Stargazing: Past and Present

by Sir Norman Lockyer

Fig. 123.—System of wires in transit eyepiece.

Let us show how this was always done some twenty or thirty years ago, and how it is sometimes done now. The transit room is kept so quiet that one can hear nothing but the ticking of the sidereal clock; the star to be observed is then carefully watched as it traverses the field of view over the wires, and the time of transit over each wire is estimated to the tenth of the time between each beat by the observer.

We reproduce in Fig. [123] a rough representation of what is seen in the field of view of a transit instrument. Now if we could be perfectly sure of making an accurate observation by means of the central wire, it is not to be supposed that astronomers would ever have cared to use this complicated system of wires in their eyepieces; but so great is the difficulty of determining accurately the time at which a star passes a wire, that we have in eyepieces introduced a system of several wires, so that we may take the transit of the star first at one wire, then at another, until every wire has been passed over.

We want one wire exactly in the middle to represent the real physical middle of the eyepiece so far as skill can do it, and then there is a similar number of wires on either side at exactly equal distances; so that the average of all the observations made at each of the wires will be much more likely to be accurate than a single observation at one wire. In this way the astronomer gives himself a good many chances against one to be right. If he lost his chance from any reason when using only one wire, he would have to wait twenty-four more sidereal hours before he could make his measure again, but by having five, or seven, or twenty-five or more wires in the eyepiece of the telescope, he increases his chances of correctness: and the way in which he works is this: While the heavens themselves are taking the stars across the wires he listens to the beating of the clock. If a star crosses one of the wires exactly as the clock is beating, he knows that it has passed the wire at some second, and he takes care to know what second that is; but if, instead of being absolutely coincident with one of the beats of the clock, it is half-way between one beat and another, or nearer to one beat than another, he estimates the fraction of a second, and by practice he has no difficulty at all in estimating divisions of time equal to tenths of a second, and at each particular wire in the eyepiece the transit of the star is thus minutely observed.

Then if the observations are complete and the mean of them is taken, it should, after the necessary corrections for instrumental errors have been applied, give the actual observation made at the central wire; if the astronomer cannot make observations at every wire, he introduces a correction in his mean to make up for the lost observations.

This is what is called the “eye and ear” method, because the observer is placed with his eye to the telescope, and he depends upon his ear to give him the exact interval at which each beat of the clock takes place, and he requires an exact power of mentally dividing the distance between each beat into ten equal parts, which are tenths of seconds. In this method of observation every observer differs slightly in his judgment of the instant that the star crosses the wire, and his estimation differs from the truth by a certain constant quantity which he must always allow for; this error is called his personal equation.

In this way then the transit instrument enables us, having true time, to determine the right ascension of a heavenly body as it transits the meridian, and, knowing the right ascension of a heavenly body, we have only to watch its transit in order to know the true time; so if the observer knows at what time a known star ought to transit, he has an opportunity of correcting his clock.

So much for the eye and ear method of transit observation. There is another which has now to a very large extent superseded it. This is called the “chronographic method”; we owe it to Sir Charles Wheatstone, who made it possible about 1840.

Figs. 124-7 are from drawings of the chronograph in use at Greenwich, and by their means we hope to make the principle of the instrument clear. In this chronograph, g is a long conical pendulum which regulates the driving clock in the case below it, through the gearing of wheel-work, as it turns the cylinder, E, gently and regularly round. On the cylinder is placed paper to receive the mark registering the observations; along the side of the cylinder or roller run two long screws, K and N, Fig. [125], which are also turned by the clock, and on them are carried electro-magnets, A, B, Fig. [125], and prickers, 35, Fig. [126]; as the screws turn, the magnets and prickers are moved along the roller, and, as the roller turns, the pointer, 36, Fig. [127], traces a fine line on the paper like the worm of a screw on the surface; and it is close to this line, which serves as a guide to the eye, that the prickers make a mark each time a current is sent through the electro-magnets; this turns each of them into a magnet, and they then attract a piece of iron which, in moving upwards, presses down its pricker by means of a lever, and registers the instant the current is sent.

The different wires are brought, first from the transit circle to work one pricker, and then from the clock to work the other, the clock sending a current and producing a prick on the roller every second.