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

by E. Walter Maunder

The two related departments, therefore, those of the Transit and the Circle, which are concerned in the work of making star-catalogues, come next in order to the Time Department. Though both departments deal with the same instrument, the transit circle, they are at present placed at opposite ends of the Observatory domain; the Circle Department being lodged in the upper computing room of the old building; the Transit Department in the south wing of the New Observatory in the south ground.

It may be asked why, if this were the purpose of the Observatory at its foundation, two and a quarter centuries ago; if, as was the case, the work was set on foot from the beginning and was carried out with every possible care, how comes it that it is still the fundamental work of the Observatory, and, instead of being completed, has assumed greater proportions at the present day than ever before?

The answer to this inquiry may be found in a special application of the old proverb, originally directed against the discontent of man: 'The more he has, the more he wants.' For, however paradoxical it may seem, it is true that the fuller a star-catalogue is, and the more accurate the places of the stars that it contains, the greater is the need for a yet fuller catalogue, with places more accurate still.

It is worth while following up this paradox in some detail, as it affords a very instructive example of the way in which a work started on purely utilitarian grounds extends itself till it crosses the undefined boundary and enters the region of pure science.

We have no idea who made the earliest census of the sky. It is written for us in no book; it is not even engraved on any monument. And yet no small portion of it is in our hands to-day, and, strangest of all, we are able to fix fairly closely the time at which it was made, and the latitude in which its compiler lived. The catalogue is very unlike our star-catalogues of to-day. The places of the stars are very roughly indicated; and yet this catalogue has left a more enduring mark than all those that have succeeded it. The catalogue simply consists of the star names.

An old lady who had attended a University Extension lecture on astronomy was heard to exclaim: 'What wonderful men these astronomers are! I can understand how they can find out how far off the stars are, how big they are, and what they weigh—that is all easy enough; and I think I can see how they find out what they are made of. But there is one thing that I can't understand—I don't know how they can find out what are their names!' This same difficulty, though with a much deeper meaning than the old lady in her simplicity was able to grasp, has occurred to many students of astronomy. Many have wished to know what was the meaning of, and whence were derived, the sonorous names which are found attached to all the brighter stars on our celestial globes: Adhara, Alderamin, Betelgeuse, Denebola, Schedar, Zubeneschamal, and many more. The explanation lies here. Some 5000 years ago, a man, or college of men, living in latitude 40° north, in order that they might better remember the stars, associated certain groups of them with certain fancied figures, and the individual star names are simply Arabic words designed to indicate whereabouts in its peculiar figure or constellation that special star was situated. Thus Adhara means 'back,' and is the name of the bright star in the back of the great Dog. Alderamin means 'right arm,' and is the brightest star in the right arm of Cepheus, the king. Betelgeuse is 'giant's shoulder,' the giant being Orion; Denebola is 'lion's tail.' Schedar is the star on the 'breast' of Cassiopeia, and Zubeneschamal is 'northern claw,' that is, of the Scorpion. So far is clear enough. The names of the stars for the most part explain themselves; but whence the constellations derived their names, how it was that so many snakes and fishes and centaurs were pictured out in the sky, is a much more difficult problem, and one which does not concern us here.

One point, however, these old constellations do tell us, and tell us plainly. They show that the axis of the earth, which, as the earth travels round the sun, moves parallel with itself, yet, in the course of ages, itself rotates so as in a period of some 26,000 years to trace out a circle amongst the stars. This is the cause of what is called 'precession,' and explains how it is that the star we call the pole-star to-day was not always the pole-star, nor will always remain so. We learn this fact from the circumstance that the old constellations do not cover the entire celestial sphere. They leave a great circular space of 40° radius unmapped in the southern heavens. This simply means that the originators of the constellations lived in 40° north latitude, and stars within 40° of their south pole never rose above their horizon, and consequently were never seen, and could not be mapped, by them. In like manner, the star census taken at Greenwich Observatory does not include the whole sky, but leaves a space some 52° in radius round our south pole. Since the latitude of Greenwich is nearly 52° north, stars within that distance of the south pole do not rise above our horizon, and are never seen here. But if we compare the vacant space left by the old original constellations with the vacant space left by a Greenwich catalogue of to-day, we see that the centre of the first space, which must have been the south pole of that time, is a long way from the centre of the second space—our south pole of to-day. The difference tells us how far the pole has moved since those old forgotten astronomers did their work. We know the rate at which the pole appears to move, by comparing our more modern catalogues one with another; and so we are able to fix pretty nearly the time when lived those old first census-takers of the stars, whose names have perished so completely, but whose work has proved so immortal.

These old workers gave us the constellation groupings and names which still remain to us, and are still in common, every-day use. Their work affords us the most striking illustration of the result of precession, but precession itself was not recognized till nearly 3000 years after their day, when a marvellous genius, Hipparchus, established the fact, and 'built himself an everlasting name' by the creation of a catalogue of over 1000 stars prepared on modern principles. That catalogue formed the basis of one which survives to us at the present time, and was made some 1750 years ago by Claudius Ptolemy, the great astronomer of Alexandria, whose work, which still bears the proud name of Almagest, 'The Greatest,' remained for fourteen centuries the one universal astronomical text-book.

A modern catalogue contains, like that of Ptolemy, four columns of entry. The first gives the star's designation; the second an indication of its brightness; the third and fourth the determinations of its place. These are expressed in two directions, which, in modern catalogues, not in Ptolemy's, correspond on the celestial sphere to longitude and latitude on the terrestrial. Distance north or south of the celestial equator is termed 'declination,' corresponding to terrestrial latitude. Distance in a direction parallel to the equator is termed 'right ascension,' corresponding to terrestrial longitude. For geographical purposes we conceive the earth to be encircled by two imaginary lines at right angles to each other—the one, the equator, marked out for us by the earth itself; the other, 'longitude nought,' the meridian of Greenwich, fixed for us by general consent, after the lapse of centuries, by a kind of historical evolution. On the celestial globe in like manner we have two fundamental lines—one, the celestial equator, marked out for us by nature; the other at right angles to it, and passing through the poles of the sky, adopted as a matter of convenience. But a difficulty at once confronts us—Where can we fix our 'right ascension nought'? What star has the right to be considered the Greenwich of the sky?