The Story of the Heavens - Sir Robert Stawell Ball (ebook reader for laptop .TXT) 📗

telescope. The lighter circle represents the disc of the sun. On that disc we see the round, sharp image of Venus, showing the characteristic appearance of the planet during the progress of the transit. The only other features to be noticed are a few of the solar spots, rather dimly shown, and a network of lines which were marked on a glass plate across the field of view of the telescope to facilitate measurements.

The adjoining sketch (Fig. 46) exhibits the course which the planet pursued in its passage across the sun on the two occasions in 1874 and 1882. Our generation has had the good fortune to witness the two occurrences indicated on this picture. The white circle denotes the disc of the sun; the planet encroaches on the white surface, and at first is like a bite out of the sun's margin. Gradually the black spot steals in front of the sun, until, after nearly half an hour, the black disc is entirely visible. Slowly the planet wends its way across, followed by hundreds of telescopes from every accessible part of the globe whence the phenomenon is visible, until at length, in the course of a few hours, it emerges at the other side.

It will be useful to take a brief retrospect of the different transits of Venus of which there is any historical record. They are not numerous. Hundreds of such phenomena have occurred since man first came on the earth. It was not until the approach of the year 1631 that attention began to be directed to the matter, though the transit which undoubtedly occurred in that year was not noticed by anyone. The success of Gassendi in observing the transit of Mercury, to which we have referred in the last chapter, led him to hope that he would be equally fortunate in observing the transit of Venus, which Kepler had also foretold. Gassendi looked at the sun on the 4th, 5th, and 6th December. He looked at it again on the 7th, but he saw no sign of the planet. We now know the reason. The transit of Venus took place during the night, between the 6th and the 7th, and must therefore have been invisible to European observers.

Kepler had not noticed that another transit would occur in 1639. This discovery was made by another astronomer, and it is the one with which the history of the subject may be said to commence. It was the first occasion on which the phenomenon was ever actually witnessed; nor was it then seen by many. So far as is known, it was witnessed by only two persons.

A young and ardent English astronomer, named Horrocks, had undertaken some computations about the motions of Venus. He made the discovery that the transit of Venus would be repeated in 1639, and he prepared to verify the fact. The sun rose bright on the morning of the day--which happened to be a Sunday. The clerical profession, which Horrocks followed, here came into collision with his desires as an astronomer. He tells us that at nine he was called away by business of the highest importance--referring, no doubt, to his official duties; but the service was quickly performed, and a little before ten he was again on the watch, only to find the brilliant face of the sun without any unusual feature. It was marked with a spot, but nothing that could be mistaken for a planet. Again, at noon, came an interruption; he went to church, but he was back by one. Nor were these the only impediments to his observations. The sun was also more or less clouded over during part of the day. However, at a quarter past three in the afternoon his clerical work was over; the clouds had dispersed, and he once more resumed his observations. To his intense delight he then saw on the sun the round, dark spot, which was at once identified as the planet Venus. The observations could not last long; it was the depth of winter, and the sun was rapidly setting. Only half an hour was available, but he had made such careful preparations beforehand that it sufficed to enable him to secure some valuable measurements.

Horrocks had previously acquainted his friend, William Crabtree, with the impending occurrence. Crabtree was therefore on the watch, and succeeded in seeing the transit; a striking picture of Crabtree's famous observation is shown in one of the beautiful frescoes in the Town Hall at Manchester. But to no one else had Horrocks communicated the intelligence; as he says, "I hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, rather preferring their hawks and hounds, to say no worse; and although England is not without votaries of astronomy, with some of whom I am acquainted, I was unable to convey to them the agreeable tidings, having myself had so little notice."

