CHAPTER XIV
THE INFERIOR PLANETS
Starting from the centre of the solar system, the first body we meet with is the planet Mercury. It circulates at an average distance from the sun of about thirty-six millions of miles. The next body to it is the planet Venus, at about sixty-seven millions of miles, namely, about double the distance of Mercury from the sun. Since our earth comes next again, astronomers call those planets which circulate within its orbit, i.e. Mercury and Venus, the Inferior Planets, while those which circulate outside it they call the Superior Planets.[11]
In studying the inferior planets, the circumstances in which we make our observations are so very similar with regard to each, that it is best to take them together. Let us begin by considering the various positions of an inferior planet, as seen from the earth, during the course of its journeys round the sun. When furthest from us it is at the other side of the sun, and cannot then be seen owing to the blaze of light. As it continues its journey it passes to the left of the sun, and is then sufficiently away from the glare to be plainly seen. It next draws in again towards the sun, and is once more lost to view in the blaze at the time of its passing nearest to us. Then it gradually comes out to view on the right hand, separates from the sun up to a certain distance as before, and again recedes beyond the sun, and is for the time being once more lost to view.
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.
Various positions, and illumination by the Sun, of an Inferior Planet in the course of its orbit.
