A Brief History of the Transit of Venus

Technology Through Time: Issue #73

By Dr. Sten Odenwald (NASA / ADNET)

Ancient History: Pre-1631 A.D.

There have been 53 transits of Venus across the Sun between 2000 B.C and 2004 A.D. History says that Jeremiah Horrocks was the first human to ever witness a transit by Venus in 1639, but could other more ancient people have also seen it too?

As seen on the Sun, Venus is as big as a large sunspot. You could see it with the naked eye if you knew exactly when to look. But, because you cannot look directly at the sun except when it is close to the horizon, you would have only a very short time to be lucky to see it, and a reason for wanting to look at the sun on the horizon in this way at all!

The ancient Babylonians were the earliest sky observers who kept detailed records. Four transits occurred during the Babylonian Era on May 20, 1641 BC, November 20, 1520 BC, November 18, 1512 BC and May 23, 1406 BC. Could any of these be seen?

In the British journal 'Monthly Notices of the Royal Astronomical Society' for November 1882 (vol. XLIII page 41) you can find a curious article written by Rev. S.J. Johnson that asks whether the ancient Assyrians had observed the Venus Transit. He said that an article in the journal Nature published a few years earlier, and written by Rev. Sayce, mentioned a broken Assyrian cuneiform tablet. The tablet was about Venus, and a translated sentence on the tablet had breaks in it which seemed to indicate that such a transit had been seen. "the planet Venus --- it passed across ---- the Sun --- across the face of the Sun" .The data of the tablet was apparently before the 16th Century B.C.. So, what was this mysterious tablet mentioned by Sayce, and had it really been translated correctly? The implication is that sometime before ca 1500 'something' involving Venus and the Sun at close quarters did occur from Babylonia. If it was perhaps one of the four transits in the list above, this would be one of the earliest astronomical phenomena ever recorded by humans that survived to the present time! Since it is impossible to tell from the articles exactly which cuneiform tablet the inscription appeared upon, we cannot subject this tablet to a modern translation to see if it's message stands up.

We do know some things about ancient Babylonian astronomy, and Venus figures prominently in their fascination with the heavens. The Venus Tables of Ammizaduga were discovered in 1850 in Nineveh by Sir Henry Layard in excavations of the library of Asurbanipal. The translations were published a few years later by Sir Henry Rawlinson and George Smith as "Tables of the movements of the planet Venus and their influences". One of the large tablets called K.160 contains 14 observations of Venus. For example, in section 1 we read "If on the 21st of Ab, Venus disappears in the east, remains absent in the sky for two months and 11 days, and in the month of Arahsamna on the 2nd day, there will be rains in the land ;desolation will be wrought". None of these tablets have any inscription suggesting a transit.

Venus can be seen in exactly the same spot in the sky every 583.9 days. This is called its Synodic Period. The tablets indicate that the Babylonians knew that every 8 solar years ( 8 x 365.24 = 2921.92 days) Venus reappears in the exact same place in the sky ( 5 x 583.9d = 2919.5 days). Because this also equals 99 lunar months (99 x 29.5 = 2920.5d) Venus returns to the same place in the sky at the same lunar month (and phase) too, but the return happens 2 1/2 days later each time (2921.92 - 2919.5 = 2.42d) . After 150.8 years the return is exact (2.42 x 150.8 = 365.24).

Getting back to our story about the Babylonians having observed a transit of Venus, the Transit of 1406 BC can be ruled out because it occurs in the 15th century, but that leaves the transits of 1520, 1512 and 1641 BC as possibilities during the time of the Assyrians. It is an easy matter to determine whether these were seen at sunrise or sunset on the horizon, so that the dark spot could be viewed with the naked eye.

How about Chinese observers?

Chinese astrologers kept close track of the sun, especially large sunspots that could be seen at sunrise and sunset before the sun became too bright to see with the unaided eye. The earliest records of sunspot sightings began around 800 BC., but their observations apparently began in earnest around 167 BC. Astronomers Zhuang and Wang (1988) compiled a list of over 270 sunspot sightings from ancient Chinese, Korean and Japanese records. A comparison by Wittman and Zu (1987) and Yao and Stephenson (1988) of sunspots and the expected Venus transits shows no examples of even near-misses. By the way, Mideaval Arab astronomers often explained dark spots on sun as transits of mercury or Venus, examples are 840, 1030, 1068 and 1130 AD, but no Venus transits occurred during these years so they were probably very large sunspots.

Did Montezuma see the Venus transit in 1518 AD?

