Instead of two stars being far apart, so that the telescope has to be moved, they may be alongside of each other, as in Fig. 2; then, the observer, by moving the cross-threads from one star to the other, and measuring the amount of the motion, can determine the angular distance between the stars, and their relation to each other. This is done by a micrometer, one kind of which will now be described.
The Filar Micrometer.—This adjunct is so called because an essential part of it consists in fine threads of spider lines stretched across the field of view, as already described. The aim of its construction is to admit of these threads being moved in a direction at right angles to their length, by a very fine screw, so that the space over which they pass may be measured by the turns of the screw. The principal appliances for effecting this are a fixed frame, A B C D, Fig. 4, in which slides another frame, E F G, moved by a fine screw at S. Across this inner frame is stretched the spider line J, and across the fixed one the spider line H. To -the head of the screw is attached a cylindrical rim, which has 100 or some other number of divi sions cut upon it. An index mark serves to show how far the screw is turned. An apparatus for measuring the number of turns of the screw is attached, but need not be described here. Then when the observer turns the screw, the movable frame of the spider lines is slowly carried along with it. The position of the spider lines as they move is then shown at every mo ment by the number of turns of the screw and the fractions of a turn. To show the accuracy with which this can be done, We remark that the screws used by astronomers may have 100 or even 125 turns to the inch. Then, each revo lution of the screw, as read off on the head, measures to a motion through this space. There being 100 graduations on the head, each grad uation may measure the motion of 1-10,000 of an inch. But the observer may estimate the tenths between the divisions, thus carrying his measurements down to the 1-100,000 of an inch. Beside the movable spider line across the frame, fixed spider lines may also be stretched across the fixed frame. Then we shall have two systems of spider lines, one movable and the other fixed. The relation of each to the other is measured by'the turns of the screw.
To determine the exact position of the grad uated circle, a filar -micrometer 0 is attached to a microscope of the form shown in Fig. 5, and the latter is finally fastened to a fixed frame in such a position that, when the ob server looks into the microscope, he sees the graduations on the circle magnified as many times as necessary, and also the threads of the micrometer. The microscope being fixed re mains at rest while the circle turns. If the instrument were geometrically perfect in every respect, one reading microscope would answer the purpose; but, as the circle cannot be cen tred with mathematical exactness, pairs of microscopes are used which are at opposite ends of diameters of the circle. For example, when the graduation 15° 20' is brought under one microscope, the graduation 195° 15' would be under the opposite one. It is customary, for greater precision, to have two such pairs at right angles to each other, or four microscopes in all.
To determine the position of the circle, and hence the direction in which the telescope at tached to it points, the observer looks into one of the microscopes and fixes upon some grad uation of the circle, turns the micrometer screw till a spider line, or the middle of a pair of lines, is central on the graduation, and then reads the indication of the head of the screw.
It will be noticed that in Fig. 5 the mark 21° is central under the miscroscope. By pointing his telescope on one star after another and reading the microscope in this way, noting on each occasion what graduation is read, the distance through which the telescope is moved, and therefore the angles between the stars, are measured with the highest precision.
The Clock.— The astronomical clock does not differ greatly in its construction from the ordinary clock. Its arrangement and the num bers on its face are, however, adapted to the measures of time used by astronomers. Mean solar time, which is the time we make use of in the affairs of daily life, is also used by the astronomer with a slightly different arrange ment. Instead of the hours being designated as A.M. and P.M., the astronomer counts through the whole 24 hours. Moreover, the count does not begin at midnight, but at noon, which is therefore the commencement of the astronom ical day. For purely scientific purposes this is the natural time to begin the day, because it is by the passage of the sun across the meridian. Therefore, any day of the month, as used by the astronomer, continues until noon of the following day, when a new day begins. For this reason the hour hand of his clock only makes one revolution in the 24 hours, the hours being numbered from 0 to 23. The as tronomer makes use of a second system of time, entirely different from that used in daily life, being based on the apparent diurnal move ment of the stars. We have explained, in the preceding article (ASTRONOMY) that the time between two passages of the same star over the meridian of a place is not quite 24 hours, but nearly four minutes less. This is the true time of rotation of the earth on its axis, be cause, in consequence of the advance of the earth in its orbit, it must go through a little more than its true revolution in order that the meridian of any place on its surface— that of Washington for example — may again pass directly under the sun. The astronomer there fore uses a °sidereal which is shorter than the day determined by the sun inthe proportion 365.24:366.24. This day is divided into 24 sidereal hours, and each hour into sidereal minutes and seconds, according to the usual system. A sidereal clock is regulated so as to gain about 3 m. 56 s. every day on our ordinary clocks and, in this way, keep time with the apparent diurnal movement of the fixed stars. It is so set that it shall read 0 h., 0 m., 0 s. at the moment when the vernal equinox crosses the meridian. As any of us, by looking at the clock, can tell, by the time of day, whether the sun is in the east, south or west, so' the astronomer, by his sidereal clock, can tell in what part of its apparent diurnal course any star may be situated. For ex ample, at 5 h. he knows th4t the constellation Auriga is on the meridian, and at 18 h. 34 m. that the beautiful Lyra is crossing the meridian.