Stargazing: Past and Present

by Sir Norman Lockyer

Fig. 119.—Plan of the Greenwich Transit Circle.

We must defer giving more explanation of the practical working of the instrument until we have considered the clock used in connection with it, and we shall then show how the observations are made. One important point to which attention should be given is the method of illuminating the wires in the eyepiece. This is the arrangement. There is a lamp at the end of one of the pivots which is hollow, the light falls on a mirror, placed in the centre of the telescope, of such a shape and in such a position that it will not intercept the light from the object-glass falling through the diaphragms on to the eyepiece. The mirror is ring-shaped, something like the brim of a hat, and is carried on two pivots, so that it can be placed diagonally in the tube, or at right angles to it; it is arranged just outside the cone of rays from the object-glass, so that when the mirror is diagonally placed the light will be grasped directly from the lamp at the end of the axis and reflected down and mixed up with the light coming from the star into the eyepiece.

In this way of course the wires can be rendered visible at night, and without such a method they would be invisible. This arrangement gives a bright field and dark wires; but there is also a method of reversing matters; for near the edge of the ring-shaped reflector are fixed prisms for reflecting the light, and when the reflector is placed square with the axis of the telescope the small prisms on the reflector send the light down through apertures in the diaphragms, so that the mirror in this position no longer sends the light down with the rays from the star, but through holes in the diaphragms themselves, to two small reflecting prisms, one on each side of the wires in the eyepiece. What has that light to do? It has simply to do this, it has to fall sideways on the wires themselves in such a manner that it does not fall on the eye except by reflection from the wires. In this way we have the means of getting a bright system of wires on a dark field, in which the wires and objects to be measured are the only things to be seen.

As with the pivots of the transit circle, and in fact of any astronomical instrument, so with the circles, certain fundamental points have to be borne in mind; and, although it is absolutely impossible to ensure perfection, still, to go as near to it as possible, the astronomer has to observe a great many times over in all sorts of positions in order to bring the error down to its minimum.

First, the circle must be placed exactly at right angles to the axis of the telescope, so that it is in the plane of the meridian. Secondly, the error of centering must be found. For instance, if the Greenwich circle were to be read by only one microscope, an error in the pivot or any part of the axis round which the circle turns would vitiate the readings; but we could get rid of that error, due to a fault of the axis, or to a want of centering, by means of two readings, at the extremities of a diameter; but even then we should not get rid of the possible error due to graduation, for even if the divisions on the circle were accurate at first, they would not long remain so, for the metal of which these circles are made is liable, like other metals, to certain changes due to temperature; and if a circle is very large the weight of the circle itself, supposing its form perfect when horizontal, will, when vertical, sag it down and deflect it out of shape, so that at Greenwich the method adopted is to use six reading microscopes. Fig. [120], which shows the Cambridge Transit Circle, indicates the arrangement of the five microscopes in use there, set round the circumference of the circle, much in the same manner as in the case of the Greenwich instrument, where there are holes through the pier in which the microscopes are placed with the eye-ends arranged in a circle at the side of it.

Fig. 120.—Cambridge (U.S.) meridian circle.

When, therefore, the transit is pointed to any particular star, not only is the time noted in order to determine the right ascension of the star, in a careful and elaborate way, but the readings of the circle are made by every one of these microscopes—reading from the next five minutes division of the circle which happens to be visible,—and there is an additional microscope giving the rough reading of the larger divisions of the circle from a certain zero.

And what, then, is this zero? There is no doubt about the reading of the zero of right ascension, it is the intersection of the two fundamental planes at the first point of Aries; but what zero shall be used in the case of the vertical circle?