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

by E. Walter Maunder

'Perhaps you may be surprised to hear me say the rule is established as true, and yet there is a departure from it. This is the way we go on in science, as in everything else; we have to make out that something is true, then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained.'

In this particular case, the disturbing cause is found in the action of our own atmosphere. The rays of light from the star are bent out of a perfectly straight course as they pass through the various layers of that atmosphere, layers which necessarily become denser the closer we get to the actual surface of the earth. Every celestial body therefore appears to be a little higher in the sky than it really is. This action is most noticeable at the horizon, where it amounts to about half a degree. As both sun and moon are about half a degree in diameter, it follows that when they have really just entirely sunk below the horizon they appear to be just entirely above it. It happens, in consequence, on rare occasions, that an eclipse of the moon will take place when both sun and moon are together seen above the horizon.

It was a great matter to discover this effect of refraction. It was soon seen that it was not constant, that it varied with both temperature and pressure. It is, indeed, the most troublesome of all the hindrances to exact observation with which the astronomer has to contend; partly because of its large amount—half a degree, as has been already said, in the extreme case—and partly because it is difficult in many cases to determine its exact effect.

The double observation with the transit circle gives us, then, the place in the sky where the star appeared to be at the moment of observation, not its true place; to find that true place we have to calculate how much refraction had displaced the star at the particular height in the sky, and at the particular temperature and atmospheric pressure at which the observation was made.

THE MURAL CIRCLE.

The transit circle is a comparatively recent instrument. In earlier times the two observations of right ascension and declination were entrusted to perfectly separate instruments. The transit instrument was mounted as the transit circle is, between two solid stone piers, and moved in precisely the same way. But the great six-foot wheel, which was made as stiff as it possibly could be, was mounted on the face of a great stone pier or wall, from which circumstance it was called the 'mural circle,' and a light telescope was attached to it which turned about its centre. This arrangement had a double disadvantage—that the two observations had to be made separately, and the mural circle, not being a symmetrical instrument, was liable to small errors which it was difficult to detect. Thus, being supported on one side only, a flexure or bending outwards of either telescope or circle, or both, might be feared.

It was for this reason that Pond set up a pair of mural circles, one on the east side of its supporting pier and the other on the west.[3] His plan was not only to have each star observed simultaneously in the two instruments, a plan by which, at the cost of some additional labour, he would have got rid, to a large extent, of the individual errors of the two separate instruments, inasmuch as, on the whole, it might have been expected that the errors of the two instruments would have been very nearly equal in amount, but of opposite character. The differences, too, between the two instruments would have afforded the means for tracing these small errors to their respective causes, and so ascertaining the laws to which they were subject.

Pond went further still. He added to the mural circle a simple instrument, the extreme value of which every astronomer recognizes to-day—the mercury trough. Not only was the star to be observed by both circles when the two telescopes were pointing directly to it, it was also to be observed by reflection; the telescopes were to be pointed down towards a basin of mercury, in which the image of the star would be seen reflected. The mercury being a liquid, its surface is perfectly horizontal; and, since the law of reflection is that the angle of incidence is equal to the angle of reflection, it follows that the telescope, when pointed down toward the mercury trough, points just at as great an angle below the horizon as, when it is set directly on the star, it points above it. If the circle, therefore, be carefully read at both settings, half the difference between the two readings will give the angular elevation of the star above the horizon. A combination, therefore, of all four observations, that is to say, one reflection and one direct with each of the telescopes, would give an exceedingly exact value for the star's altitude. The conception of this method gives a striking idea of Pond's thoroughness and skill as a practical observer, and it is a distinct blot upon Airy's justly high reputation in the same line that he discontinued the system upon his accession to office.