This view is shown inverted, as it appears in a telescope. Part of the terminator appears in the lower right-hand corner. On the plain within Plato appear the shadows of the peaks along its left-hand border.

shadows cast by mountains, the height of which is shown by the length of these shadows. In fact, our knowledge of the height of lunar mountains depends on the accurate measures which have been made of their shadows. Farther from the terminator the shadows are shorter, because the mountains which cast them are turned more directly towards the Sun; so that many lunar mountains cannot well be distinguished except when they are near the terminator. But certain bright streaks on the Moon can be seen best under full sunlight, and are consequently plainest at the time of full moon. These streaks extend every way from some particular mountains, especially from one named Tycho.

195. The planet Mercury, which we are next to consider, is about as far from the Sun as the Earth is; and consequently, by Kepler's third law, it must go round the Sun about four times in one of our years. It is rather larger and much denser than the Moon, its diameter being about as long and its mass about as great as the Earth's. The eccentricity of its orbit is about, so that at perihelion it is about as far from the Sun as at aphelion. The inclination of its orbit to the plane of the ecliptic is about 7 degrees; in other words, if there were a great globe enclosing the Sun, with the orbit of Mercury for its equator, that globe would have to make about of a rotation, round that one of its equatorial diameters which would lie in the plane of the ecliptic, in order to bring its whole equator into that plane. The line of the equatorial diameter used as the axis of this rotation is called the line of the nodes of Mercury's orbit. Mercury is probably spheroidal in shape; however, it is difficult to measure accurately the small disk which it shows us. Some observers have thought they could see indications of its rotation upon an axis. It probably does rotate, but we do not know what time its supposed rotation occupies, or, indeed, that it has any rotation. We cannot say positively, therefore, that it has an equator or poles. It is not known to have an atmosphere. Signs of mountains have been perceived upon it by some observers; but recent observations show only