The Transits of Venus 

A talk given to club members by Brian Sheen.

Introduction.

The principal characteristic of the solar system is that it is co-planar, all the planets and the Sun lie within a few degrees of one another. This means that the Inferior planets, Mercury and Venus can pass across the face of the Sun this phenomena is known as a transit. Mercury transited in May 2003. Venus has not crossed the Sun since 1882, and will not do so again until 2012. The intervals between transits are successively 8, 121.5, 8, 105.5, 8, 121.5 et seq. As can be seen they occur in pairs and these pairs alternate between June and December events. The study of transits has covered nearly 400 years and during this time a number of very significant changes have taken place.

Calendar changes. In the 17th century we used the Julian calendar (0ld Style). This means that the December transits of 1631 and 1639 took place in November! In 1750 the Julian calendar was so out of step that it was revised to the Gregorian (New Style) and the dates jumped 11 days. This gave rise to a public outcry as the people thought they were being deprived of several days of life.

The measurement of time was pretty inaccurate in those early days and it was between1730 and 1760 that John Harrison developed his accurate clocks (chronometers) for the determination of the longitude of places around the world.

The transits were used to determine the size of the solar system using parallax, and also within the UK to refine the determination of longitude, using time differences. Today this method has been replaced by bouncing radar pulses off the nearby planets.

The transits considered here are as follows 1518 June 2; 1526 June 1; 1631 Dec 7th; 1639 Dec 4; 1761 June 6; 1769 June 3; 1874 Dec 9; 1882 Dec 6; 2004 June 8; 2012 June 6. The eight year pairs occur because in the time it take for the Earth to orbit the Sun eight times is exactly the same as the time taken for Venus to orbit thirteen times. Pivotal to our understanding of the transits is the graphic illustrating the times and altitudes of the Sun during the transit periods. One obvious fact is that this is the first transit in recorded history to be visible from start to finish, from the UK.

The two 16th century events were not knowingly observed by any one. The 1631 transit was missed by Gassendi and the 1639 one seen only by Horrocks and Crabtree. The 1761 event was recorded by a few astronomers including Richard Haydon who was based in Liskeard. The 1769 was observed by Captain Cook in Tahitae and the two nineteenth century transits were in the reign of George Biddall Airy as Astronomer Royal and Edwin Dunkin of Truro as his Assistant and were recorded most carefully. One problem with timing transits in white light is that the planet always exhibits a "black drop" when entering and leaving the edge of the Sun. The 2004 transit will be the first to be observed in the H Alpha wavelength, and the "black drop" will not be apparent in this monochromatic light.

Although we will be unable to carry out meaningful measurements of the size of the solar system we will be able to determine our longitude by comparing timings with RAL scientists at Oxford and Greenwich Observatory and checking them using Global Positioning Satellite Receivers (GPS).

SAFETY CAVEATS The Sun is every bit as dangerous in 2004 as it was during the eclipse of '99. Hence all the same precautions must be taken. Proper eclipse glasses are safe, although projection methods are to be preferred. Pin hole projection will not give a satisfactory result. One problem is that Venus is big enough to be seen as a disc with the naked eye. The Sun must never be observed with telescope or binoculars without the aid of a full aperture solar filter. Blindness can and will result.

THE SEVENTEENTH CENTURY TRANSITS.

The first astronomer to recognise that Venus could cross the face of the Sun was Kepler who correctly predicted the 1631 event. Although it was watched for by Gassendii the predictive science was not refined enough for him to realise that it would occur during the hours of darkness. However the graphic shows that if it has been a June transit most of it would have been clearly visible.

NOTE. Pierre Gassendii 1592 - 1655 was Professor of Mathematics in Paris, he observed a transit of Mercury in 1631 projecting an image of the Sun using a Galilean telescope invented by Galileo some 20 years before.

However Kepler failed to predict the 1639 transit and this was left to Jeremiah Horrocks (1617 - 1641) to realise that transits occur in pairs eight years apart. He set up a dark room facing the Sun and projected the image in a similar way to Gassendii. Naturally the weather was cloudy and as it was a Sunday he was called away to church from time to time. The best tables available to Horrocks were the Rudolphine Tables (1627) of Tycho Brahe and Johannes Kepler (1571 - 1630). However they were produced using pretty basic equipment and were only accurate to a few minutes of arc. Hence he was delighted and surprised to see the disc of Venus on the Sun at about three pm. The graphic confirms that at this time the transit had only just started and as the Sun was already near to the western horizon he was able to observe for no more than half an hour. Apart from William Crabtree, Horrocks partner, no other astronomer witnessed this phenomenon, Horrocks was so determined to get support that he urged Crabtree with the following words. "I beseech you with all my strength to attend to it diligently with a telescope."

