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

by E. Walter Maunder

This 'Dallmeyer' telescope, so called from the name of its maker, is one of five identical instruments which were made for use in the observation of the transit of Venus of 1874, and which, since they are designed for photographing the sun, are called 'photo-heliographs.'

The image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. Even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. This is managed by arranging a very narrow slit in a strip of brass. The strip is made to run in a groove across the principal focus. Before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. When all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second—in midsummer for less than a thousandth of a second.

Two of these photographs are taken every fine day at Greenwich; occasionally more, if anything specially interesting appears to be going on. But in our cloudy climate at least one day in three gives no good opportunity for taking photographs of the sun, and in the winter time long weeks may pass without a chance. The present Astronomer-Royal, Mr. Christie, has therefore arranged that photographs with precisely similar instruments should be taken in India and in the Mauritius, and these are sent over to Greenwich as they are required, to fill up the gaps in the Greenwich series. We have therefore at Greenwich, from one source or another, practically a daily record of the state of the sun's surface.

More recently the 'Dallmeyer' photo-heliograph, though still retained for occasional use, has been superseded generally by the 'Thompson'; a photographic refractor of nine inches aperture, and nearly nine feet focal length, presented to the Observatory by Sir Henry Thompson. The image of the sun obtained after enlargement in the telescope, with this instrument, is seven and a half inches in diameter. The 'Thompson' is mounted below the great twenty-six-inch photographic refractor,—also presented to the Observatory by Sir Henry Thompson,—in the dome which crowns the centre of the New Observatory.

A photograph of the sun taken, it has next to be measured, the four following particulars being determined for each spot: First, its distance from the centre of the image of the sun; next, the angle between it and the north point; thirdly, the size of the spot; and fourthly, the size of the umbra of the spot, that is to say, of its dark central portion. The size or area of the spot is measured by placing a thin piece of glass, on which a number of cross-lines have been ruled one-hundredth of an inch apart, in contact with the photograph. These cross-lines make up a number of small squares, each the ten-thousandth (¹/₁₀₀₀₀ in.) part of a square inch in area. When the photograph and the little engraved glass plate are nearly in contact, the photograph is examined with a magnifying glass, and the number of little squares covered by a given spot are counted. It will give some idea of the vast scale of the sun when it is stated that a tiny spot, so small that it only just covers one of these little squares, and which is only one-millionth of the visible hemisphere of the sun in area, yet covers in actual extent considerably more than one million of square miles.

The dark spots are not the only objects on the sun's surface. Here and there, and especially near the edge of the sun, are bright marks, generally in long branching lines, so bright as to appear bright even against the dazzling background of the sun itself. These are called 'faculæ,' and they, like the spots, have their times of great abundance and of scarcity, changing on the whole at the same time as the spots.

After the solar photographs have been measured, the measures must be 'reduced,' and the positions of the spots as expressed in longitude and latitude on the sun computed. There is no difficulty in doing this, for the position of the sun's equator and poles have long been known approximately, the sun revolving on its axis in a little more than twenty-five days, and carrying of course the spots and faculæ round with him.

There are few studies in astronomy more engrossing than the watch on the growth and changes of the solar spots. Their strange shapes, their rapid movements, and striking alterations afford an unfailing interest. For example, the amazing spectacle is continually being afforded of a spot, some two, three, or four hundred millions of square miles in area, moving over the solar surface at a speed of three hundred miles an hour, whilst other spots in the same group are remaining stationary. But a higher interest attaches to the behaviour of the sun as a whole than to the changes of any particular single spot; and the curious fact has been brought to light, that not only do the spots increase and diminish in a regular cycle of about eleven years in length, but they also affect different regions of the sun at different points of the cycle. At the time when spots are most numerous and largest, they are found occupying two broad belts, the one with its centre about 15° north of the equator, the other about as far south, the equator itself being very nearly free from them. But as the spots begin to diminish, so they appear continually in lower and lower latitudes, until instead of having two zones of spots there is only one, and this one lies along the equator. By this time the spots have become both few and small. The next stage is that a very few small spots are seen from time to time in one hemisphere or the other at a great distance from the equator, much farther than any were seen at the time of greatest activity. There are then for a little time three sun-spot belts, but the equatorial one soon dies out. The two belts in high latitude, on the other hand, continually increase; but as they increase, so do they move downwards in latitude, until at length they are again found in about latitude 15° north or south, when the spots have attained their greatest development.