The Royal Observatory, Greenwich: A Glance at Its History and Work

by E. Walter Maunder

EDMUND HALLEY.
(From an old print.)

A much wider scheme of work than such merely casual observations now entered his mind, possibly suggested to him by Flamsteed's appointment to the direction of the new Royal Observatory. This was to make a catalogue of the southern stars. Tycho's places for the northern stars were defective enough, but there was no catalogue at all of stars below the horizon of Tycho's observatory. Here, then, was a field entirely unworked, and young Halley was so eager to enter upon it that he would not wait at Oxford to obtain his degree, but was anxious to start at once for the southern hemisphere.

His father, who was wealthy and proud of his gifted son, strongly supported him in his project. The station he selected was St. Helena, an unfortunate choice, as the skies there were almost always more or less clouded, and rain was frequent during his stay. However, he remained there a year and a half, and succeeded in making a catalogue of 341 stars. This catalogue was finally reduced by Sharp, and included in the third volume of Flamsteed's Historia Cœlestis.

In 1678 he was elected Fellow of the Royal Society, and the following year he was chosen to represent that society in a discussion with Hevelius. The question at issue was as to whether more accurate observations of the place of a star could be obtained by the use of sights without optical assistance, or by the use of a telescope. The next year he visited the Paris Observatory, and, later in the same tour, the principal cities of the Continent.

Not long after his return from this tour, Halley was led to that undertaking for which we owe him the greatest debt of gratitude, and which must be regarded as his greatest achievement.

Some fifty years before, the great Kepler had brought out the third of his well-known laws of planetary motion. These laws stated that the planets move round the sun in ellipses, of which the sun occupies one of the foci; that the straight line joining any planet with the sun moves over equal areas of space in equal periods of time; and, lastly, that the squares of the times in which the several planets complete a revolution round the sun are proportional to the cubes of their mean distances from it. These three laws were deduced from actual examination of the movements of the planets. Kepler did not work out any underlying cause of which these three laws were the consequence.

But the desire to find such an underlying cause was keen amongst astronomers, and had given rise to many researches. Amongst those at work on the subject was Halley himself. He had seen, and been able to prove, that if the planets moved in circles round the sun, with the sun in the centre, then the law of the relation between the times of revolution and the distances of the planets would show that the attractive force of the sun varied inversely as the square of the distance. The actual case, however, of motion in an ellipse was too hard for him, and he could not deal with it. Halley therefore went up to Cambridge to consult Newton, and, to his wonder and delight, found that the latter had already completely solved the problem, and had proved that Kepler's three laws of planetary motion were summed up in one, namely, that the sun attracted the planets to it with a force inversely proportional to the square of the distance.