One brisk wintry day in 1639, a young man named Jeremiah Horrocks — barely 20 — set up a telescope in his quarters near Preston, England and focused an indirect image of the sun onto a small card. Lack of finances had cut short his brief academic career at Cambridge, despite his intelligence, but he still had a passion for the stars, bolstered by reading the works of Johannes Kepler and Tycho Brahe.
Kepler’s writings were the reason for Horrocks’ homemade experiment. In 1627, Kepler was the first scientist to predict a transit of Venus — a very rare astronomical event during which the planet Venus passes directly between the Sun and Earth, a black dot obscuring part of of the solar disk. It’s similar to a solar eclipse by the moon — we witnessed an annular eclipse just last week! — except transits of Venus occur every 243 years. Well, it’s a bit more complicated than that, because they usually happen in pairs: the pattern is eight years, 105.5 years, eight years, and 120.5 years.
The good news is, this rare event is happening again next month! And it will be visible in much of the United States! On June 5th, weather permitting, we’ll be able to gaze upon this amazing phenomenon and we won’t even need a telescope, since Venus is large enough to view with the naked eye. But you will need to take proper precautions to avoid permanent eye damage or blindness, just as you do during an eclipse. Ideally, you should travel to Hawaii, or Alaska, or the central Pacific islands for optimal viewing. Or you can watch online, since NASA Television will air a live program tracking the event, starting at 5:30 p.m. EDT.
Anyway, back in the 17th century, Kepler predicted there would be a transit of Venus in 1631, but it was not visible in much of Europe.
Kepler thought the next transit wouldn’t be until 1761, predicting a near miss in 1639. But Horrocks found an error in the great astronomer’s calculation and realized there would be a second transit, instead of a near miss, in 1639. Horrocks, at least, was ready with his little experimental set-up.
At first it seemed like the weather wouldn’t cooperate: the day was cloudy, obscuring the sun’s light. Finally, around 3:15 pm — 30 minutes or so before sunset — the clouds parted and Horrocks was able to record the very first observation of a transit of Venus, a small black shadow moving across the surface of the sun.
He used that data to make a solid estimate of the size of Venus, and the distance between the Earth and the Sun. The latter he pegged at 59.4 million miles, whereas the actual distance is 93 million miles (149.6 million kilometers), but it was certainly a more accurate measurement that others made up to them.
Horrocks died suddenly at age 22, prompting his fellow astronomer and mentor William Crabtree, who had been very impressed with the young man’s abilities, to exclaim, “What an incalculable loss!” His observations weren’t published until 1661.
So, okay, it’s a rare occurrence, but why was it such a big deal? In 1716, Edmund Halley realized one could use transit of Venus to determine the distances between the six known planets at the time. (Uranus and Neptune had not yet been discovered, nor Pluto, although poor Pluto’s status remains a topic of heated debate).
If one could carefully record the start and end of Venus’ passage across the face of the sun from widely separated geographic points on Earth, Halley figured, then it should be possible to determine the distance to Venus and the sun, using the distance between those two points on Earth as a baseline for triangulation. And once you knew that, it should also be possible to figure out the scale of the entire solar system.
Halley didn’t live to see the next transit of Venus, but his work ensured everyone had plenty of time to prepare. The weather didn’t cooperate for the Venus transit in 1761, and the data collected were useless, so astronomers pinned their hopes on the next transit in 1769.
Several expeditions were planned, all around the globe, to ensure there were multiple measurements from widely separated geographic points, as Halley recommended. The most famous was the one led by Captain James Cook. His ship, Endeavor, set sail on August 12, 1768, with a crew that included a young naturalist named Joseph Banks and an astronomer, Charles Green. Their mission: to reach the distant island of Tahiti in time to build a makeshift observatory and witness the transit of Venus in June 1769.
It wasn’t the easiest of journeys, although the abundance of Vitamin-C-rich malt wart and sauerkraut Cook had on board at least protected the men from scurvy. (If the finicky crew didn’t eat it, they were whipped. Parents, we do not recommend this approach with your children.) It took eight months, and he lost five men during a storm on Cape Hope, plus one crewman threw himself overboard in despair — maybe he was really, really tired of sauerkraut and whippings — but otherwise the ship arrived with most of its crew intact and in good health.
Having traveled all that way, through so much hardship, you’d think that Cook and his crew might have focused on the task at hand when the day of the transit arrived. But they were in Tahiti, which Banks declared “the truest picture of arcadia (idyllic and peaceful.”
The day kicked off with a breakfast meeting with Tarroa, the island king, his sister, and “three handsome women.” The king and his entourage duly joined Cook and his men to see “the planet upon the sun,” but in total Banks’ journal entry for June 3, 1769, contains less than 100 words about this spectacular event.
Fortunately, Cook was in a more verbose mood:
“This day prov’d as favourable to our purpose as we could wish, not a Clowd was to be seen… and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the planet Venus over the Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the contacts particularly the two internal ones.”
That “dusky shade” was believed to be the result of sunlight filtering through the Venusian atmosphere. It’s actually more of an optical effect caused by the smearing of the image of Venus — usually because of turbulence in the Earth’s atmosphere, or imperfections in one’s instrument.
It’s similar to what happens when you hold your thumb and index finger in front of one eye, then gradually bring the tips together. Just before the tips meet, you should see a shadowy bridge appear. And it was enough to hamper the measurements of Cook and his astronomer, Charles Green, because it made the edge of the disk all fuzzy. It was tough to figure out exactly when the transit began and ended. Their measurements were off by about 42 seconds.
