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

by E. Walter Maunder

Except at the exact time of full moon, we do not see the entire face of our satellite; one edge or 'limb' is in darkness. As the moon therefore passes over the star, either the limb at which the star disappears, or that at which it reappears, is invisible to us. To watch an occultation at the bright limb is pretty; the moon, with its shining craters and black hollows, its mountain ranges in bright relief, like a model in frosted silver, slowly, surely, inevitably comes nearer and nearer to the little brilliant which it is going to eclipse. The movement is most regular, most smooth, yet not rapid. The observer glances at his clock, and marks the minute as the two heavenly bodies come closer and closer to each other. Then he counts the clock beats: 'five, six, seven,' it may be, as the star has been all but reached by the advancing moon. 'Eight,' it is still clear; ere the beat of the clock rings to the 'nine,' perhaps the little diamond point has been touched by the wide arch of the moon's limb, and has gone! Less easy to exactly time is a reappearance at the bright limb. In this case the observer must ascertain from the Nautical Almanac precisely where the star will reappear; then a little before the predicted time he takes his place at the telescope, watches intently the moon's circumference at the point indicated, and, listening for the clock-beats, counts the seconds as they fly. Suddenly, without warning, a pin-point of light flashes out at the edge of the moon, and at once draws away from it. The star has 'reappeared.'

Far more striking is a disappearance or reappearance at the 'dark limb.' In this case the limb of the moon is absolutely invisible, and it may be that no part of the moon is visible in the field of the telescope. In this case the observer sees a star shining brightly and alone in the middle of the field of his telescope. He takes the time from his faithful clock, counting beat after beat, when suddenly the star is gone! So sudden is the disappearance that the novice feels almost as astonished as if he had received a slap in the face, and not unfrequently he loses all count or recollection of the clock beats. The reappearance at the dark limb is quite as startling; with a bright star it is almost as if a shell had burst in his very face, and it would require no very great imagination to make him think that he had heard the explosion. One moment nothing was visible; now a great star is shining down serenely on the watcher. A little practice soon enables the observer to accustom himself to these effects, and an old hand finds no more difficulty in observing an occultation of any kind than in taking a transit.

Such an observation is useful for more purposes than one. If the position of the star occulted is known—and it can be determined at leisure afterwards—we necessarily know where the limb of the moon was at the time of the observation. Then the time which the moon took to pass over the star enables us to calculate the diameter of our satellite; the different positions of the moon relative to the star, as seen from different observatories, enable us to calculate its distance.

But if the disappearance takes place at the bright limb, the reappearance usually takes place at the dark, and vice versâ; and the two observations are not quite comparable. There is one occasion, however, when both observations are made under similar circumstances, namely, at the full. And if the moon happens also to be totally eclipsed, the occultations of quite faint stars can be successfully observed, much fainter than can ordinarily be seen close up to the moon. Total eclipses of the moon, therefore, have recently come to be looked upon as important events for the astronomer, and observatories the world over usually co-operate in watching them. October 4, 1884, was the first occasion when such an organised observation was made; there have been several since, and on these nights every available telescope and observer at Greenwich is called into action.

It may be asked why these different modes of observing the moon are still kept up, year in and year out. 'Do we not know the moon's orbit sufficiently well, especially since the discovery of gravitation?' No; we do not. This simple and beautiful law—simple enough in itself, gives rise to the most amazing complexity of calculation. If the earth and moon were the only two bodies in the universe, the problem would be a simple one. But the earth, sun, and moon are members of a triple system, each of which is always acting on both of the others. More, the planets, too, have an appreciable influence, and the net result is a problem so intricate that our very greatest mathematicians have not thoroughly worked it out. Our calculations of the moon's motions need, therefore, to be continually compared with observation, need even to be continually corrected by it.

There is a further reason for this continual observation, not only in the case of the sun, which is our great standard star, since from it we derive the right ascensions of the stars, and it is also our great timekeeper; not only in that of the moon, but also in the case of the planets. Their places as computed need continually to be compared with their places as observed, and the discordances, if any, inquired into. The great triumph which resulted to science from following this course—to pure science, since Uranus is too faint a planet to be any help to the sailor in navigation—is well known. The observed movements of Uranus proved not to be in accord with computation, and from the discordances between calculation and observation Adams and Leverrier were able to predicate the existence of a hitherto unseen planet beyond—

'To see it, as Columbus saw America from Spain. Its movements were felt by them trembling along the far-reaching line of their analysis, with a certainty hardly inferior to that of ocular demonstration.'[5]

The discovery of Neptune was not made at Greenwich, and Airy has been often and bitterly attacked because he did not start on the search for the predicted planet the moment Adams addressed his first communication to him, and so allowed the French astronomer to engross so much of the honour of the exploit. The controversy has been argued over and over again, and we may be content to leave it alone here. There is one point, however, which is hardly ever mentioned, which must have had much effect in determining Airy's conduct. In 1845, the year in which Adams sent his provisional elements of the unseen disturbing planet to Airy, the largest telescope available for the search at Greenwich was an equatorial of only six and three-quarter inches aperture, provided with small and insufficient circles for determining positions, and housed in a very small and inconvenient dome; whilst at Cambridge, within a mile or so of Adams' own college, was the 'Northumberland' equatorial, of nearly twelve inches aperture, under the charge of the University Professor of Astronomy, Professor Challis, and which was then much the largest, best mounted and housed equatorial in the entire country. The 'Northumberland' had been begun from Airy's designs and under his own superintendence, when he was Professor of Astronomy at Cambridge. Naturally, then, knowing how much superior the Cambridge telescope was to any which he had under his care, he thought the search should be made with it. He had no reason to believe that his own instrument was competent for the work.