Transits of Venus. A popular account of past and coming transits, from the first observed by Horrocks A.D. 1639 to the transit of A.D. 2012.

Front matter

t O aO-

.V d>

A.

Contents

CHAPTER PAGE

I. Transits of the Setenteenth Centurt .... 1

II. The Transit of 1761 . . . . . . . 27

CHAPTER I. TB Ay SITS OF THE SEVENTEENTH CENTURY

As soon as the Copernican theory of the solar system was established, astronomers perceived that the inferior planets^ Mercury and Yenus^ must from time to time appear to cross the face of the sun. For although on most of these occasions when either planet passes between the sun and the earth, no transit was to be expected, the planet either passing above or below the face of the sun, yet it could not but happen that, in the course of many such conjunctions, the planet would make a passage at so small a distance north or south of the sun's centre as to appear for a time to be u|)on the sun's disc. To make this clear, without entering into any nice details at this stage, let e e and Yv (fig. 1) be the paths of the Earth and Venus, respectively^ around the Sun^ s.^ Then if we suppose^ the path e e to lie in the level of the paper^ we must imagine one-half of the path v v — the half Y^ v v^ — to lie above the level of the paper^ the other half, v y y% lying below that level — not greatly above or below ; in fact^ the short white lines near v and Y show how much these parts of the path of Venus are to be supposed respectively above and below the level of the paper. btill^ it will be clear that when Venus is^

Fig. 1. — Showing the paths of Venus and the Earth, and indicating their inclination and the line of intersection of their planes.

as at v, between the Sun and the Earth at s and e^ she is not really on the line s E, but considerably below it ; and, as supposed to be seen from the Earth, she passes below the Sun. When in conjunction on the other side of v^ v\ Venus passes above the Sun. Only when she is in conjunction nearly at v^ (the Earth near E^), or nearly at v^ (the Earth near /), will she

^ E is supposed to be the place occupied by the earth at the time of the autumnal equinox.

TBAXSITS OF THE SEVENTEENTH CENTURY, 3

appear to cross the face of the Sun. But in the course of many conjunctions there must be some which take place when the two planets are thus placed; that is, either near Y^ and E^, or near v and e\ Similar remarks apply to the case of Mercury.

This was early perceived by the followers of Copernicus. The Ptolemaic system did not. indeed, preclude the possibility of such phenomena as transits ; but since the older astronomers regarded the planets as shining by their own inherent lustre, it was not to be expected that, even if a transit occurred, the planet would be discernible while crossino; the sun's face. And as in reality the Ptolemaic system gave no means of inferring the relative distances of the several planets (including the sun^), there could not even be any certain assurance that Venus or Mercury ever came between the earth and the sun. It was only by a mere assumption that the old astronomy assigned to the sun a sphere outside the spheres of Mercury and Venus.

Still we find that, even so far back as the ninth century, Mercury was supposed to have been seen as a dark spot on the face of the sun. Doubtless one of those large sun-spots, which from time to time are visible to the naked eye, had attracted attention, and was regarded by the ignorant as caused by the passage of one or other of the planets. Mercury or Venus, between the earth and the sun. Venus being probably

* The sun and moon were ' planets ' in tlie old astronomy ; and we still find traces of the nsage in some modern expressions.

B 2 conspicuous at the time, either as a morning or evening star, the less familiar planet Mercury afforded a convenient explanation of the dark spot. The account of the phenomenon accords well with the belief that a solar spot was seen, for we are told by the author of the • Life of Charlemagne ' that Mercury was visible as a black spot upon the sun for eight consecutive days in April of the year 807. Kepler, who was perfectly well aware that Mercury moves too rapidly to remain even for as many hours on the sun's disc, endeavoured to show that the expression originally used in the manuscript had not been octo dies, but octoties, a barbaric form of octies, for ^ eight times.'

It is now well known that Mercury is far too small to be seen by the naked eye when crossing the sun's disc. And this fact disposes of the statement made by the famous physician Ebn Roschd (commonly called Averroes), in his Ptolemaic Paraphrase, to the effect that he saw the planet on the sun in the year 1161, at a time Avhen Mercury really was in inferior conjunction. AYe need not, however, question the veracity of the learned doctor, seeing that Kepler him« self supposed he had seen the planet upon the sun on one occasion. When, a few years later, the existence of sun-spots was detected by the telescope, Kepler admitted that in all probability he had seen such a spot, and not the planet Mercury.

