To these various positions technical names are given. When the inferior planet is on the far side of the sun from us, it is said to be in _Superior Conjunction_. When it has drawn as far as it can to the left hand, and is then as east as possible of the sun, it is said to be at its _Greatest Eastern Elongation_. Again, when it is passing nearest to us, it is said to be in _Inferior Conjunction_; and, finally, when it has drawn as far as it can to the right hand, it is spoken of as being at its _Greatest Western Elongation_ (see Fig. 11, p. 148). The continual variation in the distance of an interior planet from us, during its revolution around the sun, will of course be productive of great alterations in its apparent size. At superior conjunction it ought, being then farthest away, to show the smallest disc; while at inferior conjunction, being the nearest, it should look much larger. When at greatest elongation, whether eastern or western, it should naturally present an appearance midway in size between the two. [Illustration: Various positions, and illumination by the Sun, of an Inferior Planet in the course of its orbit. Corresponding views of the same situations of an Inferior Planet as seen from the Earth, showing consequent phases and alterations in apparent size. FIG. 11.--Orbit and Phases of an Inferior Planet.] From the above considerations one would be inclined to assume that the best time for studying the surface of an interior planet with the telescope is when it is at inferior conjunction, or, nearest to us. But that this is not the case will at once appear if we consider that the sunlight is then falling upon the side away from us, leaving the side which is towards us unillumined. In superior conjunction, on the other hand, the light falls full upon the side of the planet facing us; but the disc is then so small-looking, and our view besides is so dazzled by the proximity of the sun, that observations are of little avail. In the elongations, however, the sunlight comes from the side, and so we see one half of the planet lit up; the right half at eastern elongation, and the left half at western elongation. Piecing together the results given us at these more favourable views, we are enabled, bit by bit, to gather some small knowledge concerning the surface of an inferior planet.