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.