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'which are nearest the sun, and least at the parts which are most distant from it; in other words, they moTe quickest in perihelion, and slowest in aphelion.
COMPARISON OF THE TWO GROUPS OF THE MAJOR
Planets. (Chambers.)—Separating the major planets into two groups, if we take Mercury, Venus, the Earth, and Mars as belonging to the interior, and Jupiter, Saturn, Uranus, and Neptune to the exterior group, we shall find that they differ in the following respects:
1. The interior planets, with the exception of the earth, and Mars (p. 171), are not attended by any satellite, while the exterior planets all have satellites. We can but consider this as one of the many instances to be met with, in the universe, of the beneficence of the Creator, and that the satellites of these remote planets are designed to compensate for the small amount of light their primaries receive from the sun, owing to their great distance from that luminary.
2. The average density of the first group considerably exceeds that of the second, the approximate ratio being 5 :1.
3. The mean duration of the axial rotations, or mean length of the day of the interior planets, is much longer than that of the exterior; the average in the former case being about twenty-four hours, but in the latter only about ten hours.
The Propertieb Of The Ellipse.—In the figure, S and S' are the foci of the ellipse; AC is the major axis; BD, the minor or conjugate axis; 0, 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.
Charaoteristics Of Planetary Orbit.—It will not be difficult to follow in the mind the additional
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 mnst rise aboTe 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
plain 1| 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
According to this scale, the daily motion of Vulcan in its orbit would be 4§ feet; of Mercury, 3 feet; of Venus, 2 feet; of the Earth, 1J feet; of Mars, 1£ feet; of Jupiter, 10£ inches; of Saturn, 1\ inches; of Uranus, 5 inches; and of Neptune, 4 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
reign of the Emperor Chuenhio. Astronomers tell ns that this actually took place Feb. 28, 2446 B. o., and that they were between 10° and 18° of Pisces. This was before the Deluge, so that the fact must