Although the number of solar eclipses occurring at some point or other of the Earth's surface is greater than that of the eclipses of the Moon, yet at any single terrestrial station the eclipses of the Moon are the more frequent. While an eclipse of the Sun is only visible on a narrow belt, which is but a very small fraction of the hemisphere then illuminated by the Sun, an eclipse of the Moon is visible from all the points of the Earth which have the Moon above their horizon at the time. Furthermore, an eclipse of the Sun is not visible at one time over the whole length of its narrow tract, but moves gradually from one end of it to the other; while, on the contrary, an eclipse of the Moon begins and ends at the very same instant for all places from which it can be seen, but, of course, not at the same local time, which varies with the longitude of the place. [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.