The Science of the Stars

by E. Walter Maunder

One great practical problem was that of finding out the position of a ship when out of sight of land. The ancient Phoenician and Greek navigators had mostly confined themselves to coasting voyages along the shores of the Mediterranean Sea, and therefore the quick recognition of landmarks was the first requisite for a good sailor. But when, in 1492, Columbus had brought a new continent to light, and long voyages were freely taken across the great oceans, it became an urgent necessity for the navigator to find out his position when he had been out of sight of any landmark for weeks.

This necessity was especially felt by the nations of Western Europe, the countries facing the Atlantic with the New World on its far-distant other shore. Spain, France, England, and Holland, all were eager competitors for a grasp on the new lands, and therefore were earnest in seeking a solution of the problem of navigation.

The latitude of the ship could be found out by observing the height of the Sun at noon, or of the Pole Star at night, or in several other ways. But the longitude was more difficult. As the Earth turns on its axis, different portions of its surface are brought in succession under the Sun, and if we take the moment when the Sun is on the meridian of any place as its noon, as twelve o'clock for that place, then the difference of longitude between any two places is essentially the difference in their local times.

It was possible for the sailor to find out when it was local noon for him, but how could he possibly find out what time it was at that moment at the port from which he had sailed, perhaps several weeks before?

The Moon and stars supplied eventually the means for giving this information. For the Moon moves amongst the stars, as the hand of a clock moves amongst the figures of a dial, and it became possible at length to predict for long in advance exactly where amongst the stars the Moon would be, for any given time, of any selected place.

When this method was first suggested, however, neither the motion of the Moon nor the places of the principal stars were known with sufficient accuracy, and it was to remedy this defect, and put navigation upon a sound basis, that CHARLES II. founded Greenwich Observatory in the year 1675, and appointed FLAMSTEED the first Astronomer Royal. In the year 1767 MASKELYNE, the fifth Astronomer Royal, brought out the first volume of the Nautical Almanac, in which the positions of the Moon relative to certain stars were given for regular intervals of Greenwich time. Much about the same period the problem was solved in another way by the invention of the chronometer, by JOHN HARRISON, a Yorkshire carpenter. The chronometer was a large watch, so constructed that its rate was not greatly altered by heat or cold, so that the navigator had Greenwich time with him wherever he went.

The new method in the hands of CAPTAIN COOK and other great navigators led to a rapid development of navigation and the discovery of Australia and New Zealand, and a number of islands in the Pacific. The building up of the vast oceanic commerce of Great Britain and of her great colonial empire, both in North America and in the Southern Oceans, has arisen out of the work of the Royal Observatory, Greenwich, and has had a real and intimate connection with it.

To observe the motions of the Moon, Sun, and planets, and to determine with the greatest possible precision the places of the stars have been the programme of Greenwich Observatory from its foundation to the present time. Other great national observatories have been Copenhagen, founded in 1637; Paris, in 1667; Berlin, in 1700; St. Petersburg, in 1725, superseded by that of Pulkowa, in 1839; and Washington, in 1842; while not a few of the great universities have also efficient observatories connected with them.

Of the directly practical results of astronomy, the promotion of navigation stands in the first rank. But the science has never been limited to merely utilitarian inquiries, and the problem of measuring celestial distances has followed on inevitably from the measurement of the Earth.

The first distance to be attacked was that of the nearest companion to the Earth, i.e. the Moon. It often happens on our own planet that it is required to find the distance of an object beyond our reach. Thus a general on the march may come to a river and need to know exactly how broad it is, that he may prepare the means for bridging it. Such problems are usually solved on the following principle. Let A be the distant object. Then if the direction of A be observed from each of two stations, B and C, and the distance of B from C be measured, it is possible to calculate the distances of A from B and from C. The application of this principle to the measurement of the Moon's distance was made by the establishment of an observatory at the Cape of Good Hope, to co-operate with that of Greenwich. It is, of course, not possible to see Greenwich Observatory from the Cape, or vice versa, but the stars, being at an almost infinite distance, lie in the same direction from both observatories. What is required then is to measure the apparent distance of the Moon from the same stars as seen from Greenwich and as seen from the Cape, and, the distance apart of the two observatories being known, the distance of the Moon can be calculated.