Montezuma, the leader of the Aztec people in pre-Columbus Mexico, was a careful observer of the sun, which he used in his divination practices. Venus was a very important celestial body in Aztec mythology as well as Mayan. The Transit of May 25, 1518 AD would have been visible to him at sunset. It is said that a jade figure at the British Museum of the god Quetzalcoatl, an aspect of Venus, wearing the Sun as his neck ornament, is a memorial of this rare event. Since Montezuma and the Aztec civilization were conquered by Cortez in 1520, this would certainly have been an ill-omen of impending doom!

We now arrive at the modern era of Venus transit observing as scientists first sorted out the shape of the solar system and planetary orbits, and then began to make very accurate forecasts of where planets would be in the sky.

Galileo Galilee in ca 1610 was the first human to actually see Venus as more than just a bright point of light in the sky. With his telescope, he made the discovery that it has a disk shape that changed its illumination phase just the way the Moon does as it circles Earth. This only made sense if Venus orbited the Sun, and so Venus played a very important role in confirming the heliocentric model of Copernicus. In September 1610, he sent an anagram to a friend of his announcing his discovery which translates as: "The mother of love [Venus] emulates the shapes of Cynthia [the Moon]".

Johannes Kepler (above), meanwhile, was shaking up the world by his meticulous use of astronomical data assembled by Tycho Brahe. The result was his discovery of three important laws of planetary motion, and later on, the publication of the Rudolphine Tables in September 1627. These tables were superior to commonly used tables based on Ptolmey's epicycle models, and included planetary position predictions to 1636. What he discovered during these laborious hand calculations was that Venus would pass in front of the Sun in 1631, so he wrote a 'Notice to the Curious in Things Celestial' to alert observers to the Venus transit of 1631 as well as a second transit to take place in the 1700's. The December 6th 1631 transit was looked for by French astronomer Gassendi from Paris but not seen. It was actually not visible from Europe at all. Kepler himself died in 1630, but he had actually missed a second transit of Venus which would occur 8 years later.

Transit of Venus December 4, 1639

Jeremiah Horrocks (born c.1619) was convinced by his own observations that available tables for planetary positions were incorrect, so he resolved to gather new data rather than to try to modify Kepler's old tables. From his revised calculations and new data, he was able to predict that a further transit of Venus would occur on 4th December 1639, 8 years after the one predicted by Kepler and much earlier than the 120 year wait predicted by Kepler. Let's see what he had to say in his own words:

"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, which, for these ornamental pursuits I could not with propriety neglect. But during all this time I saw nothing in the sun except a small and common spot, consisting as it were of three points at a distance from the center 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 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 disc on the left, so that the limbs of the Sun and Venus precisely coincided, forming an angle of contact. Not doubting that this was really the shadow of the planet, I immediately applied myself sedulously to observe it"

"...I wrote therefore immediately to my most esteemed friend William Crabtree (see above painting), 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; he therefore carefully prepared for the observation, in a manner similar to that which has been mentioned. 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, certainly between thirty and forty minutes after 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 disc. ... but Crabtree's opportunity was so limited that he was not able to observe very minutely either the distance itself; or the inclination of the planet. As well as he could guess by his eye, and to the best of his recollection, he drew upon paper the situation of Venus, which I found to differ little or nothing from my own observation;...

I wrote also of the expected transit to my younger brother, who then resided at Liverpool, hoping that he would exert himself on the occasion. This indeed he did, but it was in vain, for on the 24th, the sky was overcast, and he was unable to see anything, although he watched very carefully....I hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, preferring rather their hawks and hounds, to say no worse; and although England is not without votaries of astronomy, with some of whom I am acquainted, I was unable to convey to them the agreeable tidings, having myself had so little notice... At Goesa, in Zealand, where Lansberg lately flourished, it [the Transit] commenced at fourteen minutes past three, and the Sun set at fifty-five minutes past three, consequently it might have been seen there. But no one excepting Lansberg and his friend Hortensius, both of whom I hear are dead, would trouble themselves about the matter; nor is it probable that, if living, they would be willing to acknowledge a phenomenon which would convict their much-vaunted tables of gross inaccuracy...In short, Venus was visible in the Sun throughout nearly the whole of Italy, France, and Spain; but in none of those countries during the entire continuance of the transit. But America! O fortunatos nimium bona Si sua norit! Venus! Which riches dost thou squander on unworthy regions, which attempt to repay such favors with gold, the paltry product of their mines. Let these barbarians keep their precious metals to themselves, the incentives to evil, which we are content to do without. These rude people would indeed ask from us too much should they deprive us of those celestial riches, the use of which they are not able to comprehend. But let us cease this complaint O Venus! and attend to thee ere thou dost depart.'

