“Behold! Venus on the Sun’s Face” — Eli Maor on the Transit of Venus

On June 5, 2012, Venus will venture across the face of the Sun in a spectacle that will not be seen again for over a hundred years. Here, Eli Maor provides a little history of this astronomical event and even some observation tips.

Eli Maor is the author of many Princeton University Press books, including The Pythagorean Theorem: A 4,000 Year Historye: The Story of a Number, Trigonometric Delights, and To Infinity and Beyond. One of special interest to readers of this blog post is Venus in Transit. In this book you can read more about the previous transits, including Eli Maor’s personal pilgrimage in 2002, to the town of Much Hoole where he retraced Horrocks’s historic 1639 observation. You also might want to read two of his articles that appeared in the January and June 2012 issues of Sky & Telescope. The June issue includes detailed information on the upcoming transit.

For more information about the transit of Venus, including safety tips, suggestions for viewing sites, and even an app with which you can record your observation and participate in a citizen science project to measure the universe, visit http://www.transitofvenus.org/



“Behold! Venus on the Sun’s Face

It was early on the morning of Tuesday, June 8, 2004. Our small group of family members was perched on the edge of a steep cliff overlooking the enormous Ramon Depression, near the town of Mitzpe Ramon in southern Israel. Facing us in all directions were Imposing geologic structures that formed here millions of years ago. With a little imagination it might have been a scene on the Moon, a feeling enhanced by the fact that we were the only human beings in sight. But we didn’t come here to view the awesome terrestrial sights in front of us; we came to witness one of nature’s rarest shows: the passage of the planet Venus in front of the Sun.

Photo taken by Jan Herold, posted at Wikipedia under a GNU Free Documentation License

We positioned our little Questar telescope on the edge of the cliff, checked and rechecked its solar filter to make sure it was safely secured to the front of the scope, synchronized our watches, and waited. At precisely 8:19 a.m., right on the mark, a tiny black intrusion could be seen entering the Sun’s face and slowly making its way inward until, nineteen minutes later, it was fully immersed in the Sun’s disk: a perfectly circular, jet black dot—the image of the planet Venus delineated with razor-sharp clarity against the blazing face of our home star.


It was an emotional moment for me: we were witnessing a sight no human had seen since 1882—the last time Venus crossed the Sun’s face. In fact, this event has been observed and recorded only five times before: in 1639, 1761, 1769, 1874, and 1882. You may discern a pattern in these dates: they are separated by 121 1/2, eight, 105 1/2, eight, and, counting the transit we were just observing, again 121 1/2 years. The entire cycle—considering that pairs of Venus transits alternate between June and December—repeats after 243 years, making this event even rarer than the return of Halley’s comet every 75 years.


Note again the eight-year interval between pairs of transits. This peculiar interval is a result of the orientation of Venus’s and Earth’s orbits around the Sun. So if you missed the transit of 2004, you have one more chance to see it, and it is coming up soon: on June 5, 2012 (June 6 for Europe and Asia). After that it will be a long wait until the next transit, due on December 11, 2117.


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The story of transits is not just a case of watching a rare celestial conjunction in real time. Each of the five transits before 2004 was a tale of meticulous preparations, hairsplitting adventures, and sometimes crushing disappointments. It is a riveting story: in 1639 a young and unknown astronomer, Jeremiah Horrocks, examined a table of astronomical data by the famous astronomer Johannes Kepler (1571-1630). Kepler’s table had predicted that Venus would transit the Sun in 1631 and again in 1761; but Horrocks, to his great surprise, found out that Kepler had missed a transit in 1639—just one month away! Horrocks immediately alerted the few astronomers he knew—he was just 21 at the time—urging them to watch this event with their utmost attention, as it would offer a rare opportunity to find out Venus’s true angular size, a quantity that had always been overestimated due to the planet’s intense glare when seen at night.


Horrocks (or Horrox, as his name was spelled then) was largely self-taught, driven by his passion for mathematics and astronomy. He was among the first English scientists to embrace Copernicus’s heliocentric system, which, more than half a century after its publication, was by no means generally accepted by astronomers. Living in the hamlet of Much Hoole, some 15 miles north of Liverpool, Horrocks set up his small telescope in a darkened room and projected the Sun’s image on a screen behind the scope. He began his vigil early on the morning of December 4—a Sunday—but then was called to the local church to perform some official duty. When he returned to his telescope at 3:15 in the afternoon, he was rewarded to see a perfectly circular black dot on the Sun’s image—the silhouette of the planet Venus. This marks the first recorded observation of Venus’s passage over the Sun (the transit that Kepler had predicted for the year 1631 was not visible from Europe, and there is no evidence that anyone observed it in America).


There was just one other witness to this historic moment—Horrocks’s friend William Crabtree, who observed it from his home in nearby Manchester. The two had met as students at Cambridge some years earlier and shared a passion for mathematics and astronomy; but after leaving college they kept in touch solely by correspondence. Following the transit, they were planning at last to meet again and compare their notes, but it was not to be: Horrocks died the day before their scheduled meeting, not yet 23 years old; the cause has never been determined. Thus England lost a young talent that, had he lived longer, might have become the equal of Newton.