Cook’s wasn’t the only mission to suffer from this. Astronomers were tracking the event from 76 different points around the world, and none obtained sufficiently accurate measurements to determine the scale of the solar system. It would have to wait for a future generation of astronomers.
While the mortality rate was surprisingly low on the journey out, Cook lost 38 of the original Endeavor’s company on the trip home, including Green, the ship’s astronomer. Most succumbed to malaria and other diseases picked up during a stopover in the port of Jakarta. A 40% casualty is horrifying by today’s standards, but Cook expected half the crew to perish; he probably figured they got off easy.
Even those who survived didn’t fare too well, if the tale of the unfortunate Guillaume Le Gentil is any indication. He was part of a French expedition bound for the colony of Pondicherry in India to measure the transit of Venus there. Unfortunately, war broke out between France and England shortly after his departure, and the British decided to occupy Pondicherry. So Le Gentil and his crew were forced to try to witness the June 1761 transit from Mauritius (then known as Ile de France).
But Le Gentil found it impossible to make accurate measurements with the ship rolling about. So he figured, “Well, I’m already here, and there will be another transit in eight years. I might as well stick around for it!” I’m sure it sounds more impressive uttered in a sultry French accent. He spent some time in Madagascar, and decided Manila in the Philippines would be a terrific spot to witness the 1769 transit, but let’s just say that once he got there, he found the Spanish authorities in charge were “hostile” to the idea.
So he headed back to Pondicherry, now safely back in French hands, built a small observatory, and waited. The Big Day arrived, June 4, 1769 – and the sky was too overcast to see anything. After all that, Le Gentil failed to witness either transit of Venus. “Merde!” he no doubt muttered to himself, before packing up to return to France. Except his bad luck continued. An outbreak of dysentery delayed the ship’s departure, and then a storm broke out, so he ended up stranded at Ile Bourbon until he finally found passage home on a Spanish ship, eleven years after first setting out.
Fate spat on poor Guillaume yet again. It was not an auspicious homecoming. See, apparently Le Gentil had neglected to send word to his wife or relatives about his decision to hang out an extra eight years until the next transit. She had him declared legally dead.
In his passion to chase down the transit of Venus, Le Gentil had lost everything. Literally. Another astronomer had taken his place at the Royal Academy of Sciences, so he’d lost his job. His wife had remarried, and his relatives had “enthusiastically plundered his estate.” So he had no fortune left, either.
In the end, the king intervened and put everything right, as only an 18th century sovereign could do. Le Gentil got his job back, his estate back, found a new (hopefully more patient) wife, and lived happily until he died 21 years later. He has since been immortalized in the play Transit of Venus by playwright Maureen Hunter
That’s how passionate folks were (and are!) about science, and rare events like the transit of Venus. Many lives and fortunes were lost on this quest to measure the solar system.
By the time the next pair of transits rolled around on December 8, 1874, and December 6, 1882, respectively, photography had been invented, and astronomers could now take a picture to record the event for posterity. There were more expeditions and more measurements taken, and by taking all the measurements of the last four transits into consideration, astronomers were able to refine Horrocks’ original distance estimate of the distance between the Earth and the sun to 149.49 kilometers, very close to the actual distance of 149.6 million kilometers.
There wasn’t a transit of Venus for the entire 20th century. The last one was in 2004. These days, modern astronomers are more interested in using the transits to assist in the hunt for extrasolar planets, especially smaller ones that are difficult to detect using current methods. The idea is that by measuring the light intensity during the transit, as Venus blocks out some of the sun’s light, astronomers could confirm that this slight dimming (a mere 0.001 magnitude) is a sign of an orbiting planet. And then they can use a similar approach to search for planets in other star systems.
This time around, astronomers are making use of some of the instruments aboard the Hubble Space Telescope to view the transit in several different wavelengths (UV to near-infrared), thereby dividing the sunlight into its constituent colors, in hopes of learning something about the composition of the Venusian atmosphere. This, in turn, could help analyze the atmospheres of extrasolar planets to determine if any are hospitable to life.
But Hubble can’t stare at the sun directly, any more than we can; it would fry its cameras. So the plan is point the telescope at the surface of the moon instead, using it as a mirror to capture reflected sunlight — a high-tech 21st century version of Horrocks’ simple 17th century set-up. Then they can isolate that small fraction of the light that makes it through the Venusian atmosphere, which should contain the chemical “fingerprints” of that atmosphere.
The astronomers only have one shot at this, so the stakes are high. We won’t see another transit of Venus until December 11, 2117, and by that time, well, nobody reading this will be alive. And it won’t be visible from much of the US.
For the rest of us, the transit next month will be something we can watch, appreciate, and then reflect upon the transitory nature of human existence compared to the mind-boggling time scales of the cosmos — a sentiment Horrocks captured in a bit of verse he penned while ruminating on the long periods between Venusian transits:
“… Thy return
Posterity shall witness; years must roll
Away, but then at length the splendid sight
Again shall greet our distant children’s eyes.”
Images: (top) Jeremiah Horrocks’ sketch of the Venus transit in 1639. Public domain. (center left) The view from Point Venus, Tahiti, William Hodges (1744-1797), oil on canvas. National Library of Australia. (center right) Drawings of the 1769 transit of Venus by James Cook. Public domain. (bottom) Portrait of Guillaume Le Gentil. Public domain.