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_. [Illustration: 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. When the planet is between the earth and the sun, as at _P_, it is said to be in _inferior conjunction_. When it is in the same direction as the sun, but beyond it, as at _P''_, it is said to be in _superior conjunction_. When the planet is at such a point in its orbit that a line drawn from the earth to it would be tangent to the orbit, as at _P'_ and _P'''_, it is said to be at its _greatest elongation_. [Illustration: Fig. 145.]