After Kepler had completed his Rudolphine tables of the planetary motions he was able to arrive at tolerably accurate results as to the epochs of the

TRANSITS OF THE SEVEXTJEENTH CEXTURY, 5

transits of Mercury and Yenus over the solar disc. In fact^ he announced, in 16275 that in the year 1631 both Mercury and Venus would pass over the sun's face — Mercury on Xovember 7, and Yenus on December 6 ; and that in 1761 Yenus would again pass across the face of the sun.

As the first occasion on which the transit of an inferior planet was ever witnessed, the transit of Mercury in 1631 has an interest resembling that which attaches to the first observation of a transit of Yenus, eight years later. Therefore I think the reader will be interested to hear how Cxassendi succeeded in observing Mercury in transit.

Gassendi made preparations for the observation of the transit at Paris. The manner in which he observed the phenomenon was somewhat remarkable. Through a small aperture in a shutter the solar light was admitted into a darkened room, and an image of the sun, some nine or ten inches in diameter, was formed upon a white screen. A carefully divided circle was traced upon this screen, and the whole was so arranged that the image of the sun could be made to coincide exactly with the circle. As Gassendi was anxious to ascertain the exact moment of the ingress of the planet upon the sun's disc, or-^supposing he should fail in that respect — at least to determine the moment of egress, and as he had no trustworthy clock, he determined that the altitude of the sun should be carefully estimated several times during the progress of the transit, and particularly at the moment of egress. It was necessary, therefore, that he should have an assistant, and, further, that his assistant should work in another room ; for from the room in which Gassendi was working the sun's light, as I have said, had been carefully excluded, save at the minute aperture in the window-shutter. Accordingly, Gassendi placed his assistant in a room above him, with a large quadrant for taking altitudes, instructing him to observe the height of the sun as soon as he heard Gassendi stamp upon the floor of the room beneath. A clumsy arrangement, truly, when compared with the subtle devices of modern astronomers — with the aid which they derive from powerful telescopes, all but perfect clocks, and, where need arises for communicating with one another from distant stations, the instantaneous indications of telegraphy. Yet we cannot but admire the spirit in which Gassendi worked, the readiness with which, for want of more perfect instruments^ he set himself to invent arrangements which suited his requirements, and the skill Avith which he availed himself of those imperfect adaptations.

And if we admire these qualities in Gassendi, still more must we admire the patience with which he waited for the commencement of the phenomenon. Modern astronomy is able to announce, within three or four minutes, the instant at which a transit will commence at any given spot upon the earth's surface. But Kepler's prediction respecting Mercury's motions did not layxilaim to any accuracy of this sort. So

TI^AXSITS OF THE SEVEXTEEXTH CEXTURY, /

uncertain did the epoch of the occurrence appear to be, that Gassendi began to watch for the transit two days before the date assigned by Kepler for its occurrence.

The 5th of November proved unfavourable for observation, the day being rainy. The next day was also unsuitable, clouds having overspread the sky durino; nearlv the whole dav. The mornino^ of the 7th. the day appointed by Kepler for the transit, was also cloudy. Thus Gassendi began his watch on that day with the uncomfortable feeling that during some part of the two preceding days the planet might already have passed over the sun's disc, — perhaps that the transit had been completed but a few minutes before the clouds broke up on the morning of the 7th.

A little before eight the sun shone for a few minutes through the openings between the clouds, but there still remained enough mist to prevent Gassendi from being able to determine whether any spot existed upon the image of the sun in his observing-room. Xearly an hour passed before the sun was sufficiently clear of clouds to enable Gassendi to make any satisfactory observations. Towards nine, however, the sun became distinctly visible, and turning to the image on the screen, the astronomer perceived upon it a small black spot. He could not believe, however, that this was Mercury, as the received estimate of the planet's dimensions had led him to look for a spot nearly twice as large. As he was familiar with the nature of solar spots, and the rapid manner in which

8 TRANSITS OF VE2s'US.

they form, he concluded that one had made its appearance on the sun's surface since the preceding day. At nine o'clock he had another opportunity of observing the spot^ and he carefully estimated its position, intending to make use of it as a point of reference for determining the path of the planet in transit, if he should be fortunate enough to witness that phenomenon. Soon after, he had another view of the spot, and was surprised to find that it had moved away considerably from its former position. He felt assured that no ordinary solar spot could have moved so rapidly; but still he could not persuade himself that he was looking at Mercury in transit, having so fully satisfied his mind respecting the dimensions which the planet would exhibit. Besides, the hour had not yet arrived at which Kepler had predicted that the transit would begin.

