THE TRANSIT of the Planet VENUS over the disc of the SUN, being a most important as well as curious phaenomenon, has for a long time im­ployed the consideration of Astronomers. The first notice that I find taken of it was by the sagacious Kepler, who flourished not long after the revival of the true astronomy by Coper­nicus: and in a work published in 1604, declared, that no such thing could happen in that century; nor the next, till the year 1761. In this, however, that great man was mistaken. For an English Astronomer, Jeremiah Horrockes, then no more than 20 years of age, but of an admirable genius, having corrected Kepler's Tables by some of his own observations, predicted and soon after observ'd this most rare phaenomenon, on the 24th of November O. S. in the year 1639, at Hoole, a small village about 12 miles N. from Leverpool. Having waited several hours with great impatience, he had at last the [Page 4] satisfaction to see the Planet as a black spot, just entring on the Sun, at a quarter past three in the afternoon. He made three observations of its positions in half an hour, when he was depriv'd of a longer sight of it by the setting of the Sun. He had given notice to an astro­nomical friend, Mr. Crabtree, who, at a few miles distance just got a sight of it between the clouds, before Sun-set. These two were the only persons that ever saw Venus in the Sun, before the present year.

THESE Transits became more generally known by a Series, which the late Astronomer-Royal Dr. Halley publish'd, of all in a thou­sand years; from which it appears that none could happen between the years 1639 and 1761. But what made this of June 6, 1761, most of all famous, was a paper of the same learn­ed author in the Philosophical Transactions for the year 1705, wherein he explains an important use to be made of it. Some uses of the transits of the inferior planets were obvious. They serve much better than any other phaenomena, to perfect the theory of these planets, which is more difficult to be settled than that of the superiors; and did they happen frequently, would be of admirable use in ascertaining the longitude of places on this globe. They likewise demonstrate the truth of the Copernican System, so far as it relates to these Planets, by making it evident that they respect the Sun, and not the Earth, as the centre of their motions. But Dr. Halley point­ed [Page 5] out a new and singular use to be made of the Transit of Venus; and to which no other phaenomenon in the heavens is near so well adapted. This is, in finding the Parallax of the Sun; which is so small that the Astrono­mers have never been able to discover it with exactness in any other method. If this were once known, the distance of the Sun, and of all the Planets, and of all the Comets, would be known too; and their magnitudes would also be known, from their apparent diameters. This would give us a just idea of the vast di­mensions of the solar system, and of the mighty globes which compose it. Nor can we, but by such observations, know whether the Earth continues to revolve at the same distance from the Sun, or whether it gradually approaches him, as there is some reason to suspect; nor whether the Sun remains of the same magni­tude, or consumes away and is diminish'd by the light which he is incessantly sending forth. The Parallax of the Sun is also an element that enters into the calculation of eclipses; and of what use these are in Astronomy, in Geography, in History and Chronology, is too well known to need being insisted on at this day. It would likewise determine the quantity of matter in the Earth, or the proportion which this bears to that in the Sun; and shew us precisely the a­mazing velocity with which light is darted from the Sun and other luminous Bodies;—and pro­bably give us a deeper insight into many of the wonderful works of GOD. For such is the [Page 6] relation of the several branches of natural Phi­losophy to each other, and of all of them to mo­ral Philosophy, that whatever tends to the per­fection of one tends to perfect the other, in proportion to the connection there is between them. And a capital point once adjusted, may serve for the determination of others, which were not at first thought of. Thus, the exact measure of a degree on the meridian enabled Sir Isaac Newton to demonstrate his Theory of universal Gravity, a scheme which had never enter'd into the minds of any, when the de­gree was proposed to be measured; by which means that great man laid open the constitu­tion of the astronomical world, and pointed out the fundamental Law which the alwise CREATOR has established for regulating the several movements in this grand machine.

FROM a sense of the advantages to be de­rived from this Observation, Dr. Halley recom­mended it in the most emphatical terms, and inforced it with all the energy of language, on the Astronomers and others of the present day, that they would by no means let slip an oppor­tunity, which he then doubted whether it would offer it self again to the people of this or the following age; and on which depended the adequate solution of a Problem, in itself most noble, and inaccessible any other way. And to put the matter beyond all hazard, he advised to have a number of Observators station'd in dif­ferent parts of the Earth, that some of them [Page 7] might be sure to succeed; as a single person might be frustrated by the intervention of clouds.

