THE ASTROGRAPHIC DEPARTMENT
The two last departments mentioned, the heliographic and spectroscopic, lie clearly and unmistakably outside the terms of the original warrant of the Observatory, though the progress of science has led naturally and inevitably to their being included in the Greenwich programme. But the Astrographic Department, though it could no more have been conceived in the days of Charles II. than the spectroscopic, does come within the terms of the warrant, and is but an expansion of that work of 'Rectifying the Places of the Fixed Stars,' which formed part of the programme enjoined upon Flamsteed, the first Astronomer Royal, at the first foundation of the Observatory, and which was so diligently carried out by him, the first Greenwich catalogue, containing about 3000 stars, being due to his labours.
His immediate successors did much less in this
field, though Bradley's observations were published,
long after his death, as a catalogue of 3222 stars, in
some aspects the most important ever issued. Pond,
the sixth Astronomer Royal, restored catalogue making
to a prominent place in the Greenwich routine,
and his precedent is sedulously followed to-day. But each of these was confined to about 3000 stars. The necessity has long been felt for a much ampler census, and Argelander, at the Bonn Observatory, brought out a catalogue of 324,000 stars north of South declination 2 degrees, a work which has been completed by Schonfeld, who carried the census down to South declination 23 degrees and by the two great astronomers of Cordoba, South America, Dr. Gould and Dr. Thorne, by whom it was extended to the South Pole.
These last three catalogues embrace stars of all magnitudes down to the 9th or 10th; but certain astronomers had endeavoured to go much lower, and to make charts of limited portions of the sky down to even the 14th magnitude.
From the very earliest days that men observed the stars, they could not help noticing that 'one star differeth from another star in glory,' and consequently they divided them into six classes, according to their brightness--classes which are commonly spoken of now as magnitudes. The ordinary 6th magnitude star is one which can be clearly seen by average sight on a good night, and it gives us about one-hundredth the light of an average 1st magnitude star. Sirius, the brightest of all the fixed stars, is called a 1st magnitude star, but is really some six or seven times as bright as the average. It would take, therefore, more than two and a half million stars of the 14th magnitude to give as much light as Sirius.
It is evident that so searching a census as to embrace stars of the 14th magnitude would involve a most gigantic chart. But the work went on in more than one Observatory for a considerable time, until at last the observers entered on to the region of the Milky Way. Here the numbers of the stars presented to them were so great as to baffle all ordinary means of observation. What could be done?
Just at this time immense interest was caused in the astronomical world by the appearance of the great comet of 1882. It was watched and observed and sketched by countless admirers, but more important still, it was photographed, and some of its photographs, taken at the Royal Observatory, Cape of Good Hope, showed not only the comet with marvellous beauty of detail, but also thousands of stars, and the success of these photographs suggested to her Majesty's Astronomer at the Cape, Dr. Gill, that in photography we possessed the means for making a complete sky census even to the 14th magnitude.
The project was thought over in all its bearings, and in 1887 a great conference of astronomers at Paris resolved upon an international scheme for photographing the entire heavens. The work was to be divided between eighteen Observatories of different nationalities. It was to result in a photographic chart extending to the 14th magnitude, and probably embracing some forty million stars, and a catalogue made from measures of the photographs down to the 11th magnitude, which would probably include between two and three million stars.
The eighteen Observatories all undertook to use
instruments of the same capacity. This was to be
a photographic refractor, with an object-glass of 13
inches aperture and 11 feet focus. At Greenwich this telescope is mounted equatorially--that is, so as to follow the stars in their courses--and is mounted on the top of the pier that once supported Halley's quadrant. The telescope is driven by a most efficient clock, whose motive power is a heavy weight. The rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. A seconds pendulum is mounted in a glass case on the wall of the Observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. On one of the wheels of the clock are arranged a number of little brass points, at such intervals apart that the wheel, when going at the proper rate, takes exactly one second to move through the distance between any pair. A little spring is arranged above the wheel, so that these points touch it as they pass. If this occurs exactly as the pendulum point passes through the mercury nothing happens, but if the clock is ever so little late or early, the electric current from the pendulum brings into action a second wheel, which accelerates or retards the driving of the clock, as the case may be. The total motion, therefore, is most beautifully even.
But even this is not quite sufficient, especially as
the plates for the great chart have to be exposed
for at least forty minutes. Rigidly united with the
13-inch refractor, so that the two look like the two
barrels of a huge double-barrelled gun, is a second
telescope for the use of the observer. In its eyepiece are fixed two pairs of cross spider lines, commonly called wires, and a bright star, as near as possible to the centre of the field to be photographed, is brought to the junction of two wires. Should the star appear to move away from the wire, the observer has but to press one of two buttons on a little plate which he carries in his hand, and which is connected by an electric wire with the driving clock, to bring it back to its position.
The photographs taken with this instrument are of two kinds. Those for the great chart have but a single exposure, but this lasts for forty minutes. Those for the great catalogue have three exposures on them, the three images of a star being some 20 seconds of arc apart. These exposures are of six minutes', three minutes', and twenty seconds' duration, and the last exposure is given as a test, since, if stars of the 9th magnitude are visible with an exposure of twenty seconds, stars of the 11th magnitude should be visible with three minutes' exposure.
Thus it will be seen that in three minutes an impression is got of many scores of stars, whose places it would require many hours to determine at the transit instrument. But the positions of these stars on the plate still remain to be measured. For this purpose a net-work of lines, at right angles to each other, is printed on the photograph before its development, and, after it has been developed, washed and dried, the distances of the stars from their nearest cross-lines are measured in the measuring machine.