NOTE In 1597 Brahe moved to Prague with Kepler under the patronage of the Holy Roman Emperor Rudolph II and when the tables, the product of many years work were published in1627 they were called the Rudophine Tables in honour of their patron.

THE EIGHTEENTH CENTURY TRANSITS

The next pair of transits were due in 1761 and 1769 this time in June as Kepler predicted. Edmund Halley (1656 - 1741) realised that if the transit could be observed from two different points on the surface of the Earth then the distance of the Sun from the Earth could be measured and then using Kepler's Laws the scale of the whole Solar System could be determined. He understood that he would never live long enough to see this happen so he wrote the following;-

 "To young astronomers who may live to observe these things, a method by which the immense distance of the Sun may be truly obtained. That they may diligently apply themselves and not by unseasonable obscurity of a cloudy sky be deprived of this sight and that having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory"

The principal of using parallax to measure angles from two observing stations. Given the distance between them the distance between the baseline and the remote object is simple enough to calculate. However the actual procedure is much more complex, so much so that the best results were not obtained until 1824 when Encke analysed those results arriving at a figure of 95,000,000 miles.

The 1761 transit results are well recorded by Ferguson, The large figures in his text show clearly the difficulties of obtaining good results. The locations chosen were London, and St Helena, Bencoolen (Sumatra) and the Ganges. These pairs are on similar longitudes, in addition Hudson Bay was also chosen as it some 90 degrees west of London where Venus was to be in the middle of its transit as midnight London time. A scale figure of the disc of Venus is given and the four transit lines are shown with in it. Even today the separation is exaggerated to give the effect that the transit lines are on opposite side of the solar disc.

The work of Richard Haydon is recorded in a separate chapter, due to the local interest. His involvement in the '69 transit is noted below.

JAMES COOK Captain Cook's transit 1769. The declared purpose of James Cook's voyage to the South Seas was to observe the transit of Venus from Tahiti. (The undeclared objective was to find and secure for Great Britain the suspected southern continent - Australia.)

The James Cook University in Australia features this expedition on its excellant web site. Although it omits the actual measurements:-

1st External contact 9hrs 25 mins. 4sec morning 1st internal contact 9 44 4 2nd internal contact 3 14 8 afternoon 2nd external contact 3 50 10

Latitude 17 degrees, 29 mins. 15 secs. Longitude 149 degrees, 32 mins, 30 secs. West of Greewich.

There a number of incidents with the natives, this ensured that a fort was built to ensure the safety of the party. One of the local chiefs took Mr Bank's gun and fired it into the air, much too Bank's surprise. However it was recovered without too much trouble. (Joseph Banks was the ships botanist and the plant species Banksia is named after him.) Just before the transit the main quadrant was stolen, after much effort it was recovered, although the natives had managed to dismantle it, and restored to working order. The results obtained from this transit were considered to be excellent.

Richard Haydon's contribution to the '69 transit. John Allen in his history of Liskeard notes that Haydon had recorded the '69 transit, however Ferguson states that due to the fact that the transit only started just before sunset that no useful work was done in the UK. Further more the Royal Society who hold his records of the 61 transit can find no reference to observations made during the later event. It would appear that these references are in fact an error.

THE NINETEENTH CENTURY TRANSITS.

The two transits involved were both December events (1874 and 1882) and for the most part were not visible in the UK. Both occurred during the reign of George Biddell Airy as Astronomer Royal and with Cornish astronomer Edwin Dunkin as one of his assistants. These two transits were among the most carefully observed events of all time, with expeditions going to many corners of the Earth from many countries in Europe and beyond.

The official report of the 1874 transit was given to the Royal Institution of Cornwall, presumably by Dunkin who was at one time President of the Institution. It runs to some 500 pages and was published in 1881. As one would expect it is very carefully written and lists the places the British sent expeditions to observe the event. These included Hawaii; Egypt; Rodrigues Island, near Mauritius; Kerguelen Island north of Antarctica.

Dunkin's main role was in the selection and procurement of the telescopes for these expeditions. In 1869 he visited a number of observatories to check their 6" equatorial telescopes to see if they were suitable for purchase by the Royal Observatory. Four telescopes were inspected for the firmness of the mounting and also to see if they could be adapted for use in the southern hemisphere. The quality of the Objective glass was inspected together with all the eyepieces and micrometers. The optical quality was checked by observing stars at night and the drive was also checked as to steadiness and convenience of arrangement. All the equipment was purchased and formed an important part of the instrumental equipment in the expeditions for both the '74 and '82 transit.