In 1663, James Gregory, a Scotish mathematician and astronomer, suggested that a more accurate measurement of the Solar Parallax could be gained from observations of the transit of Venus made from various widely separate geographical locations.

During a stay on the island of Saint-Helena, Sir Edmund Halley (1656-1742) observed a Mercury transit in that year and made careful note of the times of entry and exit of Mercury over the solar disk. He realized that if a transit would be observed from different latitudes on Earth, the different observers would see Mercury cross the Sun along at a different angle. This effect is known as parallax (this is even more noticable for Venus transits, since Venus is closer to us than Mercury, which increases the difference in angles) and could be used to determine an accurate Earth-Sun distance. Halley published past and future transit predictions in 1691, then in 1716 he published a greatly refined version of a paper originally read before the Royal Society in 1691, entitled 'A new Method of determining the Parallax of the Sun, or his Distance from the Earth'. In the paper he championed the idea of scientists from various nations observing the 1761 and 1769 transits of Venus in as many parts of the world as possible. This, he argued, would result in a 'certain and adequate solution of the noblest, and otherwise most difficult problem' of accurately establishing the distance between the Earth and the Sun.

In 1716, Halley formally proposes Venus transit observations and shows how to use them to find exact value of the astronomical unit - the distance from the sun to earth. In his article published in the Philosophical Transactions and titled "A new Method of determining the Parallax of the Sun, or his Distance from the Earth"

"We therefore recommend again and again, to the curious investigators of the stars to whom, when our lives are over, these observations are entrusted, that they, mindful of our advice, apply themselves to the undertaking of these observations vigorously. And for them we desire and pray for all good luck, especially that they be not deprived of this coveted spectacle by the unfortunate obscuration of cloudy heavens, and that the immensities of the celestial spheres, compelled to more precise boundaries, may at last yield to their glory and eternal fame."

Transit of Venus, June 5, 1761

On June 5, 1761 the transit of Venus was observed by 176 scientists from 117 stations all over the world. The curious 'Black Drop Effect' was first spotted, and the Russian astronomer Mikhail Lomonosov was the first to deduce that Venus had an atmosphere because of the beautiful halo of light that surrounded its dark disk just as it crossed the edge of the sun. His drawings show some of the details that he noted about the edge of Venus near the Sun. Note that his 'Figure 6' shows the halo effect.

This transit was not one of the best ones to observe to determine the distance to Venus and the sun. It actually took nearly 50 years for the astronomer Encke to finally collect all of the observations, analyze them mathematically, and report an improved estimate for the distance of 95 million miles.

Some of the scientists were involved in spectacular international events as they carried out their observations in remote corners of the world. Most were French or English, and the transit occurred during the peak of the Seven Year's War between these two international empires. Special letters of passage were carried by these scientists so that they could safely pass into 'enemy' territory. You would never know about these adventured from reading the newspapers! Here's what the newspapers had to say about this transit in a very short announcement on June 8, 1761. By the way, written english used the letter 'f' instead of 's' in many words!:

"The transit of Venus over the Sun on Saturday laft was carefully obferred by many curiouf Gentlemen and differ'd confiderably from every Computation made of it. Its Emerfion was at about 35 Minutes after Eight, but the Morning being cloudy, was not vifible in London til it had paff'd three Fourths of the Sun's Diameter."

Transit of Venus, June 3, 1769

• 400 sightings published.
• Benjamin Franklin observes it from US.
• Capt. Cook and Tahiti chronicles
• Measurements helped to establish exactly where California was located in longitude. Older maps had placed its over 350 miles further to the west.
• Parallax narrowed to 8.5 to 8.8".

Although there were many international expeditions involved in these observations, the is perhaps the most famous expedition at the time, under the lead of Captain James Cook, who set up an observation post in Tahiti with his ship, the Endeavor.

This mission was a complete success, not only did he get pretty accurate data of the transit, but on the next leg of his journey, he discovered New Zealand and got stuck on the Great Barrier Reef for several weeks (he was the first to draw a detailed map of the Reef), and explored many of the then unknown coasts of Australia. Captain Cook set sail from Plymouth in 1768 on the H.M.S. Endeavor to circumnavigate the globe and explore the southern Pacific Ocean. On the way, they were to put in at the South Pacific island of Tahiti to observed the Transit of Venus. The expedition astronomers, with help from a detachment of Royal Marines, setup an observatory on a high point of ground above the bay still known to this day as "Point Venus" and settled down to prepare for the transit. The Black Drop effect was carefully studied and many transit measurements were made successfully. Cook and his crew completed their circumnavigation of the Earth and reached England safely and in triumph. The expedition was a terrific success, and established Cook's fame as a mariner and explorer.

