1862 to 1865.

which the south latitude decreases again.

392. In Fig. 35 is represented the path of Saturn from 1862 to 1865. A comparison of this with the preceding figure shows how the distance of a planet from the Earth influences the shape of its path. In this case, as the planet's own motion is, unlike that of Venus, apparently slow, the Earth's circular motion is as it were reflected, and between each opposition we have a loop, the ends of which are represented by the stationary points.

Moreover, it will be seen that the planet during the time was north of the ecliptic, or in that part of its orbit situated above the plane of the Earth's motion round the Sun, and that the north latitude was increasing. Still,

F. 35.-Saturn's apparent Path from for all this, it was situated south of the equator, and

its south declination, or its distance south of that line, was increasing. Hence, year by year, although it is getting more above the ecliptic, it is getting more below

the equator. Let this be compared with what was said about the motion of the Moon in Art. 370, and it will be evident that when on the meridian the planet's height

above the horizon will decrease, until the planet itself reaches that part of the ecliptic 23° south of the equator

-in fact, until its position is near that occupied by the Sun in mid-winter.

393. The apparent path of a planet, then, is moulded, as it were, by the motions of the Earth and the inclina- ' tion of its own orbit. If we examine into the position of the orbit of Mars, for instance, more closely than we have hitherto done, we shall see how the ellipticity of the orbit and its inclination affect our observations of the

physical features. Fig. 36 shows the exact positions in space of the orbits of the Earth and Mars, and the amount and direction of the inclination of their axes, and the line of Mars' nodes: both planets are represented in the positions they occupy at the winter solstice of the northern hemisphere. The lines joining the two orbits indicate the positions occupied by both planets at successive oppositions of Mars, at which times, of course, Mars, the Earth, and the Sun are in the same straight line (leaving the inclination of Mars' orbit out of the question).

394. It is seen at a glance that at the oppositions of 1830 and 1860 the two planets were much nearer together than in 1867, or than they will be in 1869.

The figure also enables us to understand that in.the case of an inferior planet, if we suppose the perihelion of the Earth to coincide in direction, or, as astronomers put it, to be in the same heliocentric longitude as the aphelion of the planet, it will be obvious that the conjunctions which happen in this part of the orbits of both will bring the bodies nearer together than will the conjunctions which happen elsewhere. Similarly, if we suppose the aphelion of the Earth to coincide with the perihelion of a superior planet, as in the case of Mars, it will be obvious that the opposition which happens in that part of the orbit will be the most favourable for observation. The Earth's orbit, however, is practically so nearly circular that the variation depends more upon the eccen

The figure also shows us that when Mars is observed at the solstice indicated, we see the southern hemisphere of the planet better than the northern one; while at those

own.

tricity of the orbits of the other planets than upon our

Fig. 7-Explanation of the varying appearances of Saturn's rings.

oppositions which occur when the planet is at the opposite solstice, the northern hemisphere is most visible. But we see the northern hemisphere in the latter case better than we do the southern one in the former, because in the

Fig. 38.-Appearance of Saturn when the plane of the ring-system passes through the Earth.

latter case the planet is above the ecliptic, and we therefore see under it better; and in the former it is below the ecliptic, and we see less of the southern hemisphere than we should do were the planet situated in the ecliptic.

Fig. 39.-Saturn, as seen when the north surface of the rings is presented to the Earth.

395. Fig. 37 shows the same effect of inclination in the case of the rings of Saturn. The plane of the rings is inclined to the axis, and, like the axis, always remains parallel to itself. A study of the figure will show that