THESE pressing recommendations of the learned Doctor did not lose their effect. The approach of this phaenomenon was attended with a general curiosity: It became a topic of conversation among Persons of every rank, of every profession, of every taste. It could not escape the notice of some of the greatest Mo­narchs of Europe, surrounded, as they were, with the tumults of war. His late MAJESTY, of happy memory, attentive to the cultivation of the Arts and Sciences, those great ornaments of civil Society;—particularly of Astronomy, on which he well knew Navigation, and of con­sequence the commerce and wealth of his Kingdoms had a necessary dependence; was graciously pleased to provide for having this observation made in the most distant parts of Africa and Asia, as well as in his own Royal Observatory at home.

NOR was America inactive on this singu­lar occasion. His Excellency FRANCIS BER­NARD, Esq Governor of the Province of the Massachusetts-Bay, inspired with a just zeal for the advancement of Literature, which he de­monstrates on every opportunity, exerted him­self to procure an observation in this quarter of the world. And as Newfoundland was the only British Plantation in which one could be made, and indeed the most western part of the [Page 8] Earth where the end of the Transit could be ob­serv'd, for this was to happen before the Sun would be risen to any other part of America, except the Savage coast of Labrador, he proposed by a Message See the Appendix. to the General Assembly of the Province then sitting at Boston, to make pro­vision for this purpose. The Assembly, ani­mated with the same public Spirit, and treading in the steps of their worthy Predecessors, who from the settlement of the Country have emi­nently distinguish'd themselves among the other Governments as the PATRONS OF LEARNING, readily enter'd into his Excellency's views, and past a Vote See the Appendix. requesting Him to order the Pro­vince-Sloop to convey me to Newfoundland on this design. The orders were issued, agree­ably; and his Excellency was pleased to give me such Letters, directed to his Majesty's Offi­cers, Commanders, and other his Subjects, and likewise to those of the French King, as were proper to facilitate the execution of the enter­prise, and to secure it from disappointment by the enemy. The Reverend the President and Fellows of Harvard College, in order to pro­mote so laudable an undertaking, granted their Apparatus of astronomical instruments, to be imploy'd in this affair. Accordingly, I carried an excellent Pendulum clock; one of Hadley's Octants with Nonius divisions, and sitted in a new manner to observe on shore as well at sea Presented to the Apparatus last year by Ezek. Goldthwait, Esq; a refracting telescope with cross wires at half [Page 9] right angles, for taking differences of Right Ascension and Declination; and a curious re­flecting telescope, adjusted with spirit-levels at right angles to each other, and having horizon­tal and vertical wires for taking correspondent altitudes; or differences of altitudes and azi­muthsPresented this year by the Hon. Thomas Hancock, Esq. And taking with me for assistants two young Gentlemen my Pupils, Messr's Williams and Rand. who had made good proficiency in mathematical studies, I embark'd at Boston, on board the before-mentioned vessel, commissioned by the Gover­nor, May the 9th, and in 13 days arrived at St. John's, Newfoundland.