But there were other fascinating stories and adventures by explorers setting up stations in other parts of the world as well. For example, the French scientist Guillaume Le Gentil set sail aboard a Spanish ship bound for Manila in May of 1766. He was accused of being a French spy, but managed to escape and make passage to Pondicherry (recently returned to France by the Treaty of Paris). On Transit day the sun rose behind clouds and stayed there all day for the transit of Venus. After 9 years abroad, and traveling nearly 70,000 miles, he wrote in his journal:

"I was more than two weeks in a singular dejection and almost did not have the courage to take up my pen to continue my journal; and several times it fell from my hands, when the moment came to report to France the fate of my operations."

It is just as well that he did not know that it was sunny in Manila that day. Following a series of harrowing travel experiences on his way home to France, he crossed the Pyrenees on October 8, 1771, after being gone, by his journal, for 11 years, 6 months, and 13 days.

Transit of Venus, December 8, 1874

Hundreds of photographs taken of this transit. This was the first use of the new technology of photography, but few photographic plates were scientifically useful. Congress allocated $75,000 for international scientific expeditions. Over $1 million expended internationally. We now begin to see far more curiosity about this phenomenon in the newspaper accounts.

December 9. The Chicago Daily Telegraph

"Hence the probability is that observations of the transit of Venus in 1874, one which more that one million dollars have been expended, and involving the equivalent of not less than 200 years of labor on the part of one man, will only reduce the uncertainty by about one-third of its present magnitude".

December 10 Chicago Tribune.

"All scientific men, and all others who are sufficiently informed to respect scientific pursuits, will be rejoiced at the news that the transit of Venus has been successfully observed at several stations. Fortunately the preparations for this great event were so complete that failure was scarcely possible"

Photographic studies of the transit were extensive, but the bottom line seemed to be that the measuring of the plates led to disappointing results in establishing the sun-earth distance. The problem was in getting the plate scale accurately enough, and the orientation of the plate to the sun.

On December 31, 1874 a spectroscopic study of Venus was first attempted, and reported in the international journal Nature. They were unable to detect anything of interest.

Eight American expeditions were fitted out in 1874, organized by the Transit of Venus Commission, with Simon Newcomb as Secretary. The U. S. Congress appropriated funds totaling an astounding $177,000 for the expeditions. Although Newcomb considered the result of the 1874 observations disappointing due to inherent difficulties in the method, at the urging of Naval Observatory astronomer William Harkness, in 1882 Congress once again appropriated some $10,000 for improving the instruments, and $75,000 for sending eight more expeditions.

Transit of Venus, December 6 1882.

There was enormous public interest in this event. Smoked glass and amateur telescopes abound. Eventually, astronomers were able to get a distance to the sun from earth of 92.4 million miles with an uncertainty of about 1 million miles. Harvard Observatory astronomers tried to use a spectroscope to identify the atmosphere of Venus but could find no spectral evidence for such an atmosphere. A moon to Venus was also searched for but not found. Astronomer Henry Draper died so suddenly that no one knew how to operate his transit equipment. Among the public reactions to this event, the newspaper accounts give lots of details:

1882 December 6 Philadelphia Enquirer. "Scores of Columbia College students wearing morter boards climbed to the top of the new law-school building to catch a glimpse."

1882 December 7. Boston Daily Globe, "Visit of Venus. She crosses the disk of the God of Day. The spectacle is viewed through telescopes and smoked glass'

1882 December 7. San Francisco Chronicle, "Transit of Venus: The Planet's Course Across the Face of the Sun. A Grand Sight From the Observatory, "Many of the residents of San Francisco were noticed yesterday with a piece of smoked glass to their eye, looking curiously at the sun, between the hours of about sunrise and noon, during which time Venus was visible; and even under these disadvantages without the aid of a suitable telescope, it was still a grand and beautiful spectacle. All who missed a view of the transit of Venus are to be commiserated, for should they live to be 100 years old the chance will not come again occur."

1882. December 16 Scientific American, "It is possibly the last time that so much scientific stress will be laid upon the transit of Venus. For before the next one in 2004, we have faith to believe that other and more accurate methods will be found for computing the sun's distance"

In May of 1883, after considerable detective work, American astronomer Simon Newcomb announced that Maximillian Hell was innocent of falsifying his data for the 1769 transit of Venus. Georgetown's Jesuit astronomer John G. Hagan, S.J. wrote to Newcomb. "By this act you have obliged the Jesuits of all times and all places. It was fitting that this act of justice should be reserved to an American astronomer, who stands aloof from the petty quarrels of the old world." In 1891, Newcomb's refined calculation of the distance between earth and sun is finally published, based on all of the assembled data from several transits. He concludes that the distance is 8.79 =/- 0.051" .

