globe from east to west, we shall exactly represent the apparent motions of the heavens to a spectator on the Earth supposed to occupy the centre of the globe, as in Figs. 22-24. The wooden horizon will represent the true horizon; and why some stars never set and others never rise in these latitudes, will at once be apparent.

341. At the present time the northern celestial pole lies in Ursa Minor, and a star in that constellation very nearly marks the position of the pole, and is therefore called polaris, or the pole-star. We shall see further on (Lesson XLIII.), that the direction in which the Earth's axis points is not always the same, although it varies so slowly that a few years do not make much difference. As a consequence, the position of the celestial pole, which is defined by the Earth's axis prolonged in imagination to the stars, varies also. One of the most striking circumpolar constellations is Ursa Major (the Great Bear), the 'Plough, or Charles' Wain, as it is otherwise called. Two stars in this are called the pointers, as they point to the polestar, and enable us at all times to find it easily.

The other more important circumpolar constellations are Cassiopea, Cepheus, Cygnus, Draco, Auriga (the brightest star of which, Capella, is very near the horizon when below the pole), and Perseus. The principal southern circumpolar constellations which never rise in this country, are Crux, Centaurus, Argo, Ara, Lepus, Eridanus, and Dorado. Nearly all the other constellations mentioned in Arts. 37–39 belong to Class III.

342. If, then, we would watch the heavens on a clear night from hour to hour, to get an idea of these apparent motions, we may best accomplish this either by looking eastward to see the stars rise, westward to see them set, northward to the pole to watch the circular movement round that point. If, for instance, we observe the Great Bear, we shall see it in six hours advance from one

of its positions shown in the accompanying figure, to

the next.

343. As the Earth's rotation is accomplished in 23h. 54m. 56s., it follows that the apparent movement of the celestial sphere is completed in that time; and were there no clouds, and no Sun to put the stars out in the day-time-eclipsing them by his superior brightness-we

Fig. 27.-The Constellation of the Great Bear, in four different positions, after intervals of 6 hours, showing the effect of the apparent revolution of the celestial sphere upon circumpolar stars.

should see the grand procession of distant worlds ever defiling before us, and commencing afresh after that period of time.

This leads us to the effect of the Earth's yearly journey round the Sun upon the apparent movement of the

stars.

LESSON XXVIII.--POSITION OF THE STARS SEEN AT MIDNIGHT. DEPENDS UPON THE TWO MOTIONS OF THE EARTH. HOW TO TELL THE STARS.

CELESTIAL GLOBE. STAR-MAPS. THE EQUATORIAL CONSTELMETHOD OF ALIGNMENTS.

LATIONS.

344. We see stars only at night, because in the daytime the Sun puts them out; consequently the stars we see on any night are the stars which occupy that half of the celestial sphere opposite to the point in it occupied at that time by the Sun. We have due south, at midnight, the very stars which occupy the celestial meridian 180° from the Sun's place, as the Sun, if it were not below the horizon in England, would be seen due north.

345. Now as we go round the Sun, we are at different times on different sides, so to speak, of the Sun; and if we could see the stars beyond him, we should see them change; but what we cannot do at mid-day, in consequence of the Sun's brightness, we can easily do at midnight; for if the stars behind the Sun change, the stars exactly opposite to his apparent place will change too, and these we can see in the south at midnight.

346. It is clear, in fact, that in one complete revolution of the Earth round the Sun every portion of the visible celestial sphere will in turn be exposed to view in the south at midnight; and as the revolution is completed in 365 days, and there are 360° in a great circle of the sphere, we may say broadly that the portion of the heavens visible in the south at midnight advances 1° from night to night, which 1o is passed over in 4 minutes, as the whole 360° are passed over in nearly 24 hours.

847. This advance is a consequence of the difference between the lengths of the day as measured by the fixed stars and by the moving Sun, as we shall explain presently. We may here say, that as the solar day is longer than the sidereal one, the stars by which the latter is measured gain upon the solar day at the rate we have seen; so that, as seen at the same hour on successive nights, the whole celestial vault advances to the westward, the change due to one month's apparent yearly motion being equal to that brought about in two hours by the apparent daily motion.

348. Hence the stars south at midnight (or opposite the Sun's place) on any night, were south at 2 A.M. a month previously, and so on; and will be south a month hence at 10 o'clock P.M., and so on.

349. The best way to obtain a knowledge of the various constellations and stars is to employ a celestial globe. We first, as seen in Arts. 337-9, place its brass meridian in the plane of the meridian of the place in which the globe is used, and make the axis of the globe parallel to the axis of the Earth, and therefore of the heavens, by elevating the north pole (in our case) until its height above the wooden horizon is equal to the latitude of the place. We next bring under the brazen meridian the actual place in the heavens occupied by the Sun at the time; this place is given for every day in the almanacs. We thus represent exactly the position of the heavens at mid-day, by bringing the Sun's place to the brazen meridian, and the index is then set at 12. The reason for this is obvious; it is always 12, or noon, at a place when the Sun is in the meridian of that place. We then, if the time at the place is after noon, move the globe on till the index and the time correspond; if the time is before noon, we move the globe back—that is, from east to west, till the index and time correspond in like manner.

350. When the globe has been rectified, as it is called, in this manner, we have the constellations which are rising on the eastern horizon, just appearing above the eastern part of the wooden horizon. Those setting are similarly near the western part of the wooden horizon. The constellations in the zenith at the time will occupy the highest part of the globe, while the constellations actually on the meridian will be underneath the brazen meridian of the globe.

351. Further, it is easy at once to see at what time any stars will rise, culminate, or set, when the globe is rectified in this manner. All that is necessary is, as before, to bring the Sun's place, given in the almanac, to the meridian, and set the index to 12. To find the time at which any star rises, we bring it to the eastern edge of the wooden horizon, and note the time, which is the time of rising. To find the time at which any star sets, we bring it similarly to the western edge of the wooden horizon and note the time, which is the time of setting. To find the time at which any star culminates, or passes the meridian, we bring the star under the brass meridian and note the time, which is the time of meridian passage.

352. In the absence of the celestial globe, some such table as the following is necessary, in which are given the positions occupied by the constellations at stated hours during each month in the year. When the positions of the constellations are thus known, some star-maps (the small ones published by the Society for Promoting Useful Knowledge are amply sufficient) should be referred to, in which various bright stars which go to form each constellation should be well studied; the constellation should then be looked for in the position indicated by the table, in the sky itself. When any constellation is thus recog nised, the star-map should again be studied, in order that the stars in its vicinity may next be traced.