THE reception I met with there was suita­ble to the consequence of my Commission, and the dignity of my Employers. Whatever was wanted, was supplied in the most obliging man­ner by the Gentlemen of that place; amon whom, I am under particular obligations to mention Capt. Ross, Commander of his Maje­sty's Garrison, and Michael Gill, Esq Chief Judge in the Courts. With their assistance we were enabled to surmount the difficulties which must be expected to attend an undertaking like ours, in a country unprovided with conveni­ences for such operations. The town of St. John's being bounded with high mountains toward the Sun-rising, so that no house in it would answer our end, we were obliged to seek farther; and, after a fatiguing and fruit­less [Page 10] attempt or two, fix'd on an eminence at some distance, from whence we could have a view of the Sun presently after his rising. As this place was open, and had no building near it, we pitch'd some tents upon it for a shelter; which, together with our Apparatus, and such materials as we had occasion to make use of, we convey'd up thither, with the labor of seve­ral days. We secured the Clock to a pillar set in the ground under a large tent. Near this tent, and within call of the Clock, we fix'd two others firmly in the ground; one, to mount the refracting telescope upon; the other, which was above 8 feet high, for a Style, hav­ing at top a plate of lead with a little hole for transmitting the Sun's rays; and we laid a large horizontal Platform on the ground, to receive these rays. This Platform we kept cover'd, to defend it from the Sun and the weather; and examin'd its position every time we made use of it, by a very long level. On this we care­fully traced a meridian line, by correspondent altitudes of the Sun, taken both with the Re­flector, and by the Sun's image on the Platform. And, in order fully to examine and verify the meridian and adjust our clock, we repeated these operations every fair day, and many times in a day; and continued them with an assi­duity which the infinite swarms of insects, that were in possession of the hill, were not able to abate, tho' they persecuted us severely and with­out intermission, both by day and by night, with their venemous stings. It would be tedious, [Page 11] as well as needless, to give a detail of the ob­servations we made with this intent: 'Tis suf­ficient, that we regulated the clock with as much exactness as we could have done at home.

THUS prepared, we waited for the critical hour, which proved favorable to our wishes. The morning of the 6th of June was serene and calm, We have since learned, that all along shore, from Cape-Race to Halifax, there was a heavy storm of rain that morning. The Sun rose behind a cloud that lay along the horizon, but soon got above it; and at 4^h 18^m we had the high satisfaction of seeing that most agreable Sight, VENUS ON THE SUN, and of shewing it in our telescopes to the Gentlemen of the place, who had as­sembled very early on the hill to behold so curious a spectacle. The Planet at first appear'd dim thro' the cloud, but in a short time be­came more distinct and better defined. Upon this, I applied my self to take the passage of the Sun's and Venus's preceding limbs by the vertical, and of their lower limbs by the hori­zontal wires in the Reflector; and made the following observations; one of my assistants counting the Clock, and the other writing down the observations, as I made them.

[Page 12]Apparent Time.

AS Venus began now to draw near, the Sun's limb, I prepared to observe her egress. The in­terior contact appeared at 4^h 47^m 17^s, and the exterior at 5^h 5^m 46^s. IN memory of so rare an observation, the Gentlemen present agreed to give the hill, where our observatory was situated, the name of Venus's Hill,

THE above observations gave me so many differences between the Sun's and Venus's alti­tudes and azimuths; from whence, by spherical trigonometry, I deducd the Planet's right Ascen­sions and Declinations; and from them, in the last place, her Longitudes and Latitudes. It would be neither of entertainment nor use to the Reader, to insert the particulars of such te­dious calculations: It will be enough to give the result of the whole, or the Planet's difference in Longitude from the Sun's centre, and her La­titude, at the time of each observation; which came out as follow.

[Page 13]

THUS in the short interval of Time that was permitted, I determin'd five positions of the Planet on the Sun; and by laying off the differences of Longitude and the Latitudes, the Planet's path over the Sun may be drawn, and its situation determin'd in respect of the ecliptic,

[Page 14] or any other circle of the sphere; as is here done for the vertical thro' the rising Sun at St. John's. In the Figure, the circle CZLN re­presents the Sun; the diameter ZN, the vertical thro' his centre S; CL, the ecliptic, in its due position with respect to this vertical at the time of sun-rising at St. John's. Then, laying off the first observ'd difference of Longitude on the ecliptic from S to D, and the corresponding Latitude in a perpendicular to the ecliptic from D to A, we have the place of the Planet on the Sun at the time of the first observation; repre­sented here by the black spot at A; which also exhibits the apparent bigness of Venus in pro­portion to the Sun's; their diameters being nearly as 1 to 33. In the same manner may the other observations be projected; and the right line BE, drawn thro' the several points thus found, which there was not room to set down in this Figure, will be the Planet's path on the Sun; in which, B is its place at the begin­ning, M at the middle, and E at the end; the numbers over the path shewing Venus's place at the several hours of the night between the 5th and 6th of June.

