The Transit of Venus
The Play’s the Thing: Venus Double-Crosses Apollo
"Uncle Bob" Mead
On June 8, 2004 a rare event called a transit was observed from most of Africa, Europe and Asia. The planet Venus passed between the Earth and our Sun and assumed a negative image: rather than shining third brightest of all objects in the sky, it was a perfect circle in black silhouette, looking like a large, all-too-regular sunspot. It crossed the sun’s disk from left to right in about six hours time. In eastern North America, including New England, those who had a view low in the east and a cloudless sunrise, were able to see the final hour or so of this transit after daybreak, that is, if they were equipped with protective solar filters.
The brief 400-year human history of the transit, the people and the events involved, mostly tragic ones although some humorously so, remind me so strongly of a drama, that I decided to sketch a little play to tell it to you. This is my title page:
The Transit of Venus, a play in five acts, in which GREED, ARMED CONFLICT, PREMATURE DEATH, MISCALCULATION, AND ABJECT FAILURE are dramatically, yet factually portrayed; with prologue and possibly an epilogue, having three intermissions of 122 years duration each; the final scenes of the last act still to be written; the title character to play herself, and, unfortunately, the rest of the cast to follow suit. Written and produced by URANIA, the MUSE OF ASTRONOMY.
Prologue. 1631. A Tale of Two Errors and a Missed Entrance. We open on Johannes Kepler, pulling out his hair, yelling in torment, “Why ellipses?” Oh yes, he knows from his employer Brahe’s data that planetary orbits are ellipses, but Newton’s explanation of them is 70 years away. Nevertheless, Kepler has the right formulas and is compiling tables of future astronomical events. One of his predicted events surprises him: he sees that Venus will pass in front of the sun from earth’s vantage point in 1631. He miscalculates that the event will not be favorable to Europe, so he exhorts mariners to observe it from the high seas. Does he know that he will not live to see or hear of it himself? The Italians, in fact, could have observed it, if only they had known. Prior to his exit, one year short of the event, Kepler makes a second error in predicting that the next Venus transit will occur in 1761. One in fact does take place that year [see Act II], but another would happen much sooner.
Act I. 1639. England, and another missed entrance. Enter Jeremiah Horrocks who calculates, with one month to spare, that a transit will occur on November 24, 1639. He and a friend, William Crabtree, plan to observe it – the first using heavy filtering cloths, and the latter, a projected telescopic image. Horrocks, a cleric in a small rural English church, and an obvious astronomical prodigy, is called away that day to afternoon services, and returns home having missed the first two contacts. Every transit has four “contacts,” being the times of the initiation and completion of the entrance and exit of the planet’s disk.
Horrocks reels from the experience, as well the first human ever to have observed and reported this sight should. He intends to write a book about it, and he begins with some lovely poetical passages. The only scientific measurement he makes is the surprisingly small size of Venus’s disk. Horrocks, a rising star if you will, does not get to finish his report, and worse, he is dead at age 22. Crabtree, who observed the transit through mostly cloudy skies, follows Horrocks to the hereafter in three short years.
Act II. 1761. An Invitation to the Dance. In which the mariners take funds and scripts from either England or France and then decide to ad lib instead; in which Venus refuses her nude scene and gets emotional; in which Science enters and Science is hijacked; and, at the end of which, one actor fails to exit.
Curtain opens on a swimming pool around which Venus and a slightly tipsy Earth do a dance. They dance around and around the pool and in the same counterclockwise direction, but Venus, on a level walkway beside the pool, is making more laps faster. Earth’s path is outside the walkway and rises and falls with the terrain. Earth is higher than Venus at one end of the pool, and lower at the other. They exit.
Science enters. Science wishes to explain to the audience why the long intermissions. She says it is due to three characters named Rate, Tilt, and Wobble, which were symbolized in the dance. Rate assigned the paths of each dancer, and he intended that Venus and Earth are on the same side of the pool only once in every 8 years. There is no transit possible if Venus is not between Earth and the pool (the Sun). Tilt has landscaped the rise and fall of the pool’s environs. This puts Venus well above or below Earth’s line of sight to the pool, thus preventing a transit in most of those eight-year close encounters; specifically, the dancers would be on the same side, and at Venus’s level, about once every 120 years; however, because the planets are not points, rather they are sizable disks, they will be on about the same level for two successive octi-ennials (eight years apart). This is the reason some of us witnessed a second transit in 2012. Thus we have Venus's "double-cross." Wobble, portrayed by Earth’s tipsiness, keeps supplying the booze, and that means that we shouldn’t count on this schedule holding forever.
