Observations of the Transit of Venus Over the Sun, June 3, 1769. In a Letter to the Reverend Nevil Maskelyne, F. R. S. Astronomer Royal, from John Winthrop, Esquire, F. R. S. Hollisian Professor of Mathematics at Cambridge, in New England

XLVI. Observations of the Transit of Venus over the Sun, June 3, 1769. In a Letter to the Reverend Nevil Maskelyne, F. R. S. Astronomer Royal, from John Winthrop, Esquire, F. R. S. Hollisian Professor of Mathematics at Cambridge, in New England. Cambridge, New-England, Sept. 5, 1769. Reverend Sir, I beg leave to lay before you my observation of the late transit of Venus, the beginning of which I observed, in this place, with all the care I was capable of. Our situation on this North American coast, I am sensible, was not the most favourable, as we could see only the first part of the transit; yet, I presume, careful observations, wherever made, will have their use in determining the grand problem of the Sun's parallax; at least, may serve as collateral evidences to the capital observations. Our apparatus having been wholly destroyed by fire, about five years ago, we have since procured a new set of astronomical instruments, made by some of the most eminent hands in London. A clock, by Mr. Mr. Ellicott, with the pendulum of his invention, having the bob supported by levers. An astronomical quadrant, of 2 feet radius, made by Mr. Sisson; and an equal altitude instrument, by Mr. Bird; each having three horizontal wires in the telescope. A reflecting telescope, of four feet focus; another of two; and another of 1 foot; fitted with an object glass micrometer, of $2\frac{1}{2}$ feet focus; all three made by the late Mr. James Short. I was obliged to remove the clock to another apartment, for the sake of the transit, which I did on the 23d of May, when I took some equal altitudes. By reason of an almost continual succession of cloudy weather till the end of that month, I could make but few material observations afterwards for regulating the clock. But, happily, the weather cleared up on the 1st of June, about noon, and continued fine for several days. As the precise knowledge of the true time is of the utmost consequence in the present case, I suppose a particular account of the observations made for this purpose, within a few days of the transit, will be most satisfactory to you; and this must be my apology for the prolixity of the detail. Observations for regulating the clock. Very cloudy till noon; the Sun hardly visible for a minute at a time; so that I could make only the following sparse observations. Equal altitudes of the Sun's limbs. | Morning | I. Afternoon | Middle Times | ☉ on the meridian | |---------|--------------|--------------|------------------| | h' | h' | h' | h' | | 7 55 | 13 | 4 0 | 6 | | II | 57 39.5 | equation—4.2 | | II | 57 35.3 | ☉ on the meridian | | II | 57 39.5 | equation—3.9 | | II | 57 35.6 | ☉ on the meridian | | II | 57 39 | equation—3.8 | | II | 57 35.2 | ☉ on the meridian | Mean II 57 35.4— or, true time of noon by the clock. I. | ☉ sup. limb | low. limb | |-------------|-----------| | 8 3 39 | 51 49 | | 4 49 | 50 38 | | 6 0 | 49 28 | | 6 30 | 48 58 | | 7 41 | 47 46 | | 8 52 | 3 46 35 | Mean II 57 43.75 Equation —3.77 ☉ on meridian II 57 40 II. | ☉ sup. limb | low. limb | |-------------|-----------| | 8 11 47 | 43 41 | | 12 58 | 42 31 | | 14 9 | 41 19 | | 14 39 | 40 49 | | 15 50 | 39 38 | | 17 1 | 3 38 27 | Mean II 57 44.1 Equation —3.7 ☉ on meridian II 57 40.4 ### Morning Afternoon Middle times | | III | | | |-------|-----|-------|-------| | | h | h | h | | | | | | | ○'sup.limb | 8 35 42 | 19 45 | 43,5 | | 2 | 36 54 | 18 33 | 43,5 | | 3 | 37 25 | 18 2 | 11 57 43,5 | | low.limb | 38 37 | 16 51 | 44 | | 2 | 39 48 | 3 15 40 | 44 | Mean 11 57 43,7 Equation -3,3 ○ on meridian 11 57 40,4 --- ### IV. | | | | |-------|-------|-------| | | h | h | | | | | | ○'sup.limb | 8 41 49 | 13 37 | 43 | | 2 | 43 1 | 12 25 | 43 | | 3 | 44 13 | 11 14 | 11 57 43,5 | | low.limb | 44 45 | 10 43 | 44 | | 2 | 45 57 | 9 30 | 43,5 | | 3 | 47 10 | 3 8 17 | 43,5 | Mean 11 57 43,4 Equation -3,2 ○ on meridian 11 57 40,2 ○ on the meridian. I. 11 57 40 II. 40,4 III. 40,4 IV. 40,2 Mean of all 11 57 40,3 or, true time of noon by the clock. --- ### I. | | | | |-------|-------|-------| | | h | h | | | | | | ○'sup.limb | 7 46 28 | .9 12 | 11 57 50 | | 2 | 47 39 | 8 1 | equat. -3,8 | | 3 | 48 9 | 7 31 | | | low.limb | 49 20 | 4 6 20 | | | 2 | | | | | 3 | | | | ○ on meridian 11 57 46,2 --- ### II. | | | | |-------|-------|-------| | | h | h | | | | | | ○'sup.limb | 7 51 38 | 4 2 | 50 | | 2 | 52 48 | 2 51 | 49,5 | | 3 | 53 59 | 1 40 | 11 57 49,5 | | low.limb | 54 29 | | | | 2 | 55 40 | 4 0 0 | 50 | | 3 | 56 50 | | | Mean 11 57 49,75 Equation -3,74 ○ on meridian 11 57 46. | Morning | Afternoon | Middle times | |---------|-----------|--------------| | h' | h" | h' | | 7 58 | 56 46 | 48,5 | | 8 0 | 55 35 | 48,5 | | 1 13 | 54 34 | 11 57 48,5 | | 53 55 | | | | 2 54 | 52 46 | 50 | | 4 5 | 3 51 35 | 50 | Mean of all: 11 57 45,9 Equation: -3,6 Or, true time of noon by the clock. At 4h 15' thermometer 85°. --- | I. | |----------| | 7 45 | 10 51 | | 46 12 | 9 40 | | 47 23 | 8 29 | | 47 53 | 7 59 | | 49 4 | 6 48 | | 50 14 | 4 5 37 | Mean: 11 57 56- Equation: -3,6 Or, true time of noon by the clock. --- | II. | |----------| | 7 52 32 | 3 20 | | 53 42 | 2 9 | | 54 12 | 1 40 | | 55 23 | 4 0 29 | | 56 34 | 3 59 18 | Mean: 11 57 56- Equation: -3,5 Or, true time of noon by the clock. --- | III. | |----------| | 7 57 38 | 58 11 | | 58 50 | 57 0 | | 8 0 2 | 55 49 | | 0 32 | 55 19 | | 1 42 | 54 8 | | 2 43 | 3 52 57 | Mean: 11 57 55,1 Equation: -3,4 Or, true time of noon by the clock. --- Observation Observation of the Transit of Venus. I chose to observe the transit with the 2 feet telescope, as I supposed most of the observations in other parts would be made with telescopes of that size; and I used a power that magnified 90 times, which gave a very distinct view of the spots then on the Sun. Soon after two o'clock, I began to look on the Sun's upper limb, where the Planet was to enter. The first impression I perceived was at $2^h\ 27'51''$, by the clock, the Sun being then perfectly clear. I then rested my eye, which was pretty much fatigued, to prepare it for the total ingress or interior contact. At $2^h\ 45'15''$, I began to be doubtful whether the internal contact was not formed; but at $20''$ was satisfied that it was past, the Sun's limb being restored to its integrity, in the place where it had been interrupted by the Planet. During this interval of near $5''$, there seemed to be a duskishness in the place of contact; my idea of which is well represented by Mr. Dunn's figure of what he calls the grey contact, in Phil. Trans. Vol. LII. Tab. vii. p. 190. By the foregoing equal altitudes it appears, that the clock was now $2'13''$ + too slow. I therefore state the observation as follows: | Apparent time | h | m | s | |---------------|---|---|---| | First visible impression of Venus upon the Sun | 2 | 30 | 4 | | Internal contact | 47 | 30 | This time of internal contact, I think, cannot differ above $2''$ from the truth, and perhaps may not differ differ 1''. But about 4'' of Venus's diameter must have entered upon the Sun before I perceived the impression. At nine in the morning, I observed the Sun's diameter, in the horizontal direction, to be 121 parts of the micrometer, = 31' 33'',2. At 5h 34' 23'', the Sun's north limb was distant from Venus's south limb 9 3 of the micrometer, = 6' 16'',2. At 5h 37' 23'', I found no sensible difference; and the Sun's north limb was then distant from Venus's north limb 7 14½ of the micrometer, = 5' 17'',6. This gives Venus's diameter 58'',6; and the least distance of centers 9' 59'',7. Hence, the true duration of the ingress should be 18' 56''; but this being here contracted 15'', by parallax, is reduced to 18' 41''. So that the first contact, strictly so called, happened 1' ¼ before the impression was discovered; and the central ingress was at 2h 38' 5''. The nearest approach was nearly, I suppose, at 5h 37'. After Venus was entered upon the Sun, I viewed her attentively several times, with a power of the great telescope which magnified 260 times, but could perceive no such duskishness round her as I saw at the internal contact, nor that imperfect light upon her disk, especially near the centre, which Mr. Dunn speaks of; neither could I discover any satellite. Soon after six, the western sky began to be overcast, so that for a considerable time before sun-set the Sun was hid. I made observations for determining some other positions of Venus upon the Sun; but as they can be of no service in the grand problem of the parallax, I think it needless to swell this letter, very long already, with them. I therefore only add, that the latitude of this this place is nearly $42^\circ 25'$ N. and the difference of meridians west from London about $4^\circ 44'$. But this may be farther ascertained by the following emersions of Jupiter's satellites, which I observed with the 2 feet reflector. **Emersions of Jupiter's first satellite.** | Apparent time | h | ' | " | |---------------|---|---|---| | 1768 April 25 | 9 | 13 | 52 | | May 18 | 9 | 27 | 27 | | June 10 | 9 | 37 | 25 | | July 3 | 9 | 45 | 54 | | | h | ' | " | |---------------|---|---|---| | 1769 May 14 | 10| 19 | 7 | | Aug. 23 | 7 | 31 | 50 | Emersion of Jupiter's second satellite. | | h | ' | " | |---------------|---|---|---| | June 7 | 9 | 1 | 15 | I submit the foregoing observations to your candid acceptance; and am, with great respect, **Reverend Sir,** Your most obedient humble servant, John Winthrop.