« PreviousContinue »
a phenomenon precisely corresponding with that which, 243 years ago, was observed but by two lads only twenty years old. In Europe and America thousands will observe the phenomena of the transit, with the finest instruments opticians can make, and in absolute certainty that the transit will begin and end within a few seconds, at the outside, of the predicted times. Less than two centuries and a half ago none of the regular astronomers knew anything of the approaching event. They did not suppose a transit would occur ; and probably had they been told about the expectations of Horrocks and his friend they would have laughed at the wasted enthusiasm of the two youths.
Horrocks's observation was a precious gift to astronomy. It remains to this day one of the fixed route marks of the planet Venus, and one of the most valued data in our knowledge of the solar system generally.
Time passed, and the value of observations of Venus in transit for determining the sun's distance was recognised by Halley, Newton's favourite disciple, and second among our Governmentastronomers. He showed fully what Horrocks had more than hinted, that Venus, being between the earth and the sun, would be projected on slightly different parts of the sun's face as seen from different parts of that hemisphere of the earth turned sunwards during transit, and that the amount of her displacement on the sun's face as observed from stations at a known distance from each other would suffice, if exactly determined, to indicate her distance from the earth, and with that the dimensions of the whole of the solar system. He recognised, however, the difficulties in the way of this direct solution of the problem of determining the sun's distance. He knew that observers far apart from each other could not readily determine the apparent place of Venus as seen by each at one and the same moment, with such accuracy that subsequently the distance between the two places could be precisely learned, which is essential to the determination of the sun's distance by this direct method.
Halley therefore devised a method by which the displacement could be indirectly deduced, as he supposed, with exceeding accuracy. Let each observer note the moments when transit begins and ends, or, in other words, the time occupied by Venus in traversing her chord of transit. From these observations the lengths of the two chords can be inferred with great precision theoretically, and then it becomes an easy problem in geometry to infer the distance between the two paths of transit.
It is essential for this method that the whole transit should be seen, or at any rate the beginning and the end (which is not precisely the same thing, for in every transit there are stations from which both the beginning and end of a transit, but not the middle, can be observed). But it is not always easy to find suitable stations for seeing the whole transit where it will last as long as possible, and other suitable stations where it will last as short a time as possible. So Delisle devised another plan by which the observations either of the beginning or end of transit would suffice. Let one observer be placed at or near that part of the earth where the transit will begin earliest, and another at or near that part of the earth where it will begin latest (somewhat as one observer of a boat race might be placed on that part of a barge or pier where the racing boats would come into view first and another on that part where they would come into view last). It is manifest that if these two observers, at two known points of the earth, note the exact moment when each sees the transit first begin, the difference between the moments so noted by each will give a means of determining the precise effect of their separation by so many miles from each other, and so enable astronomers to infer the distance of Venus with the same degree of accuracy, theoretically, as by the other method. It is equally clear that two observers might determine the distance of Venus with the same theoretical accuracy if one observed the precise moment when Venus left the sun's face, as seen from that part of the earth where this happened earliest, while another timed the same phenomenon as seen from the part of the earth where it happened latest. In each case, knowing the distance between the two stations and observing the effect of this displacement in modifying the moment of Venus's entry on or departure from the sun's face, the angular displacement of Venus can (theoretically) be inferred, and thence her distance; precisely as the angular displacement of a distant object seen from two stations separated by a known distance indicates to the surveyor the distance of that (perhaps inaccessible) object.
The chief difficulty in Delisle's method consisted in this, that each observer, either of the beginning or end of transit, would have to know the precise instant of absolute time when the phenomenon he was to observe took place; and for this purpose it was essential that the exact longitude of each place of observation should be known. For till we know the longitude we cannot translate the local time of any station into Greenwich or Paris time.
The transit of 1761 was one in which great interest was taken by astronomers, chiefly because of the ideas of Halley, who was long since dead, and of Delisle, who was alive. Expeditions were sent out by
England to Cape Town and St. Helena, while English astronomers at Madras and Calcutta were enjoined to observe it. French astronomers went to Tobolsk, Rodriguez, and Pondicherry ; Swedish astronomers to Lapland ; Russians to Tartary and China. No less than 117 stations were occupied by 176 astronomers. Both Delisle's and Halley's methods were applied ; and as at a great number of the stations fine weather fortunately prevailed, astronomers supposed they had Venus fairly in the toils, and, learning how far off she was when in transit, could deduce with confidence the dimensions of the whole solar system.
But they were doomed to disappointment. The Planet of Love had not behaved as had been expected. Theoretically, she should have appeared as a perfectly round black disc on the sun's face, and under that aspect the moments (i) when she had just fully made her entry and (ii) when she was just beginning to leave the solar disc should have been determinable within a second. For in one case a fine thread of sunlight would be seen to form between the black disc of Venus and the dark background of sky on which the sun's disc is projected, in the other a thread of sunlight, growing narrower and narrower, would break at the precise moment of contact (internal contact it is called), and in each case definite moments would be indicated, whether for measuring the duration of transit or for exactly timing the moments of earliest and latest beginning and ending. But unfortunately Venus declined at these moments of internal contact to present the fair round disc they had expected to see-she appeared pear-shaped, skittle-shaped, irregularly shaped, every kind of shape in fact except round-shaped. The fine thread of light which astronomers were to see forming in one case and breaking in the other neither formed nor broke; but instead, a longish ligament of black seemed to connect the disc of Venus with the sun's edge, lying athwart a broad irregularly shaped background of luminous surface.
