the earth to less than; (3) in the constitution of the planet itself, from a density heavier than that of the earth to one nearly that of cork. The temperature sweeps downward through a scale of over

SIZE OF SUN AS SEEN FROM THE PLANETS.

2,000° in passing from Mercury to Uranus. No human being could reside on the former, while we

cannot conceive of any polar inhabitant who could endure the intense cold of the latter. At the sun, one of our pounds would weigh 27 pounds; on our moon the pound weight would become only about 2 ounces; while on Vesta, one of the planetoids, a man could easily spring sixty feet in the air and sustain no shock. Yet while we speak of these peculiarities, we do not know what modification of the atmosphere or physical features may exist even on Mercury to temper the heat, or on Uranus to reduce the cold. With, however, all these diversities, we must admit the power of an all-wise Creator to create beings adapted to the life and the land, however different from our own. The Power that prepared a world for us, could as easily and perfectly prepare one for other races. May it not be that the same love of diversity, which will not make two leaves after the same pattern nor two pebbles of the same size, delights in worlds peopled by races as diverse? While, then, we cannot affirm that the planets are inhabited, analogy would lead us to think that they are, and that the most distant star that shines in the arch of heaven is filled with living beings under the care and government of Him who enlivens the densest forest with the hum of insects, and populates even a drop of water with its teeming millions of animalculæ.

DIVISIONS OF THE PLANETS.-The planets are divided into two classes: (1) Inferior, or those whose

orbits are within that of the earth-viz., Mercury, Venus; (2) Superior, or those whose orbits are beyond that of the earth-Mars, Jupiter, Saturn, Uranus, Neptune.

MOTIONS OF A PLANET AS SEEN FROM THE SUN.Could we stand at the sun and watch the movements of the planets, they would all be seen to be revolving with different velocities in the order of the zodiacal signs. But to us, standing on one of the planets, itself in motion, the effect is changed. To an observer at the sun all the motions would be real, while to us many are only apparent. The position of a planet, as seen from the centre of the sun, is called its heliocentric place; as seen from the centre of the earth, its geocentric place. When Venus is at inferior conjunction, an observer at the sun would see it in the opposite part of the heavens from that in which it would appear to him if viewed from the earth.

MOTIONS OF AN INFERIOR PLANET.-An inferior planet is never seen by us in the part of the sky opposite to the sun at the time of observation. It cannot recede from him as much as 90°, or the circumference, since it moves in an orbit entirely enclosed by the orbit of the earth. Twice in every revolution it is in conjunction ( 6 ) with the sun,—an inferior conjunction (A) when it comes between the earth and the sun, and a superior conjunction (B) when the sun lies between it and the earth. (See Fig. 19.)

When the planet attains its greatest distance east or west (as we see it) from the sun, it is said to be at its greatest elongation, or in quadrature (□).

E

QUADRATURE AND CONJUNCTION.

When passing from B to A it is east of the sun, and from A to B it is west of the sun. When east of the sun, it sets later than the sun, and hence is " evening star: when west of the sun, it rises earlier than the sun, and hence is "morning star." An inferior planet is never visible when in superior conjunction, as its light is then lost in the greater brilliancy of the sun.

When in inferior conjunction, it sometimes passes in front of the sun, and appears to us as a round black spot swiftly moving across his disk. This is called a transit.

Retrograde motion of an inferior planet. Suppose the earth to be at A, and the planet at B. Now, while the earth is passing to F, the planet will pass to D-the arc AF being shorter than BD, because the nearer a planet is to the sun the greater its velocity. While the planet is at B, we locate it a C on the ecliptic, in Gemini; but at D, it appears to us to be at G, in Taurus. So that the planet has retrograded through an entire sign on the ecliptic, while its course all the while has been directly for