Stargazing: Past and Present

by Sir Norman Lockyer

CHAPTER XIX.
OTHER INSTRUMENTS USED IN ASTRONOMY OF PRECISION.

In former chapters we have described the transit circle as it now exists as the result of the thought of Tycho, Picard, Römer, and Airy. This, though the fundamental instrument in a meridional observatory, is by no means the only one, and we must take a glance at the others.

In Römer’s “Observatorium Tusculaneum,” near Copenhagen, built in 1704, there was not only a transit circle in the meridian of course, but a transit instrument in the prime vertical, i.e. swinging in a vertical plane at right angles to the former one, so that while the optical axis of one always lies in a N.S. plane, that of the other lies in an E.W. one. Römer did not use this instrument much; it remained for the great Bessel to point out its value in determinations of latitude.

This instrument is, so to speak, self-correcting, because between the transit of a star over its wires while pointing to the east of the meridian, and that while pointing to the west its telescope can be reversed in its Ys, or one position may be taken for one night’s observations, and the other for the next, and so on.

In Struve’s form of this instrument the transit of stars can be observed at an interval of one minute and twenty seconds, this time only being required to raise it from the Ys, to rotate it through 180°, and lower it again. This rapid reversal, and consequent elimination of instrumental imperfections, enable observations of the most extreme precision to be made in such delicate matters as the slight differences of declination of stars due to aberration, nutation, and the like.

The intersection of the meridian with the prime vertical marks the zenith. To determine this:—first, there is the zenith sector, invented by Hooke; it consists of a telescope, carried by an axis on one side of the tube, and at right angles to it, so that the telescope swings exactly as a transit does, and it is provided with cross wires in the same manner; instead, however, of having a whole circle, it has only two segments of a circle; and as it is never required to swing the telescope far from its vertical position, there is a diagonal reflector at the eyepiece, so that the observer can look sideways, instead of upwards, in an awkward position. Its use is to determine the zenith distance of stars as they pass near that point. These distances are read off on the parts of the circle by verniers or microscopes, as in the transit circle. The zenith telescope, chiefly designed by Talcott, is the modern equivalent of the sector, and both instruments are more used in geodetical operations than in fixed observatories. At Greenwich, however, there is an instrument for determining zenith distances of very special construction. This is called the reflex zenith tube, shown in Fig. [131]. It is a sectional drawing of one of those instruments showing the path of the rays of light. A, B, is an object-glass, fixed horizontally, and below it is a trough of mercury, C, the surface of which is always of course horizontal. The light from a star near the zenith is allowed to fall through the object-glass, which converges the rays just so much that they come to a focus at F, after having been reflected from the surface of the mercury, and also by the diagonal mirror or prism, G; at F, therefore, we have an image of the star, which can be examined together with the cross-wires at the eyepiece, M. There is in this instrument no necessity for the accurate adjustments that there is in the case of the transit, the surface of the mercury being always horizontal, and so giving an unaltering datum plane.

Fig. 131.—Reflex Zenith Tube.

When the star is perfectly vertical, its image will fall on a certain known part in the eyepiece; but, as it leaves the vertical, the angle of incidence of its light on the mercury alters, and likewise that of reflection, so that the position of the image changes, and this change of position in the eyepiece is measured by movable cross-wires and a micrometer screw, similar to that employed for reading the circle in the transit circle.

At the present time γ Draconis is the star which passes very nearly through the zenith of Greenwich, and observations of this star are accordingly made at every available opportunity.