E-W. Leo, Gemini. Auriga, *Andromeda, Pegasus. NE-SW. Ursa Major, * Aries, Cetus.

SE-NW. Canis Major, Orion, Taurus, *Cassiopeia, Cepheus, Cygnus.

DEC. 21, 10 P.M. (Jan. 20, 8 P.M.; Nov. 22, midnight).
N-S. Hercules, Draco, polaris, *Perseus, Taurus,

Eridanus.

FIG. 38.-Equatorial Constellations, visible in the south on Jan. 20, at 10 P.M.

some of the constellations in the equatorial zone visible on Jan. 20th to the south.

The central constellation is Orion, one of the most marked in the heavens; when all the bright stars in this

353. In Fig. 36 some of the circumpolar constellations have already been represented. In Fig. 38 are given

the line formed by the three stars in the belt, if produced eastward, will pass near Sirius, the brightest star in the heavens.

asterism are known, many of the surrounding ones may easily be found, by means of alignments. For instance,

354. Fig. 39 represents in like manner the appearance of the heavens a little south of the zenith in May: the bright star Arcturus (a Boötis) being then nearly on the meridian. The constellation Hercules is easily recognised by means of the trapezium formed by four of its stars.

355. In Fig. 40 the square of Pegasus is a very marked object, and this once recognised in the sky, may, by means of star-maps, be made the start-point of many new alignments.

356. The first magnitude stars should be first known; then the second; and so on till the positions of all the brighter ones in the different constellations are impressed upon the memory-no difficult task after a little practice, and comparison of the sky itself with good small maps.

LESSON XXIX.-Apparent Motion of the Sun. Difference in Length between the Sidereal and Solar Day. Celestial Latitude and Longitude. The Signs of the Zodiac. Sun's apparent Path. How the Times of Sunrise and Sunset, and the Length of the Day and Night, may be determined by means of the Celestial Globe.

357. The effect of the Earth's daily movement upon the Sun is precisely similar to its effect upon the stars; that is, the Sun appears to rise and set every day; but in consequence of the Earth's yearly motion round it, it appears to revolve round the Earth more slowly than the stars; and it is to this that we owe the difference between star-time and sun-time, or, in other words, between the lengths of the sidereal and solar days.

358. How this difference arises is shown in Fig. 41, in which are seen the Sun, and the Earth in two positions in its orbit, separated by the time of a complete rotation. In the first position of the Earth are shown one observer, a, with the Sun on his meridian, and another, b, with a

star on his: the two observers being exactly on opposite sides of the Earth, and in a line drawn through the centres of the Earth and Sun. In the second position, when the same star comes to b's meridian, a sees the Sun still to the east of his, and he must be carried by the Earth's rotation to c before the Sun occupies the same apparent position in the heavens it

formerly did that is, before the Sun is again in his meridian. The solar day, therefore, will be longer than the sidereal one by the time it takes a to travel this distance.

Of course, were the Earth at rest, this difference could not have arisen, and the solar day is a result of the Earth's motion in its orbit, combined with its rotation.

359. Moreover, the Earth's motion in its orbit is not uniform, as we shall see subsequently; and, as a consequence, the apparent motion of the Sun is not uniform, and solar days are not of the same length; for it is evident that if the Earth sometimes travels faster, and therefore further, in the interval of one rotation than it does at others, the observer a has further to travel before he gets to c; and as the Earth's rotative movement is uniform, he requires more time. In a subsequent chapter it will be shown how this irregularity in the apparent motion of the Sun is obviated.

360. The apparent yearly motion of the Sun is so