Gassendi was still in doubt, and endeavouring to recall the circumstances of his former measurement, in order to convince himself that he had made no mistake, when the sun again made his appearance through the clouds, and it was apparent that the spot had moved yet farther from its original place. Xo room now remained for doubt. It was clear that the phenomenon which had been so long and so anxiously awaited by the astronomer was already in progress. He immediately stamped upon the floor to attract the notice of his assistant. But this person, whose name has not reached us, was possessed of less patience than Gassendi. He probably felt much less interest in the

TJRANSITS OF THE SEVENTEENTH CENTURY. 9

phenomenon ; possibly^ he placed very little faith in the calculations of Kepler. Whatever was the reason, he had grown weary of watching, and had left his post. Gassendi had to continue his observations alone, hoping that at least his {assistant would return before the planet had passed completely off the sun's face. Fortunately this happened ; the requisite observations were made for determining the time of egress ; and thus an important addition was made to our knowledge of the motions of the innermost planet of the solar system.

Gassendi sent an amusing account of his observations to Professor Shickhard, of the University of Tubingen. ^ The crafty god,' he wrote, ^ had sought to deceive astronomers by passing over the sun a little earlier than was expected, and had drawn a veil of dark clouds over the earth in order to make his escape more effectual. But Apollo, acquainted with his knavish tricks from his infancy, would not allow him to pass altogether unnoticed. To be brief, I have been more fortunate than those hunters after Mercury who sought the cunning god in the sun. I found him out, and saw him where no one else had hitherto seen him.' He states that the planet, as seen projected on the image of the sun, did not appear altogether black, but was greyish, and somewhat ruddy round the margin. Doubtless these peculiarities were due to the method of observation employed by the astronomer. He estimated the apparent diameter of the spot at about one-ninetieth part of the sun's apparent diameter.

lO TRANSITS OF VENUS,

an estimate considerably exceeding the true dimensions, but still more considerably below the dimensions which astronomers had been disposed to assign to the planet.

Gassendi, although he did not observe the commencement of the transit, was yet able to compute the time of its occurrence. He found that the transit had begun nearly five hours before the time assigned by Kepler.^

I have mentioned that Kepler had predicted that a transit of Venus would take place on December 6, 1631. It need hardly be said that Gassendi, after his

^ The second observed transit of Mercury took place on November 3, 1651. The observations of Gassendi had enabled astronomers to estimate the epoch of the transit much more exactly than in the former instance. It resulted from their calculations that the phenomenon would not be visible in England, or indeed in Europe ; but would be well seen over a large part of Asia. Accordingly a young Englishman, Jeremiah Shakerley, went to Surat, in India, for the purpose of witnessing the phenomenon. Such a journey undertaken for such a purpose in an age when sea-voyages were not only much more protracted, but also far more dangerous than in the present day, must be looked upon as a remarkable and commendable instance of devotion to scientific pursuits. It is pleasing to be able to record that the energy of the young Englishman was rewarded by complete success.

The third observed transit took place on May 3, 1661. It was observed by Hevelius at Dantzic, and at London by Huyghens, Street, and Mercator. Hevelius was surprised to find that the diameter of the planet was very much smaller than he had been led to expect. He found on measurement that Gassendi's estimate was nearly twice as great as the true diameter of the planet.

The fourth transit of Mercury was observed by Halley at St. Helena on November 7, 1677. He was the first astronomer who had ever observed the complete passage of the planet across the solar disc.

Later transits of Mercury have no special historical interest, though observations of considerable importance were made during the transits of 1736, 1799, and 1868.

TIlA^SITS OF THE SEVENTEESTR CENTURY, II

success in observing the transit of IMercury^ had good hopes of observing Venus to even greater advantage. It is true that, according to Kepler's calculation^ the transit might be expected to begin only towards sunset^ and it was therefore possible that the phenomenon would not be visible at all in Europe. But it was equally possible that any error in the calculation might lie in the other direction, and so the whole transit be favourably seen before sunset. Gassendi took the same measures for observing the transit as in the case of Mercury. He had proposed to observe the sun on December 4 and 5, but ^ an impetuous storm of wind and rain rendered the face of the heavens invisible on both those days. On the 6th he continued to obtain occasional glimpses of the sun till a little past three o'clock in the afternoon, but no indication of the planet could be discerned upon the sun's disc as depicted upon the white circle. On the 7th he saw the sun during the whole forenoon, but he looked in vain for any trace of the planet.' ^ It is now well known,' proceeds Prof. Grant, from whose account I have just quoted, ^that the transit of the planet took place during the night between December 6 and 7.' I do not know where any calculation of the circumstances of the transit can be found ; but an investigation of my own (sufficiently accurate for a past and unseen phenomenon) shows that in the Southeastern parts of Europe the egress might have been observed, occurring for those parts after sunrise on the morning of December 7.^ Plate II. shows the parts of the earth whence the transit might have been seen wholly or in part. The position of the line C D, which, accordmg to my calculation, marks the boundary between the places where day and night were in progress in the Northern hemisphere at egress, shows the parts of Europe whence the end of the transit might have been observed.