FROM hence we state the times of the Tran­sit as it would have been seen at St. John's, had the Sun been above the horizon during the whole continuance of it. They are as follows, on supposition that the angle of Venus's visible way with the ecliptic is 8^o 28^m, and her horary motion in that way, 4^m; though indeed a very [Page 15] small alteration in any of the elements will make a sensible one in the time of the beginning.

IT may be further collected, that the con­junction in Longitude happen'd at 14^h 4^m 32^s in 15^d 35^m 22^s of Gemini; the Planet's Lati­tude being then 9^m 28^s. The Longitude at this conjunction differs but 27^s from the calcule I made by Dr. Halley's Tables; and the Lati­tude but 23^s. Which shews to what a degree of perfection those tables are already carried.

BY several observations we found the Lati­tude of the place 47^d 32^m N; which falls be­tween the Latitudes set down in different books and maps. No eclipse of the sun or moon hap­pen'd while we were on the Island, by which we might have determin'd the Longitude; and [Page 16] Jupiter's Satellites proved of no service in that affair; as they were not risen high enough to be observ'd above an hour before day-light came on. There were but two eclipses of them that could have been visible during our stay there; and tho' I watched for both these, I was disap­pointed of both by unfavourable weather. Nei­ther was I fortunate enough to get so much as one occultation of a fix'd star by the moon, tho' I spared no pains for it. On the 11th of June we made an observation for this purpose, by taking the difference of right ascension of the moon's preceding limb, and the middle star in the border of the garment of Virgo, the 82d of that constellation in the Britannic Catalogue; which at 9^h 29^m 51^s, we found to be 12^d 7^m 29^s. Calculating from hence, the Longitude came out 50^d 14^m W. from Greenwich; which is a little less than I had supposed it in my calcula­tions.

THE variation of the needle we found to be 19^d W. which is less than it is marked on the charts.

IN the above calculations I took the semi­diameter of the Sun to be 15^m 50^s; and of Venus 29^s. This last is less than Dr. Halley, after Mr. Horrockes, supposed it; but it cannot be bigger, consistent with the foregoing obser­vation, which has convinced me, that Venus, instead of being equal in diameter to the Earth, according to the common opinion, is less, in the proportion of 5 to 4.

[Page 17]BY the foregoing observations we are now enabled to predict with certainty, what Dr. Halley, in his paper before-mentioned, only sus­pected, that Venus will again pass over the Sun on the 3d of June in the year 1769; the for­mer part of which Transit will be visible in our horizon for about 5^h in the afternoon, but the Sun will set before it is ended. That Tran­sit will be more convenient for finding the Pa­rallax than the present has been; but not be­cause ‘Venus will then describe a line nearer the diameter of the Sun;’ which is the reason assign'd in the London Magazine for June 1761. The Planet will hardly come so near. The true reason is, because Venus will then describe a more northerly path on the Sun. After that, there will not be another Transit for above 100 years; viz. not 'till the 8th of December in the year 1874. But the whole of that will be in­visible here; the middle happening about mid­night. The next after that, will not be in the year 1996, as it is in Dr. HALLEY's Series. This Series was founded on the old hypothesis of the immobility of Venus's Nodes, which ap­pears to be erroneus. The observation of the late Transit has made it certain, that these Nodes have a retrograde motion, tho' a very slow one; which is agreable also to the Theory of Gravi­ty. And from hence it follows, that the next Transit after that of 1874 will not be till the year 2004, on the 8th of June; the latter part of which will be visible here after sun-rise. How Astronomy transports us into distant Futurity!

