the equinoxes, so that this designation of a star is a variable quantity. Hence the special names which we have described are the most convenient to use in the case of well-known stars. In other cases a star is designated by its number in some well-known catalogue. But even here different astronomers choose different catalogues, so that there are still different designations for the same star. The case is one in which uniformity of practice is unattainable.

CHAPTER IV

CATALOGUING AND NUMBERING THE STARS

Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion? Canst thou bring forth Mazzaroth in his season? Or canst thou guide Arcturus with his sons?-JOB.

A

CATALOGUE or list of stars is a work giving for each star listed its magnitude and its position on the celestial sphere, with such other particulars as may be necessary to attain the object of the catalogue. If the latter includes only the more conspicuous stars, it is common to add the name of each star that has one; if none is recognised, the constellation to which the star belongs is frequently given.

The position of a star on the celestial sphere is defined by its right ascension and declination. These Right As- correspond to the longitude and latitude of cension and places on the earth in the following way : Declination. Imagine a plane passing through the centre of the earth and coinciding with its equator, to extend out so as to intersect the celestial sphere. The line of intersection will be a great circle of the celestial sphere, called the celestial equator. The axis of the earth, being also indefinitely extended in both the

north and the south directions, will meet the celestial sphere in two opposite points, known as the north and south celestial poles. The equator will then be a great circle 90° from each pole. Then as meridians. are drawn from pole to pole on the earth, cutting the equator at different points, so imaginary meridians are conceived as drawn from pole to pole on the celestial sphere. Corresponding to parallels of latitude on the earth we have parallels of declination on the celestial sphere. These are parallel to the equator, and become smaller and smaller as we approach either pole. The correspondence of the terrestrial and celestial circles is this:

To latitude on the earth's surface corresponds declination in the heavens.

To longitude on the earth corresponds right ascension in the heavens.

A little study of this system will show that the zenith of any point on the earth's surface is always in a declination equal to the latitude of the place. For example, for an observer in Philadelphia, in 40° latitude, the parallel of 40° north declination will always pass through his zenith, and a star of that declination will, in the course of its diurnal motion, also pass through his zenith.

So also to an observer on the equator the celestial equator always passes through the zenith and through the east and west points of the horizon.

In the case of the right ascension, the relation between the terrestrial and celestial spheres is not constant, because of the diurnal motion, which keeps the

terrestrial meridians in constant revolution relative to the celestial meridians. Allowing for this motion, however, the system is the same. As we have on the earth's surface a prime meridian passing from pole to pole through the Greenwich Observatory, so in the heavens a prime meridian passes from one celestial pole to the other through the vernal equinox. Then to define the right ascension of any star we imagine a great circle passing from pole to pole through the star, as we imagine one to pass from pole to pole through a city on the earth of which we wish to designate the longitude. The actual angle which this meridian. makes with the prime meridian is the right ascension of the star, as the corresponding angle is the longitude of the city on the earth's surface.

There is, however, a difference in the unit of angular measurement commonly used for right ascensions in the heavens and longitude on the earth. In astronomical practice, right ascension is very generally expressed by hours, twenty-four of which make a complete circle, corresponding to the apparent revolution of the celestial sphere in twenty-four hours. The reason of this is that astronomers determine right ascension by the time shown by a clock so regulated as to read oh. om. os. when the vernal equinox crosses the meridian. The hour-hand of this clock makes a revolution through twenty-four hours during the time that the earth makes one revolution on its axis, and thus returns to oh. om. os. when the vernal equinox again crosses the meridian. A clock thus regulated is said to show sidereal time.

Then

the right ascension of any star is equal to the sidereal time at which it crosses the meridian of any point on the earth's surface, Right ascension thus designated in time may be changed to degrees and minutes by multiplying by 15. Thus, one hour is equal to 15°; one minute of time is equal to 15' of arc, and one second of time to 15" of arc.

It may be remarked that in astronomical practice terrestrial longitudes are also expressed in time, the longitude of a place being designated by the number of hours it may be east or west of Greenwich. Thus, Washington is said to be 5h. 8m. 15s. west of Greenwich. This, however, is not important for our present purpose.

Ancient and

Mediæval Catalogues

of Stars.

The first astronomer who attempted to make a catalogue of all the known stars is supposed to be Hipparchus, who flourished about 150 B.C. There is an unverified tradition to the effect that he undertook this work in consequence of the appearance of a new star in the heavens, and a desire to leave on record, for the use of posterity, such information respecting the heavens in his time that any changes which might take place in them could be detected. This catalogue has not come down to us- at least not in its original form.

Ptolemy, the celebrated author of the Almagest, flourished A.D. 150. His great work contains the earliest catalogue of stars which we have. There seems to be a certain probability that this catalogue may either be that of Hipparchus adopted by Ptolemy