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

by E. Walter Maunder

THE NEW OBSERVATORY AS SEEN FROM FLAMSTEED'S OBSERVATORY.

On the other hand, it is hard for the ordinary man to understand how it was that Adams not only left unnoticed and unanswered for three-quarters of a year, an inquiry of Airy's with respect to his calculations, but also never took the trouble to visit Challis, whom he knew well, and who was so near at hand, to stir him up to the search. But, in truth, the whole interest of the matter for Adams rested in the mathematical problem. The irregularities in the motion of Uranus were interesting to him simply for the splendid opportunity which they gave him for their analysis. A purely imaginary case would have served his purpose nearly as well. The actuality of the planet which he predicted was of very little moment; the éclat and popular reputation of the discovery was less than nothing; the problem itself and the mental exercise in its solution, were what he prized.

But it was not creditable to the nation that the Royal Observatory should have been so ill-provided with powerful telescopes; and a few years later Airy obtained the sanction of the Government for the erection of an equatorial larger than the 'Northumberland,' but on the same general plan and in a much more ample dome. This was for thirty-four years the 'Great' or 'South-East' equatorial, and the mounting still remains and bears the old name, though the original telescope has been removed elsewhere. The object-glass had an aperture of twelve and three-quarter inches and a focal length of eighteen feet, and was made by Merz of Munich, the engineering work by Ransomes and Sims of Ipswich, and the graduations and general optical work by Simms, now of Charlton, Kent. The mounting was so massive and stable that the present Astronomer Royal has found it quite practicable and safe to place upon it a telescope (with its counterpoises) of many times the weight, one made by Sir Howard Grubb, of Dublin, of twenty-eight inches aperture and twenty-eight feet focal length, the largest refractor in the British Empire, though surpassed by several American and Continental instruments.

The stability of the mounting was intended to render the telescope suitable for a special work. This was the observation of 'minor planets.' On the first day of the present century the first of these little bodies was discovered by Piazzi at Palermo. Three more were discovered at no great interval afterwards, and then there was an interval of thirty-eight years without any addition to their number. But from December 8, 1845, up to the present time, the work of picking up fresh individuals of these 'pocket planets' has gone on without interruption, until now more than 400 are known. Most of these are of no interest to us, but a few come sufficiently near to the earth for their distance to be very accurately determined; and when the distance of one member of the solar system is determined, those of all the others can be calculated from the relations which the law of gravitation reveals to us. It is a matter of importance, therefore, to continue the work of discovery, since we may at any time come across an interesting or useful member of the family; and that we may be able to distinguish between minor planets already discovered and new ones, their orbits must be determined as they are discovered, and some sort of watch kept on their movements.

A striking example of the scientific prizes which we may light upon in the process of the rather dreary and most laborious work which the minor planets cause, has been recently supplied by the discovery of Eros. On August 13, 1898, Herr Witt, of the Urania Observatory, Berlin, discovered a very small planet that was moving much faster in the sky than is common with these small bodies. The great majority are very much farther from the sun than the planet Mars, many of them twice as far, and hence, since the time of a planet's revolution round the sun increases, in accordance with Kepler's law, more rapidly than does its distance, it follows that they move much more slowly than Mars. But this new object was moving at almost the same speed as Mars; it must, therefore, be most unusually near to us. Further observations soon proved that this was the case, and Eros, as the little stranger has been called, comes nearer to us than any other body of which we are aware except the moon. Venus when in transit is 24¹/₂ millions of miles from us, Mars at its nearest is 34¹/₂ millions, Eros at its nearest approach is but little over 13 millions.

The use of such a body to us is, of course, quite apart from any purpose of navigation, except very indirectly. But it promises to be of the greatest value in the solution of a question in which astronomers must always feel an interest, the determination of the distance of the earth from the sun. We know the relative distances of the different planets, and, consequently if we could determine the absolute distance of any one, we should know the distances of all. As it is practically impossible to measure our distance from the sun directly, several attempts have been made to determine the distances of Venus, Mars, or such of the minor planets as come the nearest to us. Three of these in particular, the little planets Iris, Victoria, and Sappho, have given us the most accurate determinations of the sun's distance (92,874,000 miles) which we have yet obtained; but Eros at its nearest approach will be six times as near to us as either of the three mentioned above, and therefore should give us a value with only one-sixth of the uncertainty attaching to that just mentioned.

The discovery of minor planets has lain outside the scope of Greenwich work, but their observation has formed an integral part of it. The general public is apt to lay stress rather on the first than on the second, and to think it rather a reproach to Greenwich that it has taken no part in such explorations. Experience has, however, shown that they may be safely left to amateur activity, whilst the monotonous drudgery of the observation of minor planets can only be properly carried out in a permanent institution.

The observation of these minute bodies with the transit circle and altazimuth is attended with some difficulties; but precise observations of various objects may be made with an equatorial; indeed, comets are usually observed by its means.

The most ordinary way of observing a comet with an equatorial is as follows: Two bars are placed in the eye-piece of the telescope, at right angles to each other, and at an angle of forty-five degrees to the direction of the apparent daily motion of the stars. The telescope is turned to the neighbourhood of the comet, and moved about until it is detected. The telescope is then put a little in front of the comet, and very firmly fixed. The observer soon sees the comet entering his field, and by pressing the contact button he telegraphs to the chronograph the time when the comet is exactly bisected by each of the bars successively. He then waits until a bright star, or it may be two or three, have entered the telescope and been observed in like manner. The telescope is then unclamped, and moved forward until it is again ahead of the comet, and the observations are repeated; and this is done as often as is thought desirable. The places of the stars have, of course, to be found out from catalogues, or have to be observed with the transit circle, but when they are known the position of the comet or minor planet can easily be inferred.