THE DOUBLE-STAR DEPARTMENT
The foregoing chapters will have shown that though the original purpose of the Observatory has always been kept in view, yet the progress of science has caused many researches to be undertaken which overstep its boundaries. Thus in the present transit room, beside the successive transit instruments we find upon the wall two long thin tubes, labelled respectively Alpha Aquilae and Alpha Cygni. These were two telescopes set up by Pond for a special purpose. Dr. Brinkley, Royal Astronomer for Ireland, had announced that he had found that several stars shifted their apparent place in the sky in the course of a year, due to the change in the position of the earth from which we view them, by an amount which would show that they were only about six to nine billions of miles distant from us; or, in other words, they showed a parallax of from two to three seconds of arc. Pond was not able to confirm these parallaxes from his observations, and to decide the point he set up these two telescopes, the Alpha Aquilae telescope being rigidly fixed on the west side of the pier of Troughton's mural circles; the Alpha Cygni telescope on another pier, the one which now forms the base of the pier of the astrographic telescope. Pond's method was to compare the position of these two stars with that of a star almost exactly the same distance from the pole, but at a great distance from it in time of crossing the meridian; in other words, of almost the same declination, but widely different right ascension. The result proved that Brinkley was wrong, and vindicated the delicacy and accuracy of Pond's observations.

These two telescopes, therefore, had their day and ceased to be. Others have followed them. An ingenious telescope was set up by Sir George Airy in order to ascertain if the speed of light were different when passing through water than when passing through air. Or, in other words, if the aberration of light would give the same value as at present if we observed through water. The water telescope, as it was called, is kept on the ground floor of the central octagon of the new observatory. The observations obtained with it were hardly quite satisfactory, but gave on the whole a negative result.

Turning back to the transit room, and leaving it by the south-west door, we come into the little passage which leads at the back of Bradley's transit room into the lower computing room. Just inside this passage, on the left-hand side, there is a little room of a most curious shape, the 'reflex zenith room.' Here is fixed a telescope pointing straight upwards, the eye-piece being fixed by the side of the object-glass. The light from a star--the star Gamma Draconis--which passes exactly over the zenith of Greenwich, enters the object-glass, passes downwards to a basin of mercury, and is reflected upwards from the surface of the mercury to a little prism placed over the centre of the object-glass, from which it is reflected again into the eye-piece. By means of this telescope the distance of the star Gamma Draconis from the zenith could be measured very exactly, and, consequently, the changes in the apparent position of the star due to aberration, parallax, and other causes could be very exactly followed, and the corrections to be applied on account of these causes precisely determined.

This particular telescope was devised by Airy, and the observations with it were continued to the end of his reign. The germ of the idea may be traced back, however, to the time of Flamsteed, who would seem to have occasionally observed Gamma Draconis from the bottom of a deep well; the precise position of the well is not, however, now known. Later, Bradley set up his celebrated 12 1/2-foot zenith sector, still preserved in the transit room, first at Wanstead and then at Greenwich, for the determination of the amount of aberration. Later, a zenith tube by Troughton, of 25 feet focus, was used by Pond in conjunction with the mural circle for observations of Gamma Draconis in order to determine the zenith point of the latter instrument.

These telescopes for special purposes have passed out of use. Observations with the spectroscope have been suspended for some years. The work of the Astrographic Department will come to an end, in the ordinary course of events, when the programme assigned to Greenwich in the International Scheme is completed.

Within the last few years a new department has come into being at Greenwich--a department which has been steadily worked at many foreign public observatories, but only recently here.

This is the Department of Double-Star Observation. The first double star, Zeta Ursae Majoris, was discovered 250 years ago. Bradley discovered two exceedingly famous double stars whilst still a young man observing with his uncle at Wanstead--Gamma Virginis and Castor. Bradley made also other discoveries of double stars after his appointment to Greenwich, and Maskelyne succeeded him in the same line, but the great foundation of double-star astronomy was laid by Sir William Herschel.

At first it was supposed that double stars were double only in appearance; one star comparatively near us 'happened' to lie in almost exactly the same direction as another star much further off. It was, indeed, in the very expectation that this would prove to be the case, that the elder Herschel first took up their study. But he was soon convinced that many of the objects were true double stars- members of the same system of which the smaller revolved round the larger--not merely apparently double, one star appearing by chance to be close to another with which it had no connection--but real double stars. The discovery of these has led to the establishment of a new department of astronomy, again scientific rather than utilitarian.

