. Plants and animals grow and die, rain falls, rivers flow. We feel we understand that.

But the sky is beyond comprehension. Two great objects travel through it, one hot and constant, the other cold and changeable. In the daytime it is moody; there may be blazing sun, or racing clouds, or darkness followed by thunder and lightning. And yet on a clear night the sky is the very opposite – predictable, if you look hard enough, with recognizable groups of stars moving in a slow but reliable manner.

Man’s interest in the sky is at the heart of three separate stories – , astrology and the calendar.

Astronomy is the scientific study of sun, moon and stars. Astrology is a pseudo-science interpreting the supposed effect of the heavenly bodies on human existence. In early the two are closely linked. The sky is the home of many of the gods, who influence life on earth. And the patterns in the sky must surely reflect that influence.

and the Babylonians: from 3000 BC

Astronomical observation begins with the early civilizations of Mesopotamia, where prominent constellations (the patterns formed by stars in the galaxy) are recognized and named soon after 3000 BC. Similarly the sky-watchers of Mesopotamia identify the five wandering stars, which with the sun and moon form the seven original ‘planets’ (Greek for ‘wanderers’).

Within Mesopotamia the Babylonians, flourishing from the 18th century BC, are the first great astronomers. The minutes and seconds of modern astronomical measurement derive from their number system. And it is the Babylonians who introduce the useful concept of the zodiac.

The Babylonians realize that the zodiac – the sequence of constellations along which the sun and the planets appear to move in their passage through the heavens – can serve as a yardstick of celestial time if divided into recognizable and equal segments. They select twelve constellations to represent these segments, many of them identified by the names of animals. The Greeks later provide the term for the zodiac when they describe it as the ‘animal circle’ (zodiakos kyklos).

The zodiac links constellations with times of the year; and the constellations have their own links with the gods. So scientific observation of positions merges with speculation about divine influence. The zodiac, as a concept, is of use to both astronomers and astrologers.
The Greeks: from the 6th century BC

The Greeks make significant advances in the fields of both astronomy and astrology. In astronomy their analytical approach to the heavens leads to early insights of great brilliance, even though they eventually blind European astronomers for more than a millennium with the elaborately observed but entirely false Ptolemaic system.

Meanwhile astrology benefits from the range and vitality of the Greek gods. Linked with the planets and constellations, these very human divinities make astrology dramatic and exciting. And Greek interest in the individual extends the astrologers’ range. Evolved originally to help in affairs of state, the art finds its lasting role in casting the fortunes of ordinary men and women.

The Pythagoreans and astronomy: 5th century BC

Followers of Pythagoras, in the 5th century, are the first to produce an astronomical theory in which a circular earth revolves on its own axis as well as moving in an orbit. The theory derives in part from the need to locate the great fire which they believe fuels the universe.

The Pythagoreans place this fire at the hidden centre of things, with the earth revolving round it more closely than any of the other bodies visible in the sky. The reason why we never see or are scorched by the fire is that we live on only half the sphere of the earth, and our half is always turned away from the fire.

This makes it necessary for the earth to revolve as it orbits (one complete revolution per orbit, just as the moon orbits round the earth showing always the same face). For the Pythagoreans this spinning of the earth explains (correctly) the pattern of night and day.

Moving outwards from the earth in the sequence of heavenly bodies, they place the moon next, then the sun, the planets and finally the stars, which are unlike the others in being fixed on an outer sphere.

Heavenly spheres: from the 5th century BC

This theory introduces the concentric circles which become the false orthodoxy of the next 2000 years, as eventually enshrined by Ptolemy. It also starts a wild goose chase which will exercise many brilliant minds: what mechanical model can explain the erratic motion of the planets? Eudoxus of Cnidus, in the 4th century, is the first to propose a series of transparent spheres in the heavens, carrying the heavenly bodies at different speeds in linked groups with slightly varying centres.

To make such machinery conform to what can be observed in the sky, ever more complex arrangements are needed. Later in the 4th century Aristotle believes he has solved it. He requires no fewer than fifty-five transparent spheres.

The Pythagoreans are too far ahead of their time in proposing their one central grain of truth – the revolving globe of the earth. But Copernicus, developing this idea, will acknowledge them as his earliest predecessors.

For most Greek astronomers there seems to be overwhelming evidence that the earth is stationary and the heavens move. This is true even of the greatest among them, Hipparchus. Like his predecessors, he believes that it must be possible to analyze the movement of the spheres. He finds the available data inadequate, so devotes himself not to cosmology but to the prime task of an astronomer – observation of individual stars.

The earth and the sun: a heresy of the 3rd century BC

A lone voice on the Greek island of Samos. In about 270 BC Aristarchus is busy trying to work out the size of the sun and the moon and their distance from the earth. His only surviving work is on this topic, and his calculations are inevitably wide of the mark.

But references in other authors make it clear that his studies have brought him to a startling conclusion.

Aristarchus believes that the earth is in orbit round the sun (quite contrary to what is plain for anyone to see). There is an attempt, which comes to nothing, to have the man prosecuted for impiety. His idea joins the many other dotty notions which enliven the history of human thought, until Copernicus mentions him, in an early draft of his great book, as someone who had the right idea first.

On reflection Copernicus drops the name of Aristarchus from later versions of the text.

Hipparchus, a scientific astronomer: 2nd century BC

An observatory is erected by Hipparchus on the island of Rhodes. Here, in 129 BC, he completes the first scientific star catalogue. He lists about 850 stars, placing each in terms of its celestial latitude and longitude and recording its relative brightness on a scale of six.

He measures the altitude of a star by means of an astrolabe, a revolving calibrated disc which will be used for this purpose for nearly two millennia. It is invented either by Hipparchus himself or by his 3rd-century predecessor, Apollonius of Perga. Hipparchus also imagines another use for his astronomical instruments, to create maps of the earth’s surface. But this is a task even more demanding than his charting of the heavens.

Hipparchus is so accurate in his placing of the stars that he becomes the first scientist to observe an important phenomenon. Although almost fixed in relation to the sun, the stars move gradually over a long period. This means that at any repeated and identifiable moment in the sun’s year, such as the equinox (when day and night are of equal length), the star positions will be seen to have shifted very slightly.

Hipparchus observes this effect in relation to the equinox, and calculates that there is a shift each year of about 45 seconds of arc. It is a phenomenon known now as precession, or the precession of the equinoxes.

Hipparchus has no way of explaining this phenomenon (which is due to a slow wobble of the earth’s axis, completing one cycle every 26,000 years), but his accuracy is astonishing. Modern measurements give a figure close to 50 seconds of arc. His 45 seconds are only about 10% out.

The works of Hipparchus are lost. They are known only through the use made of them by Ptolemy, a much less scientific astronomer whose influence derives from the encylopedic nature of his work. Ptolemy acknowledges the greatness of Hipparchus, and fails lamentably when he tries to improve on his predecessor. Attempting to make the figure for precession more accurate, he moves in the wrong direction – and comes up with 36 seconds of arc.

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