Kepler had stated that after the transit of 1631 there would be none till the year 1761. According to his calculations, Venus, when in her inferior conjunction on December 4, 1639, would pass very near to the sun's centre, but not quite near enough for a transit to occur, the planet passing beloic — that is, south of the sun. On the other hand, the tables of Lansberg, a Belgian astronomer, who followed the old system of computation, seemed to show that Venus would on this occasion transit the upper or northern part of the sun's face. Horrocks, a young and then unknown astronomer, having been led to examine the tables of Venus, found that though Kepler's were much more exact than Lansberg's, a transit would really occur, Venus passing below the centre of the sun, as Kepler had predicted, but not so low as to miss the

"Ci\ o)

^ 2

<o

THAXSITS OF THE SEVESTEEyTH CEXTVRY, I 3

sun's disc altogether. The circumstances under which Horrocks made this discovery possess considerable interest, and I propose to devote some space to his account of them.

While preparing himself for practical observation, Horrocks 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 Horrocks recognised many im- ])erfections in them, and at length, as he says, ' broke off the useless computation, resolved for the future with his 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, inasmuch as about the beginning of October it induced me, in expectation of so grand a spectacle, to observe with increased attention.' Nevertheless, his heart w^as 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 meantime, 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 to foresee the appearance of Venus in the sun. But, on the other hand, may Lansberg forgive me' (this is charming) ' 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 Crab tree. ^

^ Both these ardent students of astronomy died young. Horrox (or Horrocks, as his name is now more commonly spelt) 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

THAXSITS OF THE SEVENTEENTH CENTURY. I 5

In Avhat follows I quote the account given by Horrocks himself of the observations made upon this occasion, using the translation given by the Rev. Mr. Whatton.^

^Following the example of Gassendi/ says Horrocks, ^ I have drawn up an account of this extraordinary sight, trusting that it will not prove less pleasing to astronomers to contemplate Venus than Mercury, though she be wrapt in the close embraces of the sun :

Hail ! then, ye eyes that penetrate the inmost recesses of the heavens, and, gazing upon the bosom of the sun with your sight-assisting tube, have dared to point out the spots on that eternal luminary ! And thou, too, illustrious Gassendi above all others, hail ! thou who, first and only, didst depict Hermes' changeful orb in hidden congress Avith the sun. Well hast thou restored the fallen credit of our ancestors, and triumphed o'er the inconstant wanderer. Behold thyself, thrice celebrated man! associated with me, if I may venture so to speak, in a like good fortune. Contemplate, I repeat, this most extraordinary phenomenon, never in meet with neglect ; but Horrocks died before justice had been done him. Adams was quickly able to prove that bis work was sound, notA\dthstanding the coolness with which it had been received by official astronomers. Horrocks died in 1641, in his twenty-second year. Crabtree is supposed to have been killed at the battle of Naseby Field.

^ The memoir accompanying Mr. Whatton's translation will be found full of interest. The complete work is published by Macintosh, 24 Paternoster Kow.

our time to be seen again ! the planet Venus, drawn from her seclusion, modestly delineating on the sun, without disguise, her real magnitude, whilst her disc, at other times so lovely, is here obscured in melancholy gloom ; in short, constrained to reveal to us those important truths, which Mercury on a former occasion confided to thee.

^ How admirably are the destinies appointed ! How wisely have the decrees of Providence ordered the several purposes of their creation ! Thou, a profound divine, hast honoured the patron of wisdom and learning ; whilst I, whose youthful days are scarce complete, have chosen for my theme the queen of love, veiled by the shqde of Phoebus' light.

