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LESSON XLII.-THE TRANSIT CIRCLE AND ITS ADJUSTMENTS. PRINCIPLES OF ITS USE. METHODS OF TAKING TRANSITS. THE CHRONOGRAPH. THE EQUATORIAL.

524. When we wish to determine directly the position of a celestial body on the celestial sphere itself, a transit circle is almost exclusively used. This instrument consists of a telescope moveable in the plane of the meridian, being supported on two pillars, east and west, by means of a horizontal axis. The ends of the horizontal axis are of exactly equal size, and move in pieces, which, from their shape, are called Y s. When the instrument is in perfect adjustment, the line of collimation of the telescope is at right angles to the horizontal axis, the axis is exactly horizontal, and its ends are due east and west. Under these conditions, the telescope describes a great circle of the heavens passing through the north and south points and the celestial pole; in other words, the telescope in all positions points to some part of the meridian of the place. On one side of the telescope is fixed a circle, which is read by microscopes fixed to one of the supporting pillars. The cross wires in the eye-piece of the telescope enable us to determine the exact moment of sidereal time at which the meridian is crossed: this time is, in fact, the right ascension of the object. The circle attached shows us its distance from the celestial equator: this is its declination. So by one observation, if the clock is right, if the instrument be perfectly adjusted, and if the circle be correctly divided, we get both co-ordinates.

In Plate XV. is given a perspective view of the great

transit circle at Greenwich Observatory, designed by the present Astronomer Royal, Mr. Airy. It consists of two massive stone pillars, supporting the ends of the horizontal axis of the telescope, which rest on Y s, as shown in the case of one of the pivots in the drawing. Attached to the cube of the telescope (to which the two side-pieces, the eye-piece end and object-glass end, are screwed) are two circles. The one to the right is graduated, and is read by microscopes pierced through the right-hand pillar; the eye-pieces of these microscopes are visible to the right of the drawing. The other circle is used to fix the telescope, or to give it a slow motion, by means of a long handle, which the observer holds in his hand. The eyepiece is armed with a micrometer, with nine equidistant vertical wires and two horizontal ones.

The wheels and counterpoises at the top of the view are to facilitate the raising of the telescope when the collimators, both of which are on a level with the centre of the telescope-one to the north and one to the south -are examined.

525. As we have already seen (Art. 329), a celestial meridian is nothing but the extension of a terrestrialone; and as the latter passes through the poles of the Earth, the former will pass through the poles of the celestial sphere: consequently, in England the northern celestial pole will lie somewhere in the plane of the meridian. If the position of the pole were exactly marked by the pole-star, that star would remain immoveable in the meridian; and when a celestial body, the position of which we wished to determine, was also in the meridian, if we adjusted the circle so that it read o° when the telescope pointed to the pole, all we should have to do to determine the north polar distance of the body would be to point the tele. scope to it, and see the angular distance shown by the circle.

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526. But as the pole-star does not exactly mark the position, we have to adopt some other method. We observe the zenith distance (Art. 329) of a circumpolar star when it passes over the meridian above the pole, and also when it passes below it, and it is evident that if the observations are perfectly made, half the sum of these zenith distances will give the zenith distance of the celestial pole itself. When we have found the position of the celestial pole, we can determine the position of the celestial equator, which we know is exactly 90° away from it. As we already know the zenith distance of the celestial pole, the difference between this distance and 90° gives us the zenith-distance of the equator. Here, then, we have three points from which with our transit circle we can measure angular distances :

I. From the zenith,

II. From the celestial pole,

III. From the celestial equator,

and we may add,

IV. From the horizon,

as the horizon is 90° from the zenith. Any of these distances can be easily turned into any other.

527. In this way, then, if we reckon from, or turn our measures into distances from, the celestial equator, we get in the heavens the equivalent of terrestrial latitude. But this is not enough: as we saw in the case of the Earth (Art. 161)-a thousand places on the Earth may have the same latitude-we want what is called another coordinate to fix their position. On the Earth we get this other co-ordinate by reckoning from the meridian which passes through the centre of the transit circle at

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