It is one of the most fortunate and remarkable coincidences in the whole history of science, that at the very time that Greenwich Observatory was being called into existence, the greatest of all astronomers was working out his demonstration of the great fundamental law of the material universe—the law that every particle of matter attracts every other particle with a force which varies directly with the mass and inversely with the square of the distance.
Several other of the great minds of that time, in particular Dr. Hooke, the Gresham Professor of Astronomy, had seen that it was possible that some such law might supply the secret of planetary motion; but it is one thing to make a suggestion, and a very different matter indeed to be able to demonstrate it; and the latter was in Newton's power alone. He did much more than demonstrate it—he brought out a whole series of most important and far-reaching consequences. He showed that the ebb and flow of the tides was due to the attraction of both sun and moon, especially the latter, upon the waters of our oceans. He pointed out certain irregularities which must take place in the motion of our moon, due to the influence of the sun upon it. He showed, too, what was the cause of that swinging of the axis of the earth which gives rise to precession. He deduced the relative weights of the earth, the sun, and of Jupiter and Saturn, the planets with satellites. He proved also that comets, which had seemed hitherto to men as perfectly lawless wanderers, obeyed in their orbits the self-same law which governed the moon and planets. The whole vast system of celestial movements, which had long seemed to men irregular and uncontrolled, now fell, every one of them, into its place, as but the necessary manifestations of one grand, simple order.
This great discovery gave a new and additional importance to the regular observation of the moon and planets. They were needed now, not only to assist in the practical work of navigation, but for the development of questions of pure science. Halley, the second Astronomer Royal, and Maskelyne, the fifth, devoted themselves chiefly to this department of work, to the partial neglect of the observation of the places of stars. Airy, the seventh, whilst making catalogue-work a part of the regular routine of the Observatory, developed the observation of the members of the solar system, and especially of the moon, in a most marked degree, and collected and completely reduced the vast mass of material which the industry of his predecessors had gathered. It is pre-eminently of the work of Airy that the memorable words quoted before of Professor Newcomb, the great American mathematician and astronomer, are applicable: 'that if this branch of astronomy were entirely lost, it could be reconstructed from the Greenwich observations alone.'
A most important step taken by Airy was the construction of an altazimuth. An altazimuth is practically a theodolite on a large scale. Its purpose is to determine, not the declination and right ascension of some celestial body, as is the case with the transit circle, but its altitude, i.e. its height above the horizon, and its azimuth, i.e. the angle measured on the horizontal plane from the north point. The altazimuth, then, like the transit circle, consists of a telescope revolving on a horizontal axis, but, unlike the transit circle, both the telescope and the piers which carry its pivots can be rotated so as to point not merely due north and south, but in any direction whatsoever.
AIRY'S ALTAZIMUTH.
The observations with the altazimuth are rather more complicated than those with the transit circle. Looking in the telescope, the observer sees a double set of spider threads or 'wires'; and when a star or other heavenly body enters the field, it will generally be observed to move obliquely across both sets of wires. The observer usually determines to make an observation either in altitude or azimuth. In the former case he presses the little contact button, which, as in the transit circle, is provided close to the eyepiece, as the star reaches each of the horizontal wires in succession. If in azimuth, it is the times of crossing the vertical wires that are in like manner telegraphed to the chronograph. The transit over, the appropriate circle is read; for the telescope itself is rigidly attached to a vertical wheel having a carefully engraved circle on its face and read by four microscopes, whilst the entire instrument carries another set of microscopes, pointing to a fixed horizontal circle, and upon which the azimuth can be read. A complete observation involves four such transits and sets of circle readings, two of altitude, and two of azimuth; for after one of altitude and one of azimuth the telescope is turned round, and a second observation is taken in each element.
The observation gives us the altitude and azimuth of the star. These particulars are of no direct value to us. But it is a mere matter of computation, though a long and laborious one, to convert these elements into right ascension and declination.