In order that the line of sight of the in strument may describe the true meridian, it is necessary that, when the instrument is turned in the proper direction, the line shall pass through the celestial pole. This is effected by the following arrangement: In the course of its apparent diurnal motion, a star near the pole will cross the meridian of any place twice in the course of a sidereal day, first above the pole and then below it. Let the dotted circle in Fig. 11 be its apparent diurnal circuit around the pole P. Let the vertical line M R be the true meridian passing through the pole, and the other line A B that marked out by the line of sight of the transit instrument, supposed not to be exactly in the meridian. Then the star will take a less time in passing around from A to B on the left than in the other part of its course from B to A. Therefore by ob serving the transit both above and below the pole, across the middle thread of the instru ment, the observer determines whether the line of sight of the instrument passes east or west of the pole and may adjust it accordingly. It may be said that, in astronomical practice, no instrument is ever assumed to be perfectly ad justed. The clock of the astronomer is never assumed to be correct, nor his transit instru ment to be in the true meridian. What he does is, assuming them wrong, to make his observations, determine the errors, and correct his observations accordingly. This is called the observation. We have already explained that, when a star is exactly in the same hour circle with the vernal equinox, its right ascension is 0 h., 0 m., 0 s. Since the clock, assumed to be correct, then reads exactly 0 h., it follows that the star in question will cross the meridian at this time by the clock. Then, as the sphere revolves, the right ascen sions of the stars are all equal to the sidereal time at which they cross the meridian. Thus the observer, bynoting these times, measures the right ascensions of the heavenly bodies. This system of using the clock instead of a divided circle for determining right ascensions constitutes one of the greatest advances ever made in astronomical measurement. It de pends upon the perfect uniformity of the earth's rotation and the excellence with which a clock can be made.
In this manner we may obtain the right ascension of any star. If we suppose that the error of the clock is known and that the mid dle wire is exactly in the meridian, the cor rected (sidereal) clock time when the star is bisected by the middle wire will be precisely the same as the right ascension of the star. The clock error is determined by observing the transits of a selected number of funda mental stars whose right ascensions are as sumed to be known; the deviations of the middle wire from the meridian are also deter mined from these transits, and finally, the combined effect of these errors and the error of level is readily computed and applied to the recorded time of transit of the star whose right ascension is desired.
A far more troublesome source of error, however, and one which it is exceedingly difficult to completely eliminate is known to astronomers as the Personal Equation. By this is meant the difference between the time when the star is actually bisected and the time at which the observer presses the chrono graphic key. The latter is usually later by about 02 second, an amount which approxi mates to the ((Reaction time" of the psycholo gists, but this is by no means always the case.
Moreover, as the apparent motion of equatorial stars across the field is very rapid compared with that of stars near the pole, there occurs a marked variation in the observer's personal equation, depending on the declination of the star observed. It is found that a minute vari ation also occurs which depends upon the brightniss of the star, and finally, the personal equation varies with the physical condition, fatigue, etc., of the observer, though with an observer of long experience who takes care to work as far as is possible only under normal conditions, the last source of error should be slight.
As meridian work is the most important and fundamental of all observation for the purposes of exact astronomy, every effort is made to either determine or eliminate the ef fects of personal equation. In the United States Naval Observatory, it is directly determined for each observer by means of an artificial star which is made to cross the field at all varying rates of speed and automatically to record upon the chronograph the instants when it was bisected by each wire; the ob server's records appear upon the same sheet, whence the personal equation follows by a di rect measurement. Here also a screen is placed before the objective of the telescope by which all stars observed are made to appear equally bright during the transit. There are two devices already coming into considerable use which almost completely eliminate this troublesome source of error. In the first, called the Transit Micrometer, a moving wire is kept accurately bisecting the star as the lat ter crosses the field. The frame which carries the movable wire automatically makes an electric contact and records on the chrono graphic sheet when this movable wire reaches the successive transverse wires indicated in Fig. 10, which latter are therefore omitted. The second device is the Photo-chronograph. Here the eye is replaced by a small photo graphic plate on which the star photographs itself as a straight line, or An elec tro-magnet is so arranged that at every sec ond or fourth beat of the clock the plate is displaced slightly upward or downward; the trail thus appears on the developed plate as a series of short dashes arranged alternately in two parallel lines. The image of the wires is afterward obtained by holding a light for a few seconds in front of the object glass. By a later measurement of the developed plate under a microscope a result is obtained which is practically free from the effects of personal equation.
The Meridian Circle is the transit instru ment, just described, with one or two grad uated circles on its axis of rotation. The method of using it and determining the arc through which the circle has moved at any time has already been explained. The in quiring reader may wish to know how, by such readings, the astronomer can determine the declination of stars. If the celestial pole were a visible point in the heavens this would be very simple; the observer would turn his instrument until it pointed exactly at the pole and then read his microscopes. Then as one star after another crossed the meridian he would make a similar pointing, reading his microscope for the transit of each star. The difference between the reading on the pole and that on the different stars would show their distances from the pole. Subtracting each of these from 90° would give the declina tion of the stars as seen in the instrument.