We might naturally think from [Fig. 46] that Venus must pass at every revolution directly between the earth and the sun; and therefore it might appear that what is called the transit of Venus across the sun ought to occur every time between the appearance of the planet as the evening star and the next following appearance as the morning star. No doubt on each of these occasions Venus seems to approach the sun closely; but the orbits of Venus and the Earth do not lie quite in the same plane, and hence the planet usually passes just over or just under the sun, so that it is a very rare event indeed for her to come right in front of the sun. But this does sometimes happen. It happened, for instance, in the year 1874, and again in the year 1882; but, alas! I cannot hold out to you the prospect of ever seeing another such spectacle. There will be no further occurrence of the transit of Venus until the year 2004, though there will be another eight years later, in 2012.
It seems rather odd that one transit of Venus should be followed by another after an interval of eight years, and that then a period of much more than a century should have to elapse before there will be a repetition of a similar pair. This is in consequence of a curious relation between the motion of Venus and the motion of the Earth, which I must endeavor to explain with the help of a little illustration.
Let us suppose a clock with ordinary numbers round the dial, but so arranged that the slowly moving short hand requires 365.26 days to complete one revolution round the dial, while the more rapidly moving long hand revolves in 224.70 days. The short hand will then go round once in a year, and the long hand once during the revolution of Venus. Let us suppose that both hands start together from XII, then in 224.70 days the long hand is round to XII again, but the short hand will have only advanced to about VII, and by the time it reaches XII the long hand will have completed a large part of a second circuit. It happens that the two numbers 224.70 and 365.26 are very nearly in the ratio of 8 to 13. In fact, if the numbers had only been 224.8 and 365.3 respectively, they would be exactly in the proportion of 8 to 13. It, therefore, follows that eight revolutions of the short hand must occupy very nearly the same time as thirteen revolutions of the long hand. After eight years the short hand will of course be found again at XII; and at the same moment the long hand will also be back at XII, after completing thirteen revolutions.
We can now understand why the transits, when they do occur, generally arrive in pairs at an interval of eight years. Suppose that at a certain time Venus happens to interpose itself directly between the earth and the sun, then, when eight years have elapsed, the earth is, of course, restored for the eighth time since the first transit to the same place, and Venus has returned to almost the same spot for the thirteenth time. The two bodies are practically in the same condition as they were at first, and, therefore, Venus again intervenes, and the planet is beheld as a black spot on the sun’s surface. We must not push this argument too far; the relation between the two periods of revolution, though nearly, is not exactly 8 to 13. The consequence is that when another eight years have elapsed, the planet passes a little above the sun or a little below the sun, and thus a third occurrence of the transit is avoided for more than a century. The next transit will take place at the opposite side of the path.
We were fortunate enough to be able to see the transit of Venus in 1882 from Great Britain. Perhaps I should say a part of the transit, for the sun had set long before the planet had finished its journey across the disk. Venus looked like a small round black spot, stealing in on the bright surface of the sun and gradually advancing along the short chord that formed its track.
Fig. 51.—Venus in Transit across the Sun.
An immense deal of trouble was taken in 1882, as well as in 1874, to observe this rare occurrence. Expeditions were sent to various places over the earth where the circumstances were favorable. Indeed, I do not suppose that there was ever any other celestial event about which so much interest was created. The reason why the event attracted so much attention was not solely on account of its beauty or its singularity; it was because the transit of Venus affords us a valuable means of learning the distance of the sun. When observations of the transit of Venus made at opposite sides of the earth are brought together, we are enabled to calculate from them the distance of Venus, and knowing that, we can find the distance of the sun and the distances and the sizes of the planets. This is very valuable information; but you would have to read some rather hard books on astronomy if you wanted to understand clearly how it is that the transit of Venus tells us all these wonderful things. I may, however, say that the principle of the method is really the same as that mentioned on pp. 19–25. When you remember that not we ourselves, nor our children, and hardly our grandchildren, will ever be able to see another transit of Venus, you will, perhaps, not be surprised that we tried to make the most of such transits as have occurred in our time.
VENUS AS A WORLD.
Though Venus exhibits such pretty crescents in the telescope, yet I must say that in other respects a view of the planet is rather disappointing. Venus is adorned by such a very bright dress of sunbeams that we can see but little more than those sunbeams, and we can hardly make out anything of the actual nature of the planet itself. We can sometimes discern faint marks upon the globe, but it is impossible even to make a conjecture of what the Venus country is like. This is greatly to be regretted, for Venus approaches comparatively close to the earth, and is a world so like our own in size and other circumstances that we feel a legitimate curiosity to learn something more about her.