William Harkness, an astronomer at the U.S. Naval Observatory, also spent a considerable part of his career analyzing the data, producing a value for the solar parallax in his lengthy monograph "The Solar Parallax and its Related Constants," published in 1891 in Washington Observations. Harkness reported on October 11, 1888 that the value for solar parallax deduced from 1,475 photographs was 8.847" plus or minus 0.012". This corresponded to a distance of the Sun of 92,385,000 miles, with a probable error of 125,000 miles. The following year this was refined to 8.842" plus or minus 0.0118", or 92,455,000 miles, with a probable error of 123,400 miles (Table 7.1). But he discovered that he could greatly improve this estimate by considering the measurements of other astronomical constants that relied on its value. The crowning achievement of a lifetime of work, was published in 1891 as his monograph "The Solar Parallax and Its Related Constants."[42] In his Presidential Address "On the Magnitude of the Solar System", presented before the American Association for the Advancement of Science in 1894, Harkness concluded that the best estimate for solar parallax was 8.809" ±0.0059, yielding a Sun-Earth distance of 92,797,000 miles, with a probable error of 59,700 miles. How important, then, were the transit of Venus observations? In answering this question we need to recall that the transits of Venus were only one method for determining the solar parallax. The man whose system of astronomical constants, including the solar parallax, would be used for most of the 20th century was Simon Newcomb. And Newcomb gave the Venus transits very low weight. Harkness must have been disappointed by Newcomb's low ranking of his work.

In the end Newcomb, not Harkness, had the final say, for it was his system of constants that was approved for international use; the value for solar parallax adopted at the 1896 Conference Internationale des Etoiles Fondamentales, held in Paris, was Newcomb's 8.80 arcseconds, not Harkness's 8.809. Considering the probable errors, Newcomb's system and Harkness's system actually overlapped in their values for solar parallax, and Newcomb came closest to overlapping the modern value of 8.794146. Simon Newcomb in fact used his final value in his famous volume 'The Elements of the Four Inner Planets and the Fundamental Constants of Astronomy' published in 1895. And that was the end of this story until radar measurements could be made in the 1960's to measure the distance to Venus to better than one-kilometer accuracy.

Yet, Harkness may have had the 'final word' on this transit in another way than in the details of its measurement. One of the lovliest comments made by a scientist during this time was when Harkness noted that the next transit of Venus will occur, "when the June flowers are blooming in 2004," . The longer version of this quote reads:

We are now on the eve of the second transit of a pair, after which there will be no other till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit season occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day. As for ourselves, we have to do with the present ..." (Address by William Harkness," Proceedings of the AAAS 31st meeting ... August, 1882 (Salem, 1883), 77.)

Another similar quote that is often attributed to Harkness reads:

"There will be no other [transit of Venus] till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows.

But there is a third quote following the 1882 transit attributed to the Astronomer Royal of Ireland, Robert Stawell Ball (1840-1913) in his 1885 book 'The Story of the Heavens'. He gave over 700 popular lectures on astronomy, and was in great demand in Ireland and Britain. An excerpt from this book, which was published three years after Harkness's lecture and the above quotes reads:

"While steadily looking at the exquisitely beautiful sight of the gradual advance of the planet, I became aware that there were other objects besides Venus between me and the sun. They were the snowflakes, which again began to fall rapidly. I must admit the phenomenon was singularly beautiful. The telescopic effect of a snowstorm with the sun as a background I had never before seen. It reminded me of the golden rain which is sometimes seen falling from a flight of sky-rockets during pyrotechnic displays; but I would gladly have dispensed with the spectacle, for it necessarily followed that the sun and Venus again disappeared from view. The clouds gathered, the snowstorm descended as heavily as ever, and we hardly dared to hope that we should see anything more; 1 hr. 57 min. came and passed. the first internal contact was over, and Venus had fully entered on the sun. We had only obtained a brief view, and we had not yet been able to make any measurements or other observations that could be of service. Still, to have seen even a part of a transit of Venus is an event to remember for a lifetime, and we felt more delight than can be easily expressed at even this slight gleam of success. But better things were in store...The sun was already beginning to put on the ruddy hues of sunset, and there, far in on its face, was the sharp, round, black disk of Venus. It was then easy to sympathise with the supreme joy of Horrocks, when, in 1639, he for the first time witnessed this spectacle. The intrinsic interest of the phenomenon, its rarety, the fulfillment of the prediction, the noble problem which the transit of Venus helps us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the June of A. D. 2004."