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Enter now Edmond Halley (1656-1742), one of England’s most illustrious scientists, who in his long life made major discoveries in mathematics, physics, geography, and astronomy, leading to his appointment to the position of Astronomer Royal, the second person to hold this prestigious office. Halley is chiefly remembered today for predicting the return of the famous comet that bears his name, but his other contributions to astronomy were no less important. In 1716, in a paper delivered to the Royal Society in London, he suggested using future transits of Venus to determine the mean distance between the Earth and the Sun. This distance, known as the Astronomical Unit (AU), was a crucial yardstick in establishing the scale of the solar system and the universe beyond.


Halley’s idea was to have the transit observed from a large number of stations, widely separated around the globe. Each station would record the exact moment of Venus’s entrance on the Sun’s disk, and again the moment of its exit. From these times, the planet’s path across the Sun could be determined for each observing station. Due to the effect of parallax, these paths would be shifted slightly as seen from different locations. By measuring the angular distance between different paths, and knowing the geographical position of each station, it should be possible to calculate the AU, using basic trigonometry.


Spurred by Halley’s call, hundreds of astronomers fanned out over the entire globe to watch the next two transits, in 1761 and 1769. Their adventures, successes, and frequent misfortunes became the stuff of legend. Topping them all is the story of a Frenchman by the impossibly long name Guillaume-Joseph-Hyacinthe-Jean-Baptiste Gentil de la Galaisière (1725-1792), commonly known as Le Gentil. His destination was the town of Pondicherry on the southeastern coast of India. Le Gentil left France on March 26, 1760, more than a year before the transit. En route he stopped at the island of Mauritius in the Indian Ocean, where he learned that Pondicherry was under siege by the British—it was the height of the Seven-Year War between Britain and France. So he boarded a ship headed for the coast of Coromandel in India. But the captain was in no hurry to leave port; it was not until March 1761—less than three months before the transit—that the ship finally set sail. On their way they encountered persistent calms, and worse, Le Gentil had a brush with dysentery. The boat finally reached India in May, but then the Captain got news that Pondicherry had just fallen to the British, so he turned his ship around and sailed straight back to Mauritius. June 6, 1761 found Le Gentil observing the transit from the deck of his rolling ship, making his timings practically useless.


Undaunted, he resolved to make the best of a bad situation. Rather than returning home, he spent the next eight years crisscrossing the islands of the Indian Ocean, observing their flora and fauna, and recording his impressions. But the approaching transit of 1769 was always foremost on his mind. He decided to watch it from Manila, capital of the Philippines, reaching it on August 10, 1766. But then he was instructed by his sponsors, the French Academy of Sciences in Paris, to head back to Pondicherry, his 1761 destination, now that the Seven-Year War was over. Le Gentil did as instructed and arrived there in March 1768, still over a year before the transit. He spent the remaining time making all preliminary observations in order to determine the exact longitude of his location.


For a full month prior to the transit the skies were crystal clear. But on the crucial day, June 3, 1769, an unexpected cloud hid the Sun from his view, and he missed the all-important moment of Venus’s exit from the Sun. It took him two weeks until he could put pen to paper and report his failure to his sponsors in Paris. To add insult to injury, he later learned that at Manila the sky was perfectly clear during the entire transit!


But his troubles were not quite over yet. Slowly making his way back home, he finally arrived in Paris in October 1771, being away for nearly 12 years—only to find out that he had been assumed dead; his heirs were already dividing up his estate, and the Academy demoted him to the rank of a retiree. He had to take legal action to restore his position and get back his property. Then he got married and spent his remaining years raising their daughter and writing his memoirs.


I often think of Le Gentil and find solace in his tribulations whenever I miss some astronomical event due to poor weather, as happened during the total solar eclipse of July 11, 1991—one of the longest of the twentieth century. Our group went to the Big Island of Hawaii, having been promised 360 days of sunshine there. Unfortunately we got one of the remaining five: on Eclipse Day the sky was completely overcast, and we saw nothing. It took two days until our leader found the courage to talk to our group again.


In anticipation of the pair of transits to occur in 1874 and 1882, British astronomer Richard A. Proctor wrote Transits of Venus, a classic account of past and future transits that is still being quoted today. He ended his book with these words:


We cannot doubt that when the transits of 2004 and 2012 are approaching, astronomers will look back with interest on the operations conducted during the present transit-season. The astronomers of the first years of the twenty-first century, looking back over the long transitless period which will then have passed, will understand the anxiety of astronomers in our own time to utilise to the full whatever opportunities the coming transits may afford; and I venture to hope that should there then be found, among old volumes on their book- stalls, the essays and charts by which I have endeavoured to aid in securing that end (perhaps even this little book in which I record the history of the matter), they will not be disposed to judge over-harshly what some in our own day may have regarded as an excess of zeal.


That remote time is now upon us, and the second and last chance to watch our closest planetary neighbor cross the Suns’ face in our lifetime is just a few weeks away. I wish you good luck and clear skies on Transit Day!


Warning: When looking at the Sun, you need to use an approved protective filter, such as can be obtained from a reputable outlet of astronomical equipment. Failure to do so may result in permanent damage to your eyes. Sunglasses, or a smoked glass, are not considered safe and should not be used. In general, the same safety precautions should be taken as when watching the partial phases of a solar eclipse.


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  1. Sandra Jack says:

    It will be a history event. If you will have some news please write. I will be check yours blog. Thx for info!