Popular Science Monthly/Volume 3/July 1873/Venus on the Sun's Face

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By R. A. PROCTOR, B. A.,

EACH evening during the month of April the planet of Love could be seen in the west for a few hours after sunset. She set earlier and earlier each successive night—overtaking the sun, as it were—and toward the end of April she could no longer be detected. On the 5th of May she had overtaken the sun, passing him at a distance of about three times his own breadth above or to the north of his disk. When these lines appear she will be a morning star. This passage by the sun is the last made by Venus (at least when on the hither side of him) before the long-desired and now famous transit of December 9, 1874, when, instead of passing by the sun, either above or below his disk, as she usually does, she will pass right across his face.

So much has been said of late respecting this approaching phenomenon, and so much importance is deservedly attached to it, that my readers will probably be interested by a brief and simple account of the matter. In particular, some may desire to know what has been the special aim of the controversy recently and still in progress. Before entering on these matters, I shall make a few remarks on the history of former transits.

The first occasion on which Venus was ever seen on the sun's face was on November 24, 1639 (Old Style), corresponding to December 4th (New Style). It is rather singular that then, somewhat as at present, doubts had arisen, owing to a difference of opinion between an astronomer of established reputation and one less known to the scientific world. The Belgian astronomer Lansberg had stated in his "Tables of the Motion of Venus" that no transit would occur in 1639. Young Horrox, while preparing himself for practical observation, undertook (apparently from sheer love of science) the computation of Venus's motions from the tables of Lansberg. These tables were so highly valued by their author that he had spoken of them as superior to all others—"quantum lenta solent inter viburna cupressi." But Horrox recognized many imperfections in them, and at length, as he says, "broke off the useless computation, resolved for the future with my own eyes to observe the positions of the stars in the heavens; but, lest so many hours should be entirely thrown away," he made use of his results to predict the positions of the planets. "While thus engaged, I received," he proceeds, " my first intimation of the remarkable conjunction of Venus and the sun; and I regard it as a very fortunate occurrence, insomuch as about the beginning of October it induced me, in expectation of so grand a spectacle, to observe with increased attention." Nevertheless, his heart was wroth within him against Lansberg, insomuch that he could not refrain from the extreme step of "forgiving" him in the following agreeable terms: "I pardon, in the mean time, the miserable arrogance of the Belgian astronomer who has overloaded his useless tables with such unmerited praise, and cease to lament the misapplication of my own time, deeming it a sufficient reward that I was thereby led to consider and foresee the appearance of Venus in the sun. But, on the other hand, may Lansberg forgive me" (this is exquisite) "that I hesitated to trust him in an observation of such importance, and from having been so often deceived by his pretensions to universal accuracy that I disregarded the general reception of his tables. . . . Lest a vain exultation should deceive me," he proceeds, "and to prevent the chance of disappointment, I not only determined diligently to watch the important spectacle myself, but exhorted others whom I knew to be fond of astronomy to follow my example; in order that the testimony of several persons, if it should so happen, might the more effectually promote the attainment of truth, and because by observing in different places our purpose would be less likely to be defeated by the accidental interposition of clouds, or any fortuitous impediment." He was particularly anxious because Jupiter and Mercury seemed by their positions to threaten bad weather. "For," says he, "in such apprehension I coincide with the opinion of the astrologers, because it is confirmed by experience; but in other respects I cannot help despising their puerile vanities." Among the astronomers to whom he wrote was his friend Crabtree.[1]

Horrox calculated that the transit would begin at three o'clock in the afternoon of November 24th; but "being unwilling to depend entirely on his own opinion," he began his watch on Saturday, November 23d. On Sunday morning he resumed it, only interrupting it to go to church—so, at least, I interpret his remark that he "was called away by business of the highest importance, which, for these ornamental pursuits," he "could not with propriety neglect. . . . About fifteen minutes past three," he proceeds, "when I was again at liberty to continue my labors, the clouds, as if by divine interposition, were entirely dispersed, and I was once more invited to the grateful task of repeating my observations. I then beheld a most agreeable spectacle, the object of my sanguine wishes, a spot of unusual magnitude and of a perfectly circular shape, which had already fully entered upon the sun's disk on the left, so that the edges of the sun and Venus perfectly coincided, forming an angle of contact." I pass over his observations, to quote his account of the feelings with which Crabtree witnessed the spectacle of "Venus on the sun's face." "I had written," he says, "to my most esteemed friend William Crabtree, a person who has few superiors in mathematical learning, inviting him to be present at this Uranian banquet, if the weather permitted; and my letter, which arrived in good time, found him ready to oblige me. . . . But the sky was very unfavorable, being obscured during the greater part of the day with thick clouds; and, as he was unable to obtain a view of the sun, he despaired of making an observation, and resolved to take no further trouble in the matter. But a little before sunset—namely, about thirty-five minutes past three—the sun bursting forth from behind the clouds, he at once began to observe, and was gratified by beholding the pleasing spectacle of Venus upon the sun's disk. Rapt in contemplation, he stood for some time motionless, scarcely trusting his own senses, through excess of joy; for we astronomers have, as it were, a womanish disposition, and are overjoyed with trifles, and such small matters as scarcely make an impression upon others; a susceptibility which those who will may deride with impunity, even in my own presence; and, if it gratify them, I too will join in the merriment. One thing I request: let no severe Cato be seriously offended with our follies; for, to speak poetically, what young man on earth would not, like ourselves, fondly admire Venus in conjunction with the sun, pulchritudinem divitiis conjunctam?"

