TRANSIT CIRCLE or MERIDIAN CIRCLE, a tele scope for measuring the time when a star passes across the meridian and at the same time measuring its altitude. These two measurements determine respectively the right ascension and declination, and hence the place of the star on the celestial sphere. Because of its fundamental importance in positional astronomy, and as the source of standard time furnished to the community, the transit circle is traditionally regarded as the leading instru ment, at least in the great national observatories; but in modern times important observatories have grown up whose work lies wholly in other directions. The transit circle looks rather like a big gun on a gun-carriage, and to the stranger the surprising thing about this kind of telescope is that it cannot be turned to all parts of the sky ; it only moves up and down in the north and south line, looking oat through a slit in the walls and roof. Nor can it be used for prolonged study; the observation is a snap-shot as the object passes rapidly across the field of view.
Suppose that we are going to remeasure the exact position of some star. We choose from the catalogues a star which is due to pass across the north-south line in a minute or so ; and by means of a setting circle we elevate the telescope to the proper angle to catch it. The aperture of the object-glass is probably 8 or 9 in., and it will easily show stars to the ninth magnitude. Whilst waiting, let us examine the field of view. Turning on a faint artificial illumina tion, we see stretched across the field a number of "wires" which are actually made of spider-web ; there is one horizontal wire which we can move up or down by turning a micrometer screw, and a considerable number of vertical wires (perhaps ten). Our star now enters the field—at the extreme right since the telescope inverts—and is seen travelling rapidly towards the left. We shift the telescope slightly so as to bring the star almost on to the horizontal wire, and then clamp the instrument at that altitude. Now the star is approaching the series of vertical wires ; as it passes each, we press a key which records the instant on a chrono graph on which the standard sidereal clock is recording the seconds.
Near the centre of the field there is an intermission which gives us a few moments for making the altitude observation. Turning the micrometer screw, we make the horizontal wire bi sect the star, repeating this two or three times to reduce acci dental errors. There is no time to read the micrometer head, but it is furnished with some printing or recording device which will keep a record of our settings. We now finish tapping off the pas sage over the vertical wires, and the star disappears from view.
We have next to go round to the side of the instrument and read four long microscopes which are viewing a graduated circle attached to the telescope. Their combined reading will ultimately tell us at what elevation the telescope was clamped; we must add on to that the reading of the micrometer head so as to include the extra displacement given to the wire to bring it on to the star.
The whole observation takes two minutes or less, but there is a great deal of work in store for the computer. He must sort out the taps on the chronograph belonging to this star, and also the records of the micrometer head. The error and rate of the stand ard clock must be worked up from the "clock stars" observed during the night. The large correction for refraction must be com puted and applied to the altitude. Various observations for ad justment will have been taken, and from these the collimation, azimuth, level, and zenith-point corrections must be deduced and applied. There will be further corrections, not specially con nected with the instrument, to be included before the final place of the star is obtained in the form of mean right ascension and declination. This typical procedure may be varied somewhat. The most important modification is the use of the travelling-wire micrometer (see MICROMETER), which is now generally employed in first-rate work. Attempts have been made to substitute some photographic or photoelectric method for visual observation.
In olden times the altitude observation and the time observa tion were made with different instruments, called respectively the mural circle and the transit instrument. The mural circle is obso lete; but the transit instrument survives as a small portable in strument used for determining longitudes and for the most accu rate determinations of time. The transit instrument is virtually a transit circle shorn of its graduated circles and microscopes.
The transit circle is used for determining the positions day by day of the sun, moon and planets. For stellar work its scope is limited to the brighter stars, roughly those brighter than mag. 9.o (there are more than Ioo,000 of these). There is no advantage in observing fainter stars with it, since the work can be done more speedily and accurately by photography; but photographic meas ures are relative, and the positions can only be made absolute if the photograph contains some "reference stars" whose absolute positions are already known from meridian work. Thus transit circle observation is extended only so far as to provide accurate positions of a reasonable number of reference stars on all photo graphs.