It was not till long afterwards that the full importance of the transit of Venus was appreciated. Nearly a century had rolled away when the great astronomer, Halley (1656-1742), drew attention to the subject. The next transit was to occur in 1761, and forty-five years before that event Halley explained his celebrated method of finding the distance of the sun by means of the transit of Venus.[15] He was then a man sixty years of age; he could have no expectation that he would live to witness the event; but in noble language he commends the problem to the notice of the learned, and thus addresses the Royal Society of London:--"And this is what I am now desirous to lay before this illustrious Society, which I foretell will continue for ages, that I may explain beforehand to young astronomers, who may, perhaps, live to observe these things, a method by which the immense distance of the sun may be truly obtained.... I recommend it, therefore, again and again to those curious astronomers who, when I am dead, will have an opportunity of observing these things, that they would remember this my admonition, and diligently apply themselves with all their might in making the observations, and I earnestly wish them all imaginable success--in the first place, that they may not by the unseasonable obscurity of a cloudy sky be deprived of this most desirable sight, and then that, having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory." Halley lived to a good old age, but he died nineteen years before the transit occurred.

The student of astronomy who desires to learn how the transit of Venus will tell the distance from the sun must prepare to encounter a geometrical problem of no little complexity. We cannot give to the subject the detail that would be requisite for a full explanation. All we can attempt is to render a general account of the method, sufficient to enable the reader to see that the transit of Venus really does contain all the elements necessary for the solution of the problem.

We must first explain clearly the conception which is known to astronomers by the name of _parallax_; for it is by parallax that the distance of the sun, or, indeed, the distance of any other celestial body, must be determined. Let us take a simple illustration. Stand near a window whence you can look at buildings, or the trees, the clouds, or any distant objects. Place on the glass a thin strip of paper vertically in the middle of one of the panes. Close the right eye, and note with the left eye the position of the strip of paper relatively to the objects in the background. Then, while still remaining in the same position, close the left eye and again observe the position of the strip of paper with the right eye. You will find that the position of the paper on the background has changed. As I sit in my study and look out of the window I see a strip of paper, with my right eye, in front of a certain bough on a tree a couple of hundred yards away; with my left eye the paper is no longer in front of that bough, it has moved to a position near the outline of the tree. This apparent displacement of the strip of paper, relatively to the distant background, is what is called parallax.

Move closer to the window, and repeat the observation, and you find that _the apparent displacement of the strip increases_. Move away from the window, and the displacement decreases. Move to the other side of the room, the displacement is much less, though probably still visible. We thus see that the change in the apparent place of the strip of paper, as viewed with the right eye or the left eye, varies in amount as the distance changes; but it varies in the opposite way to the distance, for as either becomes greater the other becomes less. We can thus associate with each particular distance a corresponding particular displacement. From this it will be easy to infer that if we have the means of measuring the amount of displacement, then we have the means of calculating the distance from the observer to the window.

It is this principle, applied on a gigantic scale, which enables us to measure the distances of the heavenly bodies. Look, for instance, at the planet Venus; let this correspond to the strip of paper, and let the sun, on which Venus is seen in the act of transit, be the background. Instead of the two eyes of the observer, we now place two observatories in distant regions of the earth; we look at Venus from one observatory, we look at it from the other; we measure the amount of the displacement, and from that we calculate the distance of the planet. All depends, then, on the means which we have of measuring the displacement of Venus as viewed from the two different stations. There are various ways of accomplishing this, but the most simple is that originally proposed by Halley.

From the observatory at A Venus seems to pursue the upper of the two tracks shown in the adjoining figure (Fig. 47). From the observatory at B it follows the lower track, and it is for us to measure the distance between the two tracks. This can be accomplished in several ways. Suppose the observer at A notes the time that Venus has occupied in crossing the disc, and that similar observations be made at B. As the track seen from B is the larger, it must follow that the time observed at B will be greater than that at A. When the observations from the different hemispheres are compared, the _times_ observed will enable the lengths of the tracks to be calculated. The lengths being known, their places on the circular disc of the sun are determined, and hence the amount of displacement of Venus in transit is ascertained. Thus it is that the distance of Venus is measured, and the scale of the solar system is known.

The two transits to which Halley's memorable researches referred occurred in the years 1761 and 1769. The results of the first were not very successful, in spite of the arduous labours of those who undertook the observations. The transit of 1769 is of particular interest, not only for the determination of the sun's distance, but also because it gave rise to the first of the celebrated voyages of Captain Cook. It was to see the transit of Venus that Captain Cook was commissioned to sail to Otaheite, and there, on the 3rd of June, on a splendid day in that exquisite climate, the phenomenon was carefully observed and measured by different observers. Simultaneously