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

by E. Walter Maunder

He was not absolutely correct, however, for the great twenty-four-hour clock does not mark the exact meridian of Greenwich. To find the instrument which marks it out and defines it we must step inside the Observatory precincts, and just within the gate we see before us on the left hand a door which leads through a little lobby straight into the most important room of the whole Observatory—the Transit Room.

THE GREAT CLOCK AND PORTER'S LODGE.
(From a photograph by Mr. Lacey.)

This room is not well adapted for representation by artist or photographer. Four broad stone pillars occupy the greater part of the space, and leave little more than mere passage room beside. Two of these pillars are tall, as well as broad and massive, and stand east and west of the centre of the room, carrying between them the fundamental instrument of the Observatory, the transit circle. The optical axis of this telescope marks 'Longitude Nought,' which is further continued by a pair of telescopes, one to the north of it, the other to the south, mounted on the third and fourth of the pillars alluded to above.

This room has not always marked the meridian of Greenwich, for it stands outside the original boundary of the Observatory. But it is only a few feet to the east of the first transit instrument which was set up by Halley, the second Astronomer Royal, in the extreme N.-W. corner of the Observatory domain, a distance equivalent to very much less than one-tenth of a second of time, an utterly insensible quantity with the instruments of two hundred years ago.

It would be a long story to tell in detail how the Greenwich transit room has come to define one of the two fundamental lines that encircle the earth. The other, the equator, is fixed for us by the earth itself, and is independent of any political considerations, or of any effort or enterprise of man. But of all the infinite number of great circles which could be drawn at right angles to the equator, and passing through the north and south poles, it was not easy to select one with such an overwhelming amount of argument in its favour as to obtain a practically universal acceptance. The meridians of Jerusalem and of Rome have both been urged, upon what we may call religious or sentimental grounds; that of the Great Pyramid at Ghizeh has been pressed in accordance with the fantastic delusion that the Pyramid was erected under Divine inspiration and direction; that of Ferro, in the Canaries, as being an oceanic station, well to the west of the Old World, and as giving a base line without preference or distinction for one nation rather than another.

The actual decision has been made upon no such grounds as these. It has been one of pure practical convenience, and has resulted from the amazing growth of Great Britain as a naval and commercial power. Like Tyre of old, she is 'situate at the entry of the sea, a merchant of the people for many isles,' and 'her merchants are the great men of the earth.' To tell in full, therefore, the steps by which the Greenwich meridian has overcome all others is practically to tell again, from a different standpoint, the story of the 'expansion of England.' The need for a supreme navy, the development of our empire beyond the seven seas, the vast increase of our carrying trade—these have made it necessary that Englishmen should be well supplied with maps and charts. The hydrographic and geographic surveys carried on, either officially by this country, or by Englishmen in their own private capacity, have been so numerous, complete, and far-reaching as not only to outweigh those of all other countries put together, but to induce the surveyors and explorers of not a few other countries to adopt in their work the same prime meridian as that which they found in the British charts of regions bordering on those which they were themselves studying. Naturally, the meridian of Greenwich has not only been adopted for Great Britain, but also for the British possessions over-sea, and, from these, for a large number of foreign countries; whilst our American cousins retain it, an historic relic of their former political connection with us. The victories of Clive at Arcot and Plassy, of Nelson at the Nile and Trafalgar, the voyages and surveys of Cook and Flinders, and many more; the explorations of Bruce, Park, Livingstone, Speke, Cameron, and Stanley; these are some of the agencies which have tended to fix 'Longitude Nought' in the Greenwich Transit Room.

There are two somewhat different senses in which the meridian of Greenwich is the standard meridian for nearly the entire world. The first is the sense about which we have already been speaking; it constitutes the fundamental line whence distances east and west are measured, just as distances north and south are measured from the equator. But there is another, though related sense, in which it has become the standard. It gives the time to the world.

There are few questions more frequently put than, 'What time is it?' 'Can you tell me the true time?' A stickler for exactitude might reply, 'What kind of time do you mean?' 'Do you mean solar or sidereal time?' 'Apparent time or mean time?' 'Local time or standard time?' There are all these six kinds of time, but it is only within the last two generations, within, indeed, the reign of our Sovereign, Queen Victoria, that the subject of the differences of most of these kinds of time has become of pressing importance to any but theorists.