The measuring machine is constructed to hold
two plates, one half its breadth higher than the other.
In fact, in each of the two series of photographs the whole sky is taken twice, but the two photographs of any region are not simply duplicates of each other. The centre of each plate is at a corner of four other plates, and in the micrometer the stars on the quarter common to two plates are measured simultaneously.
In this way will be carried out a great census of the sky that will exceed Flamsteed's ten thousand fold. And just as Flamsteed's was but the first of many similar catalogues, so, no doubt, will this be followed by others--not superseded, for its value will increase with its age and the number of those that follow it, by comparison with which it will prove an inexhaustible mine of information concerning the motions of the stars and the structure of the universe.
There is a great difference between the work of the observer with the 'Astrographic Telescope,' as this great twin photographic instrument is called, and the work of the transit observer. The latter sees the star gliding past him, and telegraphs the instant that the star threads itself on each of the ten vertical wires in succession. The astrographic observer, on the other hand, sees his star shining almost immovably in the centre of his field, threaded on the two cross wires placed there, for the driving-clock moves the telescope so as to almost exactly compensate for the rotation movement of the earth. The observer's duty in this case is to telegraph to his driving-clock, when it has in the least come short of or exceeded its duty, and so to bring back the 'guiding star' to its exact proper place on the cross wires.
So far, the work of the Astrographic Department has been, as mentioned above, a development on an extraordinary scale, but a development still, of the original programme of the Observatory. But the munificent gift of Sir Henry Thompson has put it within the power of the Astronomer Royal to push this work of sidereal photography a stage further. Sir Henry Thompson gave to the Observatory, not merely the photographic refractor of 9 inches' aperture, now used for solar photography, and known as the 'Thompson photo-heliograph,' but also one of 26 inches' aperture and 22 1/2 feet focal length. This instrument was specially designed of exactly double the dimensions of the standard astrographic telescope used for the International Photographic Survey, the idea being that, in the case of a field of special interest and importance, a photograph could be obtained with the larger instrument on exactly double the scale given by the smaller. It has rather, however, found its usefulness in a slightly different field. The observation of the satellites of Jupiter was suggested by Galileo as a means of determining the longitude at sea. As already pointed out, the suggestion did not prove to be a practical one for that purpose, but observations of the satellites have been made none the less with a view simply to improving our knowledge of their movements, and of the mass of Jupiter. The utilitarian motive for the work having fallen through, it has been carried on as a matter of pure science.
And the work has not stopped with the satellites
of Jupiter; eight satellites were in due time discovered
to Saturn, four to Uranus, and two to Mars; and
though these could give not the remotest assistance
to navigation, they too have been made the subjects
of observation for precisely the same reason as those
of Jupiter have been.
In just the same way, when the discovery of Neptune was followed by that of a solitary companion to it, this also had to be followed. The difficulties in the way of observing the fainter of all these satellites were considerable, and the work has been mostly confined to two or three observatories possessing very large telescopes. As the largest telescope at Greenwich was only 7 inches in aperture up to 1859, and only 12 3/4 inches up to 1893, it is only very recently that it has been able to take any very substantial part in satellite measures. But since the Thompson photographic telescope was set up, it has been found that a photograph of Neptune and its satellite can be taken in considerably less time than a complete set of direct measures can be made, whilst the photograph, which can be measured at leisure during the day, gives distinctly the more accurate results.
So, too; the places of the minor planets can be got more accurately and quickly by means of photographs with this great telescope than by direct observation, and photographs of the most interesting of them all, the little planet Eros, have been very successfully obtained. So that, though doing nothing directly to improve the art of navigation, or to find the longitude at sea, the great photographic refractor takes its share in the work of 'Rectifying the Tables of the Planets.'
The reflector of 30 inches' aperture, which acts
as a counterpoise to the sheaf of telescopes of the
Thompson, is intended for use with the spectroscope,
the quality which mirrors possess of bringing all
rays, whatever their colour, to the same focus being
of great importance for spectroscopic work. But the
experiments which have been made with it in celestial
photography have proved so extremely successful as
to cause the postponement of the recommencement
of the spectroscopic researches. Chief amongst these photographs are some good ones of the moon, and more recently some exceedingly fine photographs of the principal nebulae.
In no department of astronomy has photography brought us such striking results as in regard to the nebulae. Dr. Roberts' photograph of the great nebula in Andromeda converted the two or three meaningless rifts--which some of the best drawings had shown--into the divisions between concentric rings; and what had appeared a mere shapeless cloud was seen to be a vast symmetrical structure, a great sidereal system in the making. The great nebula in Orion has grown in successive photographs in detail and extent, until we have a large part of the constellation bound together in the convolutions of a single nebula of the most exquisite detail and most amazing complexity. The group of the Pleiades has had a more wonderful record still. Manifestly a single system even to the naked eye, and showing some faint indications of nebulosity in the telescope, the photographs have revealed its principal stars shining out from nebulous masses, in appearance like carded wool, and have shown smaller stars threaded on nebulous lines like pearls upon a string.
Such photographs are, of course, of no utilitarian value, and at present they lead us to no definite scientific conclusions. They lie, therefore, doubly outside the limits of the purely practical, but they attract us by their extreme beauty, and by the amazing difficulty of the problems they suggest. How are these weird masses of gas retained in such complex form over distances which must be reckoned by millions of millions of miles? By what agency are they made to glow so as to be visible to us here? What conceivable condition threads together suns on a line of nebula? What universes are here in the making, or perhaps it may be falling into ruin and decay?