Astronomy for Everybody/Part 2/Chapter 5

It is commonly supposed that the principal work of an astronomer is to study the stars as he sees them in his telescope. This is true only in the sense that a telescope is a necessary part of almost every astronomical instrument. But the mere studying of a star with a telescope is a very small part of the astronomer's work. The most important practical use of astronomy to our race consists in the determination of the latitudes and longitudes of points on the earth's surface, so that we may know where towns and cities are situated and be able to make a map of a state or country. This requires a knowledge of the exact positions of the stars in the heavens, that is to say, of their right ascension and declination. We have shown in a former chapter how these quantities correspond to longitude and latitude on the earth's surface. Through that correspondence an observer may determine his latitude by the star's declination and his longitude by its right ascension, combined with a knowledge of the sidereal time at a place of known longitude.

The figures and dimensions of the planets, the motions of the satellites, the orbits of planets and comets, the structure of nebulae and clusters of stars—all these offer fields of astronomical investigation to which there is ​no end, and in order to make these investigations other instruments besides the telescope are necessary.

The problem which demands most attention from the working astronomer in an observatory is the determination of the positions of the heavenly bodies. The prin-

cipal instrument for making these determinations is the meridian circle, called also a meridian instrument. This consists of a telescope supported on a horizontal east and west axis, at right angles to its length, so that its line of sight can move only along the meridian. If it points ​exactly south you can turn it on the axis until the line of sight passes through the zenith, and still farther until it passes through the pole on the north horizon; but you cannot turn it east or west. This might seem to restrict its usefulness, but it is on this restriction of its motion that its usefulness depends. The great value of this instrument is that it enables us to determine the right ascension of a star without taking any measurement but one of time. In a former chapter we described sidereal time, the units of which are slightly shorter than those of our ordinary time, so that a sidereal clock gains about two hours every month on an ordinary clock. The sidereal time at which a star crosses the meridian is the same as its right ascension; the problem of determining the latter, therefore, is the simplest in the world. We start our sidereal clock, set it on the exact sidereal time, point the telescope of the meridian circle to various stars as they are about to cross the meridian, and note the exact moment at which each star passes. In the instrument the meridian is shown by a very fine fibre or spider’s web fixed in the focus of the telescope. The moment when the image of the star as seen in the telescope crosses this spider line is that of passing the meridian. The time by the sidereal clock then shows the star’s right ascension. If the clock could be set with perfect exactness and the instrument revolved exactly in the plane of the meridian, right ascensions would be determined in the very simple way we have described.

It unfortunately happens, however, that no clock can be set with such exactness as to satisfy the requirements ​of the astronomer, who wants to know the time down to the tenth or even to the hundredth of a second. Moreover, no meridian circle can have its axis set so exactly east and west that the instrument shall not deviate a little from the meridian. The astronomer must therefore make allowances for the error of his clock and for the deviation of his instrument; and these require much careful observation and calculation. Even when he does the best he can, a single observation will always be liable to little errors which he wishes to make as small as possible. He does this by repeatedly determining the position of every star which he puts upon his list. He generally has to be satisfied with three or four observations on the great mass of the stars, but on the more important stars he makes them by scores or hundreds.

To determine the declination of a star, a graduated circle is necessary. This consists of a brass or steel circle, much like a carriage wheel, of which the axis is the same as that on which the telescope of the meridian instrument turns. The circle is firmly attached to the axis so that it must turn with the telescope as the latter sweeps along the celestial meridian. The graduations of the circle consist of very fine marks or lines all round its circumference. The latter being divided into three hundred and sixty degrees, every degree is marked by such a line. Between these it is common to mark thirty intermediate lines, which are therefore two minutes apart. Attached to one or both the stone piers which support the instrument are four microscopes, so fixed that the graduations on the circle are seen through them. When ​the instrument is turned on its axis, all these graduations pass successively under each microscope, so that they can be seen by the observer looking through the latter. The position of the star is determined by measures with the microscope on the graduation which happens to be under it when the telescope is pointed at a star.

The equatorial telescope and the meridian circle are the two principal instruments in the astronomical outfit of an observatory. Many other instruments are more or less in use for special purposes, but they are not of great interest, save to one who is making a special study of astronomy and who must therefore refer to books specially written for the professional student of the subject.

The precision with which a practised observer can note the time of transit of a star over the thread of his instrument is remarkable. One method of doing this consists in listening to and counting the beats of the clock as the star approaches and crosses the thread. He watches the exact position of the star at the beat before the transit, and again at the beat following. By comparing in his mind the opposite distances of the star from the thread at the two clock beats, he estimates the number of tenths of the second at which the transit took place, and records the time in his notebook.

This method is now superseded in most observatories by that of registration on a chronograph. This instrument consists of a revolving cylinder, covered with paper, having a pen-point resting upon it, so that, as the cylinder revolves, the pen leaves a trace on the paper. ​The pen is so connected with an electric current passing through the clock, and through a key held by the observer, that every beat of the clock and every pressure of the key by the observer makes a notch in the trace left by the pen. When the observer sees that a star has reached the thread of his instrument he presses the key, and the position of the notch thus made in the pen-trace between two notches made by the clock gives the moment at which the key was pressed.

The astronomer's clock must be of the highest attainable perfection, running for a whole day or more without a deviation of one tenth of a second. With a common house clock, the change in the length of the pendulum produced by changes of temperature between the day and night would cause deviations of several seconds. Hence in the astronomical clock these changes must be neutralised. This is done by making the pendulum of such a combination of different materials that the unequal expansions of the latter shall neutralise each other. The most common combination is that of a steel rod bearing at its lower end a steel or glass jar of quicksilver, which serves as the bob of the pendulum. Then, when the temperature rises, the upward expansion of the quicksilver compensates the downward expansion of the steel.