PLATE VII.—PARTIAL ECLIPSE OF THE MOON.
Observed October 24, 1874
The partial eclipse of the Moon, represented on Plate VII., shows quite plainly the configuration of our satellite as seen with the naked eye during the eclipse, with its bright and dark spots, and its radiating streaks. This eclipse was observed on October 24th, 1874.
THE PLANETS
Around the Sun circulate a number of celestial bodies, which are called "Planets." The planets are opaque bodies, and appear luminous because their surfaces reflect the light they receive from the Sun.
The planets are situated at various distances from the Sun, and revolve around this body in widely different periods of time, which are, however, constant for each planet, so far as ascertained, and doubtless are so in the other cases.
The ideal line traced in space by a planet in going around the Sun, is called the orbit of the planet; while the period of time employed by a planet to travel over its entire orbit and return to its starting point, is called the sidereal revolution, or year of the planet. The dimensions of the orbits of the different planets necessarily vary with the distance of these bodies from the Sun, as does also the length of their sidereal revolution.
The distance of a planet from the Sun does not remain constant, but is subject to variations, which in certain cases are quite large. These variations result from the fact that the planetary orbits are not perfect circles having the Sun for centre, but curves called "Ellipses," which have two centres, or foci, one of which is always occupied by the Sun. This is in accordance with Kepler's first law.