1882 Transit in Ireland. Dunsink Observatory, Dublin. Although the graphic makes plain the fact that none of the 74 transit was observable from the UK the first part of the 82 event was. It was recorded in true astronomer fashion by Sir Robert Stawell Ball. "The morning of the eventful day appeared to be as unfavourable for a grand astronomical spectacle as can be imagined with snow two inches thick and more falling." "At length the struggling beams of the Sun pierced through the gloom" "The snow flakes falling during the brief periods of sunshine reminded me of the golden rain seen during pyrotechnic displays." Eventually the clouds cleared to allow some measurements to be taken until the Sun sank so low to the horizon as to make further recording uncertain.

He concluded with "The intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of Venus enables us to solve, are all present to our thoughts when we look at this pleasing picture, a repartition of which will not occur again until the flowers are blooming in the June of AD 2004."

Rev Richard Haydon's account of the 1761 transit as seen from Liskeard.

Transcribed from the Philosophical Transactions of the Royal Society 1761.

To the Rev. Thomas Birch DD Secretary to the Royal Society. Dated Nov. 12th 1761.

Dear Sir,

I send to you enclosed the Rev. Haydon's observation of the late transit of Venus; and should have waited on you with it long ere now; as I promised, but that I unluckily got a fall, which still confines me at home. I presume this observation may deserve the notice of the Society, as the best circumstanced of any I have yet seen made in England for several of the phases are earlier than earlier than those at Greenwich, or ours at Savile-House, taken with a telescope of Mr Short's armed with Mr. Dollond's new micrometer, and his time accurately ascertained.

Dear Sir, Your most obedient and affectionate humble servant J.Bevis. Clerkenwell-Close, Nov12th, 1761

By many comparisons of different observations, I make Mr. Haydon's latitude to be 500 26' 55", and his longitude west of London in time, 16 minutes 10 seconds nearly; though he, from a memorandum he made some years ago, supposed it near two minutes more.

To John Bevis, Doctor of Physic.

SIR, Liskeard, June 9, 1761.

I should, with great pleasure, have pursued, in every particular, the method you recommended to me in observing the late transit of Venus, but unfortunately, had it not in my power to do so. The low situation of my house, and a small hill at a distance to the N.E of it, would not allow me, even from my garret windows, a view of the Sun, till it was 11 degrees or 12 degrees above the horizon. By this means, I was deprived of an opportunity of making two of the principal observations. It was almost half an hour after five, when I could first get a sight of the Sun. I was in hopes, from what you had wrote me, that the planet had not at that time passed its nearest distance from the centre; but soon the vexation to find myself disappointed. I, however, continued to observe the distance of Venus from the limb of the Sun with as much accuracy as I could, an account of which you have on the next leaf. I think there cannot be an error of more than two or three seconds in the time of the interior contact, and not one of the total egress.

It was but two days before I received the favour of your letter, that I came down stairs for the first time, after a six weeks severe fit of the gout. During my illness, my clock was run down, and stopped. I immediately set it going again, as nearly to the time as I could then guess. The next day, being the first of this month, at night I observed the transit of a star over the horizontal hair in the hair of my telescope of my quadrant. The third day, I repeated the same, and again last night: by which you will see my clock measures time correctly enough. Thursday, the 4th, was very hot and sultry all day; the evening, hazy and foggy. Fearing the night following might prove the same (as it unluckily did), and that I should not be able to take the equal altitudes of any of the stars before and after they had passed the meridian, I observed, on Friday, several correspondent altitudes of the upper limb of the Sun, in the morning and afternoon; by which time they may be precisely enough ascertained. At the bottom, I trouble you with an account of these observations, as also of some made the day following.

It gives me much concern, that I cannot herein answer your expectations, in a more perfect and satisfactory manner; but I assure you, Sir, I did every thing in my power for that purpose, and should most readily embrace any opportunity of testifying the respect I owe you. I beg you'll present my compliments to Mr Short; and am,

SIR, &c. R.Haydon.

Corrispondent altitudes of the upper limb of the Sun. June the 5th, 1761

By the above, my clock too fast in apparent time 9' 34"

June 6th, upper limb of the Sun.

By these, clock too fast 9' 40"

Star crossed the hair of my telescope, &c.

N.B. Mr. Haydon informs me, in a subsequent letter, that "on comparing his observations with those made in London, his interval between the internal contact and total egress was considerably longer than any of those others. Wherefore, he examined his notes again, but could not find he had made any mistake in transcribing them." He adds that "being obliged to observe from an upper window, his regulator being fixed below, but within hearing, he got a lad, of about fourteen, whom he strictly charged to be particularly attentive to the second shewn by the clock, whenever he should call to him; in which respect, he is of opinion, he made no mistake, though, possibly, he might make one with regard to the minute, by setting down one too many at the egress; which he now thinks there is some cause to believe he did.

J.B

Nearest distance of Venus from the limb of the Sun, N.B. The diameter of Venus is included.