As mentioned above, it is only recently that

Greenwich has taken any appreciable part in this work. Under Airy, the largest equatorial of the time had been furnished with a good micrometer, and observations of one or two double stars been made now and again; but Airy's programme of work was far too rigid, and kept the staff too closely engaged for such observations to be anything but extremely rare. And, indeed, when the micrometers of the equatorials were brought into use, they were far more generally devoted to the satellites of Saturn than to the companions of stars. In the main, double-star astronomy has been in the hands of amateurs, at least in England. But the discovery in recent years of many pairs so close that a telescope of the largest size is required for their successful observation, has put an important section of double stars beyond the reach of most private observers, and therefore the great telescope at Greenwich is now mainly devoted to their study. The Astronomer Royal, therefore, soon after the completion of the great equatorial of 28-inches aperture placed in the south-east dome, added this work to the Observatory programme.

The 28-inch equatorial is a remarkable-looking instrument, its mounting being of an entirely different kind to that of the other equatorials in the Observatory, with the solitary exception of the Shuckburgh, which is set up in a little dome over the chronograph room. The Shuckburgh was presented to the Observatory in the year 1811, by Sir G. Shuckburgh. It was first intended to be mounted as an altazimuth, but proved to be unsteady in that position, and was then converted into an equatorial without clockwork, and mounted in its present position.

The position is about as hopelessly bad a one as a telescope could well have, completely overshadowed as it is by the trees and buildings close at hand. The dome is a small one, and the arrangements for the shutters and for turning the dome are as bad as they could possibly be. It has practically been useless for the last forty years.

Its only interest is that the method of mounting employed is a small scale model of that of the great telescope in the S.-E. dome. In the German or Fraunhofer form of mounting for an equatorial there is but a single pillar, which carries a comparatively short polar axis. At the upper end of the polar axis we find the declination axis, and at one end of the declination axis is the telescope, whilst at the other end is a heavy weight to counterpoise it. The German mounting has the advantage that the telescope can easily point to the pole of the heavens; its drawbacks are that, except in certain special forms, the telescope cannot travel very far when it is on the same side of the meridian as the star to which it is pointed, the end of the telescope coming into contact under such circumstances with the central pier, whilst the introduction of mere deadweight as the necessary counterpoise, is not economical. It has been already pointed out that the present Astronomer Royal has not only considerably modified the German mounting in the great collection of telescopes in the Thompson dome, but has used a powerful reflector as a counterpoise to the sheaf of refractors at the other end of the declination axis.

The English equatorial requires two piers. Between these two piers is a long polar axis. Both in the little Shuckburgh and in the great 28-inch equatorial the frame of the polar axis consists of six parallel rods disposed in two equilateral triangles, with their bases parallel to each other, the telescope swinging in the space between the two bases. The construction of this form of equatorial, therefore, is expensive, as it requires two piers. It takes much more room than the German form, and the telescope cannot be directed precisely to the pole. But the instrument is symmetrical, there is no deadweight, and the telescope can follow a star from rising to setting without having to be reversed on crossing the meridian. The great stability of the English form of mounting, therefore, commended it very highly to Airy, and he designed the great Northumberland equatorial of the Cambridge Observatory on that plan, as well as one for the Liverpool Observatory at Bidston, and in 1858 the S.-E. equatorial at Greenwich. The telescope at first mounted upon it had an object-glass of 12 3/4 inches' aperture, and 18 feet focal length. That was dismounted in 1891, and is now used as the guiding telescope of the Thornpson 26-inch photographic refractor. Its place was taken by an immensely heavier instrument, the present refractor of 28 inches' aperture, and 28 feet focal length; and that this change was effected safely was an eloquent testimony to the solidity of the original mounting.

