It follows from the foregoing statement that the inferior planets can never be seen in those regions of the heavens which are, as it is technically called, in “Opposition” to the Sun; that is, which are on the meridian at midnight whilst the Sun is on the meridian in its midday splendour to places on the opposite side of the Earth. On the other hand, the two inferior planets on stated, though rare, occasions exhibit to a terrestrial spectator certain phenomena of great interest and importance in which no superior planet can ever take part. I am here referring to the “Transits” of Mercury and Venus across the Sun. If these planets and the Earth all revolved round the Sun exactly in the plane of the ecliptic, transits of these planets would be perpetually recurring after even intervals of only a few months; but the fact that the orbit of Mercury is inclined 7°, and that of Venus about 3½, to the ecliptic, involves such complications that transits of Mercury only occur at unequal intervals of several years, whilst, in extreme cases, more than a century may elapse between two successive transits of Venus. For a transit of an inferior planet over the Sun to take place, the Earth and the planet and the Sun must be exactly in the same straight line, reckoned both vertically and horizontally. Twice in every revolution round the Sun an inferior planet is vertically in the same straight line with the Earth and the Sun; and it is said to be in “inferior conjunction” when the planet comes between the Earth and the Sun; and in “superior conjunction” when the planet is on the further side of the Sun, the Sun intervening between the Earth and the planet. But for all three to be horizontally in the same straight line is quite another matter. It is the orbital inclinations of Mercury and Venus which enable them, so to speak, to dodge an observer who is on the lookout to see them pass exactly in front of the Sun, or to disappear behind the Sun; and so it comes about that a favourable combination of circumstances which is rare is needed before either of the aforesaid planets can be seen as round black spots passing in front of the Sun. A passage of either of these planets behind the Sun could never be seen by human eye, because of the overpowering brilliancy of the Sun’s rays, even though an Astronomer might know by his calculations the exact moment that the planet was going to pass behind the Sun. When an inferior planet attains its greatest angular distance from the Sun, as we see it (which I have already stated to be about 27° in the case of Mercury and 47° in the case of Venus), such planet is said to be at its “greatest elongation,” “east” or “west,” as the case may be. At eastern elongation or indeed whenever the planet is east of the Sun, it is, to use a familiar phrase, an “evening star”; on the other hand, at western elongation, or whenever it is on the western side of the Sun, it is known as a “morning star.”