Biographical Details-

The Rev Richard Haydon, born 1706 was the Master of the Liskeard Grammer School 1741 - 71. He was educated at Pembroke College, Cambridge and gained his MA in 1738. He opened a Latin School in 1755 and it is recorded that he received 5/- from the boys who broke the glass at the school. He acquired a profound knowledge of mathematics and astronomy; and in these the most exact and noble of sciences, he was not content with theory, and with the practical result of labours carried on by others. He provided himself with various instruments of a size and accuracy then rarely possessed by individuals, and made important observations on the transit of Venus in 1769 (in addition to 1761) and for along time all the longitudes of places in the west of England were deduced from Rev Haydon's determinations at Liskeard. He retired to the family living in Oakford, Devon in 1771. He died on the 25th January 1788. No records of his parentage have been found. The Rev Haydon took charge of the Grammer School before 1741, a position he held for 30 years. In 1741 his salary is recorded as £30.00 per year. His account of the transit of Venus June 6th 1761 took the form of a letter to John Bevis MD dated 9th June 1761. It was published in the Philosophical Transactions vol. LII, part 1 p 202 - 208 for the year 1761. published 1762.

The only reference I can find in the public domain to Rev Haydon's 1761 observations is in Ferguson J. Astronomy, ps. 497, 498 11th edition 1803.

"The Rev Richard Haydon at Liskeard in Cornwall (16 minutes 10 seconds in time west from London, as stated by Dr. Bevis) observed the internal contact to be at 8 hours 0 minutes 20 seconds, which by reduction was 8 hours 16 minutes 30 seconds at Greenwich: so that he must have seen it 2 minutes 30 seconds sooner in absolute time than it was seen at Greenwich - a difference by much too great to be occasioned by the difference in parallaxes. But by a memorandum of Mr Haydon's some years before, it appears that he then supposed his west longitude to be near two minutes more; which brings his time to agree within half a minute of the time at Greenwich; to which the parallaxes will very nearly answer."

The biographical details are compiled from the following 3 papers.

1 Boase G and Courtney W, Bibliotheca Cornubiensis, Vol 1 p. 224 pub. Longmans, Green, Reader and Dyer, 1874

2 Allen J., History of the Borough of Liskeard p 250, 349, 508, 509. Pub W & F Cash, 1856.

3 Boase G. Collectanea Cornubiensia p 1387. Pub Netherton & Worth, Truro, 1890

The astronomical report is to be found p 202 - 208 Philosophical Transactions of the Royal Society Vol LII, part 1, it relates to the year 1761 but was published in 1762 and takes the form of a letter to Dr John Bevis.

Acknowledgements;- My thanks for assistance with this work go to Kim Cooper, Cornish Studies Library, Redruth and to Peter Hingley Librarian of the Royal Astronomical Society, London Brian Sheen - Roseland Observatory.

References and Acknowledgements

1) Ball, R.S., The Story of the Heavens, Cassell & Co. 1893 2) Hingley P.D. & Daniel. T.C., A far off vision - E.Dunkin's biography, Royal Institution of Cornwall 1999. 3) Hoskin. M., The Cambridge illustrated History of Astronomy, Cambridge UP.1997 4) Low. C.R., Captain Cook's Three voyages round the world, George Routledge and Sons Ltd. 1897 5) Chapman A., Gods in the Sky, Channel 4 Books, 2002 6) Porter R. & Ogilvie M., Hutchinson Dictionary of Scientific Biography, Helicon Publishing Ltd. 2000 7) Sorbel D, Longitude, Fourth Estate, London, 1995. 8) Moore. P., 1991 Yearbook of Astronomy, Sidgwick & Jackson, London. 1990 9) Bonnycastle J. Introduction to Astronomy. J.Nunn et al, London, 1816. 10) Ferguson. J., Astronomy. A.Strahan, London, 1803 11) Keill, J. Introduction to the true Astronomy, J.F & C Rivington et al. London. 1778. 12) Sheehan, W. "The Transit of Venus", Sky & Telescope - May 2004.

Acknowledgements.

Frank Johns - Roseland Observatory for the Venus Transit Period graphic. 
Peter Hingley - Librarian Royal Astronomical Society details of 1761 transit. 
Kim Cooper Cornish Studies Library, Redruth, information on Richard Haydon. 
Clara Anderson - Royal Society, for help with the 1769 transit.

Web sites Roseland Observatory - www.roselandobservatory.com 
NASA & Goddard - http://sunearth.gsfc.nasa.gov/eclipse.html 
Fred Espenaks eclipse site. James Cook University Australia http://www.jcu.edu.au/school/mathphys/astronomy/jamescook.shtml excellent background information on James Cook and for a drawing of Tahiti. They also run a distance learning course in astronomy, see 'Astronomy Now'.