and S' are the foci of the ellipse; AC is the major axis; BD, the minor or conjugate axis; O, the centre: or, astronomically, OA is the semi-axis-major or mean distance, OB the semi-axis-minor: the ratio of OS to OA is the eccentricity; the least distance, SA, is the perihelion distance; the greatest distance, SC, the aphelion distance.

Fig. 14.

B

S

D

AN ELLIPSE.

CHARACTERISTICS OF PLANETARY ORBIT.—It will not be difficult to follow in the mind the additional

Fig. 15.

PLANETARY ORBITS.

characteristics of a planet's orbit. The orbit or ellipse just given is laid on a plane surface. Now,

incline it slightly, as compared with some other fixed plane ring, as in the cut. The astronomical fixed plane is the ecliptic. Imagine a planet following the inclined ellipse; at some point it must rise above the level of the fixed plane: this point is called the ascending node, and the opposite point of intersection is termed the descending node. A line connecting the two nodes is called the line of the nodes. The longitude of the node is its distance from the first point of Aries, measured on the ecliptic, eastward. In this way we can get a very correct idea of a planetary orbit in space.

COMPARATIVE SIZE OF PLANETS. (Chambers.)-The following scheme will assist in obtaining a correct notion of the magnitude of the planetary system. Choose a level field or common; on it place a globe two feet in diameter for the Sun: Vulcan will then be represented by a small pin's head, at a distance of about 27 feet from the centre of the ideal sun; Mercury by a mustard-seed, at a distance of 82 feet; Venus by a pea, at a distance of 142 feet; the Earth, also, by a pea, at a distance of 215 feet; Mars by a small pepper-corn, at a distance of 327 feet; the minor planets by grains of sand, at distances varying from 500 to 600 feet. If space will permit, we may place a moderate-sized orange nearly one-quarter of a mile distant from the starting point to represent Jupiter; a small orange twofifths of a mile for Saturn; a full-sized cherry threequarters of a mile distant for Uranus; and lastly, a

plum 14 miles off for Neptune, the most distant planet yet known. Extending this scheme, we should find that the aphelion distance of Encke's comet would

be at 880 feet; the aphelion distance of Donati's comet of 1858 at 6 miles; and the nearest fixed star at 7,500 miles.

According to this scale, the daily motion of Vulcan in its orbit would be 4 feet; of Mercury, 3

Earth, 13 feet; of inches; of Saturn, and of Neptune, 4

feet; of Venus, 2 feet; of the Mars, 1 feet; of Jupiter, 10 7 inches; of Uranus, 5 inches; inches. This illustrates the fact that the orbital velocity of a planet decreases as its distance from the sun increases.

CONJUNCTIONS OF PLANETS.-The grouping together of two or more planets within a limited area of the heavens is a rare event. The earliest record we have is the one of Chinese origin, already mentioned on page 16, wherein it is stated that a conjunction of Mars, Jupiter, Saturn, and Mercury occurred in the

VENUS AND JUPITER IN CONJUNCTION, JANUARY 30, 1868.

reign of the Emperor Chuenhio. Astronomers tell us that this actually took place Feb. 28, 2446 B. C., and that they were between 10° and 18° of Pisces. This was before the Deluge, so that the fact must

have been afterward calculated and chronicled in their records. In 1859, Venus and Jupiter came so near each other that they appeared to the naked eye as one object. In 1725, Venus, Mercury, Jupiter, and Mars appeared in the same field of the telescope.

ARE THE PLANETS INHABITED?--This question is one which very naturally arises, when we think of the planets as worlds in so many respects similar to our own. We can give no satisfactory answer. Many think that the only object God can possibly have in making any world is to form an abode for man. Our own earth was evidently fitted up, although perhaps not created, for this express purpose. Everywhere about us we find proofs of special forethought and adaptation. Coal and oil in the earth for fuel and light, forests for timber, metals in the mountains for machinery, rivers for navigation, and level plains for corn. Our own

bodies, the air, light, and heat are all fitted to each other with exquisite nicety. When we turn to the planets, we do not know but God has other races of intelligent beings who inhabit them, or even entirely different ends to attain. Of this, however, we are assured, that, if inhabited, the conditions on which life is supported vary much from those familiar to us. We shall notice these more especially as we speak of the different planets. We shall see (1) how they differ in light and heat, from seven times our usual temperature to less than Too; (2) in the intensity of the force of gravity, from 2 times that of