' Whilst I was meditating in what manner I should commence my observation of the planet Venus, so as effectually to realise my expectations, the recent and admirable invention of the telescope afforded me the greatest delight, on account of its singular excellence and superior accuracy above all other instruments. For although the method which Kepler recommends in his treatise on C)ptics, of observing the diameter and eclipse of the sun through a plain aperture without the aid of glasses, is very ingenious, and in his opinion, on account of its freedom from refraction, preferable to the telescope ; yet I was unable to make use of it, even if I had wished to do so, inasmuch as it does not show the sun's image exactly, nor with sufficient distinctness, unless the distance IVom the aperture be very great, which the smallness

TBAXSITS OF THE SEVEXTEEXTH CEXTURY. 1 7

of my apartment would not allow. Moreover, I was afraid to risk the chance of losing the observation ; a misfortune which happened to Schickard and Mostling, the astronomer to the Prince of Ilesse^ as Gassendi tells us in his " Mercury : " for they, expecting to find the diameter of Mercury greater* than it was reasonable to anticipate, made use of so large an aperture that it was impossible to distinguish the planet at all, as Schickard himself has clearly proved ; and even though Venus gave promise of a larger diameter, and thereby in some measure lessened this apprehension, and 1 was able to adapt the aperture to my own convenience, yet in an observation that could never be repeated I preferred encountering groundless fears to the certainty of disappointment. Besides, 1 possessed a telescope of my own, of such power as to show even the smallest spots upon the sun, and to enable me to make the most accurate division of his disc ; one which, in all my observations, I have found to represent objects with the greatest truth.

' This kind of instrument, therefore, I consider ought always to be preferred in such experiments.

' Having attentively examined Venus with my instrument, I described on a sheet of paper a circle Avhose diameter was nearly equal to six inches, the narrowness of the apartment not permitting me conveniently to use a larger size. This, however, admitted of a sufficiently accurate division ; nor could the arc of a quadrant be apportioned more exactly, even with a radius of fifty feet, which is as great a one as any astronomer has divided; and it is in my opinion far more convenient than a larger, for although it represents the sun's image less, yet it depicts it more clearly and steadily. I divided the circumference of this circle into 360° in the usual manner, and its diameter into thirty equal parts, which gives about as many minutes as are equivalent to the sun's apparent diameter ; each of these thirty parts was again divided into four equal portions, making in all 120 ; and these, if necessary, may be more minutely subdivided ; the rest I left to ocular computation, which, in such small sections, is quite as certain as any mechanical division. Suppose, then, each of these thirty parts to be divided into 60'', according to the practice of astronomers. AYhen the time of the observation approached I retired to my apartment, and having closed the windows against the light, I directed my telescope, previously adjusted to a focus, through the aperture towards the sun and received his rays at right angles upon the paper already mentioned. The sun's image exactly filled the circle, and I watched carefully and unceasingly for any dark body that might enter upon the disc of light.

' Although the corrected computation of Venus's motions which I had before prepared, and on the accuracy of which I implicitly relied, forbade me to expect anything before three o'clock in the afternoon of the 24th ; yet since, according to the calculations of most astronomers, the conjunction should take place sooner— by some even on the 23rd— I was unwilling to

TRAXSITS OF THE SEVENTEEXTH CENTURY. 1 9

depend entirely on my own opinion^ which was not sufficiently confirmed^ lest by too much self-confidence I might endanger the observation. Anxiously intent, therefore, on the undertaking through the greater part of the 23rd^ and the whole of the 24th, I omitted no available opportunity of observing her ingress. I watched carefully on the 24th from sunrise to nine o'clock, and from a little before ten until noon, and at one in the afternoon, — being called away in the intervals by business of the highest importance, wdiich for these ornamental pursuits I could not wdth propriety neo-lect.^ But durino* all this time I saw nothino; in the sun except a small and common spot, consisting as it were of three points at a distance from the centre towards the left, which I noticed on the preceding and following days. This evidently had nothing to do with Venus. About fifteen minutes past three in the afternoon, when I was again at liberty to continue my labours, the clouds, as if by Divine interposition, were entirely ^dispersed, and I was once more incited to the

^ Presumably, as JVIr. Whatton points out,^ 'the business of the highest importance ' here referred to was the duty of conducting divine service, as November 24, Old Style, was a Sunday. Mr. Whatton quotes the following passage from one of Thomas Hearne's pocket-books, dated February 8, 1723: 'Mr. Horrox, a young man, minister of Hoole, a very poor pittance, within four miles of Preston, in Lancashire, was a prodigy for his skill in astrojiomy, and had he lived, in all probability he would have proved the greatest man in the whole world in his profession. He had a very strange unaccountable genius, and he is mentioned ^A\h great honour by Hevelius upon account of his discovery of Venus in the sun, upon a Sunday ; but being called away to his devotions and duty at church, he could not make such observations as otherwise he would have done.'