The three most conspicuous craters from which these lines radiate are Tycho, Copernicus, and Kepler. Tycho is seen at the bottom of Figs. 143 and 130. Kepler is a little to the left of Copernicus in the same figures.
It has been thought that these bright streaks are chasms which have been filled with molten lava, which, on cooling, would afford a smooth reflecting surface on the top.
123. Tycho.—This crater is fifty-four miles in diameter, and about sixteen thousand feet deep, from the highest ridge of the rampart to the surface of the plateau, whence rises a central cone five thousand feet high. It is one of the most conspicuous of all the lunar craters; not so much on account of its dimensions as from its being the centre from whence diverge those remarkable bright streaks, many of which may be traced over a thousand miles of the moon's surface (Fig. 143). Tycho appears to be an instance of a vast disruptive action which rent the solid crust of the moon into radiating fissures, which were subsequently filled with molten matter, whose superior luminosity marks the course of the cracks in all directions from the crater as their common centre. So numerous are these bright streaks when examined by the aid of the telescope, and they give to this region of the moon's surface such increased luminosity, that, when viewed as a whole, the locality can be distinctly seen at full moon by the unassisted eye, as a bright patch of light on the southern portion of the disk.
III. INFERIOR AND SUPERIOR PLANETS.
Inferior Planets.
124. The Inferior Planets.—The inferior planets are those which lie between the earth and the sun, and whose orbits are included by that of the earth. They are Mercury and Venus.
Fig. 144.
125. Aspects of an Inferior Planet.—The four chief aspects of an inferior planet as seen from the earth are shown in Fig. 144, in which S represents the sun, P the planet, and E the earth.