The results of calculation were consequently not very trustworthy. All sorts of solar distances were determined, ranging between 77,846,110 miles and 96,162,840 miles. This was manifestly a very unsatisfactory result.
· If the reader prefers scientific (but to most of those who are not astronomers unmeaning) verbiage, he can have it. I see, in fact, that Prosessor Harkness, following in other respects very closely the statements made in my Transits of Venus, departs from me where I add the estimated distances of the sun to the scientific statements of the solar parallax. It would degrade science, some official astronomers seem to think, to speak of the sun's distance ; so readers are told that values of the solar parallax were obtained during the transit of 1761, which ranged from 8.49 seconds to 10:10 seconds-a statement which is as Goojurati Hindu even to many well-instructed persons, and certainly as Greek to the “general reader.”
It was generally supposed by astronomers that this wide range of error arose from too much reliance having been placed on Delisle's method, though Halley's had been also to some degree employed. So they determined in 1769 to employ Halley's method more fully. Preparations were made for sending observers to the South Sea, California, Mexico, Lapland, and Kamschatka. The King of Denmark invited Father Hell, an eminent German astronomer, to observe the transit at Wardhuus in Lapland, whither he went with Bor. greving, the Danish astronomer. England sent Captain Cook to Otaheite, France sent Chappe d'Auteroche to Lapland. Many observations were sent also to other stations in Europe, North America, the East Indies, and China.
But again astronomers were disappointed, though they did not find out the full measure of their disappointment till the middle of the present century. The values of the best computors ranged between about 964 millions of miles and 92 millions; a range of discrepancy too wide to be satisfactory.
In 1825-27, Encke discussed the transits of 1761 and 1769 very fully, and in 1835, having gone carefully over his work, he published that estimate of the sun's “mean equatorial horizontal parallax (this is for the dignity of science), corresponding to a distance of 95,365,000 miles (this is below the dignity of science), which so long did duty in our books of astronomy as the true distance of the sun, within a thousand miles or so,
But about the middle of the century other methods of determining the sun's distance showed such serious discrepancies that Encke's result began to be looked upon with grave suspicion. The moon's motions, observations of Mars, and other methods seemed to agree in showing that the sun's distance must be less than Encke's calculations seemed to indicate, though they did not agree very closely inter se. Distances ranging between 91 and 93 millions of miles began to be in vogue, and when Powalky, Stone, and Newcomb, treating the observations of 1769 in different ways, deduced different results, none of them even near Encke's, astronomers began to suspect that the observations made in 1769 could have had but little real value.
Yet did they not despair of obtaining highly satisfactory results from the observation of the transits of 1874 and 1882. They opined that the astronomers of last century owed their defeat partly to the inferiority of their instruments, and partly to their want of experience in observation. They devised new methods for observing the coming transits; and they looked forward to results of great value and importance.
So far back as 1857, Sir G. Airy (then Professor Airy) called attention to what he supposed to be the fact that the transit of 1882 was the one of the pair which could alone be observed by Halley's method ; and later, in 1868, he called together the chief captains and chartists of the Admiralty to get their opinion about the antarctic observations necessary for the due utilisation of the transit now imminent. With cheerful alacrity Commander Davis, Admiral Ommanney, Captain Richards (hydrographer to the Admiralty), and others, attended his call, proved incontestably that the proposed antarctic expeditions were feasible and desirable, and gave promise to all the world, by every sentence they uttered, that those expeditions should be undertaken.
Unfortunately the then Astronomer-Royal was mistaken. The earlier, not the later, transit was the one to be observed by Halley's method. How his error had arisen it would take long to say ; it is all fully explained elsewhere, and though in words he never admitted that he had made any mistake at all (officials never have done such a thing), yet in action he admitted the largest part of his error, and events demonstrated the rest so unanswerably that he might as well have admitted that too.
The advantage of the earlier transit lay not only in the greater observable differences of duration (on which of course the value of Halley's method depends), but in the greater accessibility of the stations at which the method could be employed. The very best southern stations would have been those already advocated earnestly by the Astronomer-Royal and his subordinates at the Admiralty. But they were not essential to the application of Halley's method in 1874, though they would have been in 1882. was now suddenly discovered that the antarctic stations, which had been described as accessible and even eulogised as convenient, were altogether inaccessible and utterly uninhabitable, a general official eating of words taking place about this time which indicated very strong digestion all round. Unfortunately, other stations remained in the south which were not only well suited for Halley's method and easily accessible, but had been already indicated for Delisle's method. It was not very difficult, however, to assert that their value for observing duration had been recognised from the beginning, though nothing had been said on the subject ; officials always do see everything (“it is their duty, and they do"), and it would be naturally taken for granted that they had seen everything in this case as in all others, despite the utter absence of a word referring to this particular and rather important point. As for the northern stations for observing