[Page 18]FROM these numbers a question naturally arises, How it comes to pass, that there will be another Transit after so short a term as 8 years, when there has not been one for above 120 years past, nor will there be another for above 100 years after the next? This question deserves an answer. I shall give it one, deduced from the first principles, in as clear and succinct a man­ner as I am able. It is to be observed then, that the Earth revolves round the Sun in 365 days; and Venus in 224; and moving in a smaller orbit included within the Earth's, she may pass between the Sun and the Earth; which aspect is called an inferior Conjunction. By reason of this quicker revolution, Venus must overtake the Earth in certain intervals of time, which I find to contain 584 days; so that Venus, hav­ing once passed between the Sun and the Earth, will return to pass between them in 584 days. Now if her orbit lay in the same plane with the Earth's, at every one of these inferior conjunc­tions Venus would pass between the Sun and the Earth, in the strictest sense of the word; and, her unilluminated side being then turned to the Earth, would appear to us like a black spot passing over the dise of the Sun. Had this been the position of her orbit, at the end of every period of 584 days there would have been a Transit of Venus; and the Transits would all have been central, the Planet every time des­cribing a diameter of the Sun. But the plane of Venus's orbit is inclined to the Earth's in an angle of [...] and so far might Venus be [Page 19] seen to the N. or S. of the ecliptic, if view'd from the Sun; but view'd from the Earth, to which she is much nearer in the inferior con­junctions, she may be seen above 9 ¼^d from the ecliptic; and therefore, as the Sun's semidiame­ter is but ¼ of a degree, Venus in her inferior conjunctions may pass 9^d to the N. or S. of the Sun's limbs. This Planet can appear on the face of the Sun, only in those inferior conjunc­tions, when her distance from the ecliptic, as seen from the Earth, is less than the semidia­meter of the Sun. For this effect, she must be within 5^d of her Nodes, which are the two op­posite points in which her orbit crosses the ecliptic; and this are she describes in 3 days. About the present age, the Sun is in one of these Nodes on the 5th of June, and in the other on the 6th of December. So that the Transits must happen between the 2d and 8th of June; or the 3d and 9th of December. To find at what intervals they may follow one ano­ther, we must find when the aforesaid period of conjunctions, viz. 584 days, taken some number of times will amount to some number or years, within 3 days more or less: for that will bring the conjunctions to happen at the same time of the year. Now five of these periods amount to eight years, wanting two days; and this is shortest interval in which the Transits can fol­low one another. No other number of these periods less than 147 will answer this conditi­on; and this number amounts to 235 years.— At the end of the period of 8 years in June, [Page 20] Venus passes almost 20^m more northerly than before; and as the semidiameter of the Sun is near 16^m, if in a Transit in June Venus passes above 4^m to the S. of the Sun's centre, there will be another Transit in 8 years; otherwise, not till 235 years. And from hence it appears, that these Transits cannot succeed one another in two consecutive periods of 8 years; for in the last of them, Venus will pass near 40^m more nor­therly than in the first; which is more than the whole diameter of the Sun. The like may be said of the Transits at the other Node in December.—The last time this period of 8 years took place, was in 1631 and 1639; and the next time, after the present, will be in 2004 and 2012.

AS there was some reason to suspect that Venus had a Satellite, partly from two observa­tions of M. Cassini at Paris in the years 1672 and 1686, but principally from one that was made by Mr. Short at London on the 23d of October 1740, I entertained hopes of seeing it pass over the Sun; and with this thought, view'd the Sun with great attention in the Re­flector, both on the 5th and 6th of June; but in vain. There were several spots then on the Sun, but none that I saw could be a Satellite.

TO conclude. The path of Venus, above determin'd, is her visible path, as affected by parallax at St. John's; which will be different from her visible path in distant parts of the [Page 21] Earth, where parallax has a different effect. This visible path, and the apparent motion of Venus in respect of the Sun, are here rightly determin'd, even if the elements imploy'd in the calculation, such as the latitude and longi­tude of the place, and those supplied by the astronomical tables, should not be exactly such as we supposed them. Any small difference in these particulars must equally affect the mo­tion both of the Sun and Venus; and there­fore, tho' it would alter their places in respect of the equinoctial points, it would produce no sensible difference in the relative motion of one to the other; nor any error in the situation of Venus in respect of the Sun. The comparison of the observations made in the N. W. parts of the world with those in the S. E. when all of them come to be laid toge­ther, will give the true path of Venus, ab­stracted from parallax; by which means, the quantity of the parallax will at length be dis­covered. The right determination of which point will render this year 1761 an ever-memorable aera in the annals of astronomy.