in lat. 45° its altitude was 45°. In London, therefore, in lat. 5110, its altitude will be 51°, and hence stars of less than that distance from the pole will always be visible, as they will be above the horizon when passing below the pole. All the stars, therefore, within 51° of the north pole will form Class I.; all those within 51° of the south pole Class II.; and the remainder—that is, all stars from lat. 39° N (90°-51°=39°) to 39° S.-will form Class III.

337. In these and similar inquiries the use of the terrestrial and celestial globes is of great importance in clearing our ideas.

To use either properly we must begin by making each a counterpart of what is represented—that is, the north pole must be north, the south pole south, and moreover the axis of either globe must be made parallel with the Earth's axis.

338. This is accomplished generally by the use of a compass, the indication of that instrument being corrected by its known variation. This variation at present is about 18° to the west of the true north; therefore the true north lies 18° to the east of magnetic north, and the brazen meridian of the globe must be placed accordingly. Secondly, the wooden horizon of the globe must be level; it will then represent the horizon of the place.

339. This done, the pole—the north pole in our case— must be elevated to correspond with the latitude of the place where the globe is used. At the poles this would be 90°, at the equator oo, and at London 5110.

340. If we then turn the terrestrial globe from west to east, we shall exactly represent the lie, and the direction of motion of the Earth. If we then turn the celestial 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. 32-34. 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.

M

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

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

FIG. 35.-Southern Circumpolar Constellations. (These constellations are invisible in high and middle north latitudes, and therefore in Europe.)

varies also.

One of the most striking circumpolar con

stellations is Ursa Major (the Great Bear), the Plough, or Charles' Wain, as it is otherwise called.

Two stars

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,

FIG. 36.-The Northern Circumpolar Constellations.

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 Cassiopeia, Cepheus, Cygnus, Draco, Auriga (the brightest star of which, Capella, is very near the horizon when below the pole), and Perseus. The principal

FIG. 37.-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.

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 eastwards to see the stars rise, westward to see them set, or 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 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 Constellations. Method of Align

ments.

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