Celestial Cartography

Article excerpt

FROM very early times, the stars in the firmament have held a strong fascination for man. In the movements of the stars the Ancients saw the operation of supernatural forces which could influence human destiny. Believing that there were portents to be read in the stars, our distant ancestors began to observe and make precise descriptions of the heavens.

Thus astronomy has a very long history. Vestiges of astronomical observatories and instruments dating back to 3000 BC have been found in Sumer, Babylon, China, Egypt, Mexico, Peru and the United Kingdom.

The earliest known star catalogues were the work of Babylonian astronomers and date back to about 1700 BC, during the reign of the great Babylonian monarch Hammurabi. Calculations concerning the movements of the Moon and of the planets, especially of Venus, were almost certainly made for astrological purposes. For the Egyptians, Sirius, the Dog star, to which they accorded divine attributes as "Sothis", or the "Star of Isis", was "the bringer of the new year and of the floodwaters of the Nile". In fact, the day of the year on which Sirius was first visible on the horizon, just before sunrise, usually coincided more or less with the rising of the Nile waters, on which subsequent sowing and then harvesting of crops depended.

It was not long before the Ancients became aware of the slow progression of the Sun, the Moon and the five great planets of the solar system (Venus, Mercury, Mar, Jupiter and Saturn) along a regular path across the heavens. This belt around the sky, first observed by the Babylonians and later by the Greeks, is divided into twelve segments, or astrological signs, which correspond to the constellations which occupied these segments some 2,000 years ago (Taurus the bull, Cancer the crab, Leo the lion, etc.). The Zodiacal belt corresponds to what modern astronomers call the "ecliptic", the apparent annual path followed by the Sun.

The Greek astronomer Hipparchus (161 to 127 BC) was the first to draw up a star catalogue worthy of the name. He indicated the position of about a thousand stars, attributing to each of them a magnitude based on its luminosity, or brightness.

Star catalogues

An accurate listing of the stars was essential for navigation on the open sea and for making large-scale land measurements. Until the end of the Middle Ages the Almagest, an astronomical and mathematical encyclopedia compiled by Ptolemy around 160 AD, remained the authoritative reference book for astronomers. The Almagest, which survived in an Arabic translation, is a digest of the mathematical knowledge of Antiquity. It contains a catalogue of forty-eight constellations and 1,022 stars with their ecliptic co-ordinates and magnitudes.

The first really usable astronomical atlas, however, was the German astronomer Johann Bayer's Uranometria, which was drawn up in 1603. It consisted of fifty-one maps and listed 1,277 stars, with the stars in each constellation being designated in decreasing order of brightness by the letters of the Greek alphabet. When further letters were needed, the Latin alphabet was used.

With the aid of the telescope at the Greenwich Royal Observatory, the English astronomer John Flamsteed added considerably to the number of stars listed. His Historia Coelestis Britannica, published in 1725, designated some 3,000 stars by number and exceeded all earlier catalogues both in accuracy and in the number of stars listed.

For a long time observation was limited to the northern skies. The explorers and navigators of the sixteenth century were the first Europeans to see the southern skies and constellations in their totality. The first catalogue of the stars of the southern skies, complete with the positions of 341 of them, determined by telescope, was drawn up in 1676 by the English astronomer Edmond Halley from the south Atlantic island of St. Helena.

During an expedition (1750 to 1754) to the Cape of Good Hope, the French astronomer Nicolas Louis de la Caille listed some 10,000 stars grouped in fourteen constellations, as well as several new nebulae. …