The clock that drives this great instrument, so that it can follow a star or other celestial object in its apparent daily motion across the sky, is in the basement of the S.-E. tower. It is a very simple looking instrument, a conical pendulum in a glass case. The pendulum makes a complete revolution once in two seconds. Below it in a closed case is a water turbine. A cistern on the roof of the staircase supplies this turbine with water, having a fall of about thirty feet. The water rushing out of the arms of the turbine forces it backward, and the turbine spins rapidly round, driving a spindle which runs up into the dome, and gears through one or two intermediate wheels with the great circle of the telescope; the extremely rapid rotation of the spindle, four times in a second, being converted by these intermediate wheels into the exceedingly slow one of once in twenty-four hours. Just above the centre of motion of the turbine is a set of three small wheels, all of exactly the same size, and of the same number of teeth. Of these the bottom wheel is horizontal, and is turned by the turbine. The top wheel is also horizontal, and is turned by the pendulum. The third wheel gears into both these, and is vertical. If the top and bottom wheels are moving exactly at the same rate, the intermediate wheel simply turns on its axis, but does not travel; but if the turbine and pendulum are moving at different rates, then the vertical wheel is forced to run in one direction or the other, and, doing so, it opens or closes a throttle valve, which controls the supply of water to the turbine, and so speedily brings the turbine into accord with the pendulum. The control of the motion of the great telescope is therefore almost as perfect as that of the astrographic and Thompson equatorials, though the principle employed is very different. And the control needs to be perfect, for, as said above, the great telescope is mostly devoted to the observation of double stars, and there can be no greater hindrance to this work than a telescope which does not move accurately with the star.

There is a striking contrast between the great telescope and all the massive machinery for its direction and movement, and the objects on which it is directed--two little points of light separated by a delicate hair of darkness.

The observation is very unlike those of which we have hitherto spoken. The object is not to ascertain the actual position in the sky of the two stars, but their relative position to each other. A spider's thread of the finest strands is moved from one star to the other by turning an exquisitely fine screw; this enables us to measure their distance apart. Another spider thread at right angles to the first is laid through the centres of both stars, and a divided circle enables us to read the angle which this line makes to the true east and west direction. Such observations repeated year after year on many stars have enabled the orbits of not a few to be laid down with remarkable precision; and we find that their movements are completely consistent with the law of gravitation. Further, just as Neptune was pre-recognized and discovered from noting the irregularities in the motion of Uranus, so the discordances in the place of Sirius led to the belief that it was attracted by a then unseen companion, whose position with respect to the brighter star was predicted and afterwards seen. Gravitation thus appears, indeed, to be the Bond

of the Universe, yet it leaves us with several weighty problems. The observation of the positions of stars shows that though we call them fixed they really have motions of their own. Of these motions, a great part consists of a drift away from one portion of the heavens towards a point diametrically opposite to it, a drift such as must be due, not to a true motion of the individual stars, but to a motion through space of our sun and its attendant system. The elder Herschel was the first to discover this mysterious solar motion. Sir George Airy and Mr. Edwin Dunkin, for forty-six years a member of the Greenwich staff, and from 1881-1884 the Chief Assistant, contributed important determinations of its direction.

What is the cause of this motion, what is the law of this motion, is at present beyond our power to find out. Many years ago a German astronomer made the random suggestion that possibly we were revolving in an orbit round the Pleiades as a centre. The suggestion was entirely baseless, but unfortunately has found its way into many popular works, and still sometimes is brought forward as if it were one of the established truths of astronomy. We can at present only say that this solar motion is a mystery.

There is a greater mystery still. The stars have their own individual motions, and in the case of a few these are of the most amazing swiftness. The earth in its motion round the sun travels nearly nineteen miles in a second, say one thousand times faster than the quickest rush of an express train. The sun's rate of motion is probably not quite so swift, but Arcturus, a sun far larger than our own, has a pace some twenty times as swift as the orbital motion of the earth. This is not a motion that we can conceive of as being brought about by gravitation, for if there were some unseen body so vast as to draw Arcturus with this swiftness, other stars too would be hurtling across the sky as quickly. Such 'runaway stars' afford a problem to which we have as yet no key, and, like Job of old, we are speechless when the question comes to us from heaven, 'Canst thou guide Arcturus and his sons?'

It will be seen then that, fundamentally, Greenwich Observatory was founded and has been maintained for distinctly practical purposes, chiefly for the improvement of the eminently practical science of navigation. Other inquiries relating to navigation, as, for instance, terrestrial magnetism and meteorology, have been added since. The pursuit of these objects has of necessity meant that the Observatory was equipped with powerful and accurate instruments, and the possession of these again has led to their use in fields which lay outside the domain of the purely utilitarian, fields from which the only harvest that could be reaped was that of the increase of our knowledge. So we have been led step by step from the mere desire to help the mariner to find his way across the trackless ocean, to the establishment of the secret law which rules the movements of every body of the universe, till at length we stand face to face with the mysteries of vast systems in the making, with the intimate structure of the stellar universe, with the apparently aimless, causeless wanderings of vast suns in lightning flight; with problems that we cannot solve, nor hope to solve, yet cannot cease from attempting, problems to which the only answer we can give is the confession of the magicians of Egypt--'This is the finger of God.'