Many years passed before another transit of Venus took place. This was the transit of 1761, and it affords striking evidence of the interest with which, even at this early epoch, astronomers regarded the transits of Venus, that Dr. Halley, the first Astronomer Royal, prepared a dissertation on the subject of the transit of 1761 forty-five years before it took place. Considering all the circumstances, he made a very fair prediction—in fact, the calculated time when Venus was to be at her nearest to the middle of the sun's face was only about half an hour in error, whereas the epochs first announced by our present Astronomer Royal for the entrance and exit of Venus during the transit of 1874 were one hour and three-quarters of an hour, respectively, in error. I do not propose here, however, to touch on any of the mathematical matters dealt with by Halley, and I shall content myself with quoting the remarks which he made on the importance of observing Venus with due care for the sake of determining the sun's distance:

"I could wish," he says (I follow Ferguson's translation), "that many observations of the same phenomenon might be taken by different persons at several places, both that we might arrive at a greater degree of certainty by their agreement, and also lest any single observer should be deprived by the intervention of clouds of a sight which I know not whether any man living in this or the next age will ever see again, and on which depends the certain and adequate solution of a problem the most noble, and at any other time not to be attained to. 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 to the making this observation; 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."

A few years before the transit of 1761, Delisle, the French astronomer, undertook a careful analysis of all the circumstances of the approaching phenomenon. It had been ascertained that the transit of 1761 was only the first of a pair of transits, the second occurring in 1769; and it was found that the method by which Halley had proposed to utilize the earlier transit would not, on this occasion, be altogether suitable. I shall presently describe the methods respectively suggested, but it is necessary to mention them here, in order that the chronological sequence of the events may be recognized. For many who have heard Delisle's method lately spoken of and insisted upon (as in Parliament by Mr. Goschen) have been led to imagine that it is a recent invention, and, again, that it possesses great advantages over Halley's, whereas it was known and discussed before the transits of 1761 and 1769, and, while very properly adopted for the first transit, was as properly superseded by Halley's in the case of the second.

The transit of 1761 (like that which will occur on December 6, 1882) was partially visible in England. It was observed at Greenwich by the Rev. M. Bliss, Astronomer Royal, and at Savile House, near London, by Mr. Short, "in presence," says the account, "of his Royal Highness the Duke of York, accompanied by their Royal Highnesses Prince William, Prince Henry, and Prince Frederick." A great number of observations[2] were made also in different parts of the world, and a sufficiently satisfactory determination of the sun's distance was deduced therefrom.

It was, however, in 1769, that the real attack was made. It was then that the famous expedition of Captain Cook, in the Endeavor, was made, England being the only country which had a station in the Pacific. The arctic regions were visited also, a station being selected at Wardhus, in North Lapland, where the following notable peculiarity was presented—the beginning of the transit was observed before sunset, and the end after sunrise. There were also stations at Kola, Yakutsk in Siberia, Peking, Manila, Batavia, Hudson's Bay, St. Petersburg, St. Joseph in California, and many other places. In all there were no less than 74 observing-stations, whereof 50 were in Europe.

The reader need hardly be reminded that the determination of the sun's distance which was until lately in use in our text-books of astronomy was based on the observations made during the transit of Venus in 1769. Nevertheless it has been shown that those observations, rightly interpreted, give a determination of the sun's distance according well with those which have been obtained by the best modern methods, whether these have depended on observations of the sun himself, or the moon, or Mars—or, lastly, of the swift flight of light.

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  1. Both these ardent students of astronomy died young. Horrox (or Horrocks, as his name is now more commonly spelled) was but twenty years old when he calculated the transit, so that his feat may not inaptly be compared to that of Adams in calculating the place of the unknown planet Neptune within a few months of taking his degree. Each instance of an early mastery of difficult problems was fated to meet with neglect; but Horrox died before justice had been done him. Adams was quickly able to prove that his work was sound, notwithstanding the coolness with which it had been received by the Astronomer Royal. Horrocks died in 1641, in his twenty-second year. Crabtree is supposed to have been killed at the battle of Naseby Field.
  2. There were 68 observing-stations in all, thus distributed: 13 in North Europe, 8 in England, 15 in France, 6 in Spain, Portugal, and Italy, 16 in Germany, and 3 in other places.