Greenwich, which meridian passes through all places, whatever their latitude, north or south, which have the same longitude as Greenwich.

So in the heavens we reckon from the position occupied by the Sun at the vernal equinox. The astronomer not only has a telescope and a circle, but, as we have seen (Art. 425), he has a sidereal clock, adjusted to the apparent movement of the stars, actually to the Earth's rotation. On its dial are marked 24 hours. The time shown by this clock is called sidereal, or star time, and is so regulated that the exact interval between two successive passages of the same star over the meridian of the same place is divided into twenty-four sidereal hours, these into sixty sidereal minutes, and so on; and this time is reckoned, not from any actual star, but from the point in the heavens called "the first point of Aries," which we have mentioned; when that point is on the meridian of any place the sidereal clock shows oh. om. os., and then it goes on indicating the twentyfour sidereal hours till the same point comes on the meridian again.

528. Now it follows, that as right ascension and sidereal time are both reckoned from the first point of Aries, a sidereal clock at any place will denote the right ascension of the celestial meridian visible in the transit circle at that moment; and if we at the same moment, by means of the circle, note how far any celestial body is from the celestial equator, we shall know both its right ascension and declination, and its place in the heavens will be determined that is to say, the Earth itself, by its rotation, performs the most difficult part of the task for us, and every star will in turn be brought into the meridian of our place of observation; all we have to do is to note its angular distance either from the zenith or from the celestial equator, and note the sidereal

time one enables us to determine, or actually gives us, its declination; the other gives us its right ascension.

529. Of course, the method which is good for determining the exact place of a single heavenly body is good for mapping the whole heavens, and in this manner the position of each body has been determined, until the whole celestial sphere has been mapped out, the right ascension and declination of every object having been determined.

The most important of the catalogues in which these positions are contained is due to the German astronomer Argelander. This catalogue contains the positions of upwards of 324,000 stars, from N. Decl. 90° to S. Decl. 2°. Bessel also has published a catalogue of upwards of 32,000 stars. The Astronomer Royal and the British Association have also published similar lists. There are also catalogues dealing with double stars and variable stars exclusively.

530. In order that the angular distance from the zenith, and the time of meridian passage, may be correctly determined, observations of the utmost delicacy are required.

531. The circle of the Greenwich transit, for instance, is read in six different parts of the limb at each observation by the microscopes, the eye-pieces of which are shown in Plate XV., and the recorded zenith distance is the mean of these readings.

The other co-ordinate,—that is, the right ascension,— is obtained with equal care. The transit of the star is watched over nine equidistant wires, in the micrometer eye-piece (called in this case a transit eye-piece), the middle one being exactly in the axis of the telescope. The following table of some objects observed at Greenwich on Aug. 7, 1856, will show how the observation made at this central wire is controlled and corrected by the observations made at the other wires on either side of it.

NAME OF
OBJECT.

I.

Seconds of Transit over the Wires.

Concluded

Transit

over the

II. III IV. V. VI. VII. VIII. IX. Centre Wire.

532. There are two methods of observing the time of transit over a wire, one called the eye and ear method, the other the galvanic method. In the former, the observer, taking his time from the sidereal clock, which is always close to the transit circle, listens to the beats and estimates at what interval between each beat the star passes behind each wire. An experienced observer in this manner mentally divides a second of time into ten equal parts with no great effort.

533. In the second method a barrel covered with paper is made to revolve at an equal rate of speed. By means of a galvanic current, a pricker attached to the keeper of an electro magnet is made at each beat of the sidereal clock to make a puncture on the revolving barrel. The pricker is carried along the barrel, so that the line of punctures forms a spiral, the pricks being about half an inch apart. Here then we have the flow of time fairly recorded on the barrel. At the beginning of each minute the clock fails to send the current, so that there is no

confusion. What the clock does regularly at each beat the observer does when a star crosses the wires of his transit eye-piece. He presses a spring, and an additional current at once makes a puncture on the barrel. The time at which the transit of each wire has been effected is estimated from the position the additional puncture occupies between the punctures made by the clock at each second.

534. By this method, which is also termed the chronographic method, the apparatus used being called a chronograph, the observer is enabled to confine his attention to the star, and after observing with the telescope can at leisure make the necessary notes on the punctured paper, which is taken off the barrel when filled, and bound up as a permanent record.

535. With the transit circle the position of a body in the celestial sphere can only be determined when that object is on the meridian. The equatorial enables this to be done, on the other hand, in every part of the sky, though not with such extreme precision. The object is brought to the cross wires of the micrometer eye-piece, and the declination circle at once shows the declination of the object. The right ascension is determined as follows:-At the lower end of the polar axis is a circle divided into the 24 hours of right ascension. This circle is not fixed. Flush with the graduation are two verniers ; the upper one fixed to the stand, the lower one moveable with the telescope. The fixed vernier shows the position occupied by the telescope, and therefore by the moveable vernier, when the telescope is exactly in the meridian. Prior to the observation, therefore, the circle is adjusted so that the local sidereal time, or, in other words, the right ascension of the part of the celestial sphere in the meridian, is brought to the fixed vernier. The circle is then carried by the clockwork of the instru

S

ment, and when the cross wires of the telescope are adjusted on the object, the moveable vernier shows its right ascension on the same circle.

LESSON XLIII.-CORRECTIONS APPLIED TO OBSERVED PLACES. INSTRUMENTAL AND CLOCK ERRORS. CORRECTIONS FOR REFRACTION AND ABERRATION. CORRECTIONS FOR PARALLAX. CORRECTIONS FOR LUNI-SOLAR PRECESSION. CHANGE OF EQUATORIAL INTO ECLIPTIC CO-ORDINATES.

536. After the astronomer has made his observations of a heavenly body-and has freed them from instrumental and clock errors, if his telescope is not perfectly levelled or collimated, or his circle is not perfectly centred, or if the clock is either fast or slow-he has obtained what is termed the observed or apparent place. This, however, is worth very little he must, in order to obtain its true place, as seen from his place of observation, apply other corrections rendered necessary by certain properties of light. These properties have been before referred to in Arts. 450 and 451, and are termed the refraction and aberration of light. Refraction causes a heavenly body to appear higher the nearer it is to the horizon; in the zenith its action is nil; near the horizon it is very decided, so decided that at sunset, for instance, the sun appears above the horizon after it has actually sunk below it.

It will be seen, therefore, that refraction depends only upon the altitude of the body on the sphere of observation.

537. The correction for refraction is applied, therefore, by means of some such table as the following::