Science, speaking in much the manner of Bob Dole, now explains why Science is a central character. After Horrocks observed a tiny Venus against the Sun, he probably figured that the distances among the three bodies were vaster than imagined. How vast? The relative positions of the known planets out from the Sun are known from Kepler’s work, but there is no giant yardstick to measure actual distances. Edmund Halley, of comet fame, devises a measuring stick which utilizes a Venus transit. He had observed a transit of Mercury (another “inside job”) in 1677 and became inspired. At age 60, knowing he would not be alive for the next transit, he submits a plan to the British Royal Society. I paraphrase:
We must send ships to far northern and southern climes to observe and record sightings of the 1761 transit. Using an estimate of the parallax angle, and the distance between locations, and a bit o’ trigonometry, England will find the distance to Venus, and using one of Kepler’s formulas, she will reveal the size of every orbit in the solar system.
Parallax, by the way, is merely the change in apparent position of a close object against a more distant background when it is observed from two separate vantages. Science and Halley get France and England into a lather over the project. The two countries plan six expeditions to the (six?) corners of the globe, and then they proceed to declare war – on each other! Science boards the vessel commanded by Alexandre Pingre which arrives at its destination in time. The audience should recall that travel time in the 18th century was not what it is today: we’re talking months, if not years. Pingre is able to see the middle of the transit, but clouds prevent the all-important timing of first and last pairs of contacts. His ship is attacked by the British on its return, and Pingre and Science are forced to walk home, more or less, from Lisbon.
Two other notables to venture forth are astronomer Charles Mason and his surveying assistant Jeremiah Dixon, who are destined to draw a line in R.E. Lee’s sand, just below Gettysburg, U.S.A. Portsmouth (England) is barely out of sight when they come under heavy French fire: eleven are killed, the ship is seriously damaged, and they limp back to port. The delay makes it doubtful that Mason will reach his viewing post in Sumatra, so when, on the second attempt, he survives as far as the Cape of Good Hope, he says, “Good Enough,” and there he makes the sole unobstructed observation of the complete transit from below the equator. Combine this with the fact that a French expedition observes the entire event from Siberia, far to the north, and Halley’s method should have calculated the Venus distance well.
But soft! There is a whimper coming from the star’s dressing room. Venus has had a glowing, shimmering robe made for her transit scene. Oh, let’s call it her aura..., or how about her atmosphere? As she enters stage left, Mason observes this cloaking and is not prepared for it. Does he time the robe’s entrance, or that of the body beautiful, which is hidden under the robe regardless? Never mind though, because an accurate timing might still be made using the inside contact, the instant when Venus’s complete silhouette is entirely visible on stage. Recall, however, that this play is thoroughly overcast in doom. Just as Venus is completing her entrance, which takes a good 15 minutes or so, a second problem arises. The robe catches on an ill-placed straight pin in the curtain. Venus’s trailing “atmosphere” appears distended, and so the second contact is just as imprecise as the first. This optical distortion, called the “black drop effect,” is one that will repeat itself in all of her performances. Rum show!
Through the entire second act, another player named Le Gentil crosses three times at the proscenium. He is undecided about where to dock his ship for the sighting. Word reaches him that the British have seized his intended destination, and after having crossed most of the Indian Ocean, he about-faces and sails west. He changes his mind with each war bulletin that reaches his ears. He heads east again, but receives more bad news and turns back west. These actions put him exactly smack in the water, somewhere, for the transit, and this renders all of his timings utterly useless. He seems reluctant to return to France with the bad news. Also, he has read ahead in the script. The next eclipse is only eight years away in Act III. He decides to sit it out on stage and pretend to make some other useful observations.
Act III. 1769. The Second Time Around. In which the wandering Gentil finally returns home to a mean estate battle; in which even more governments launch even more expensive missions to time the transit, all of this in spite of evidence that an accurate timing would never be possible due to the visual vagaries observed by Mason, and in spite of the dismal returns of 1761, namely, the earth-sun distance, called the astronomical unit, was calculated to be in the range of 77 to 96 million miles – not nearly the precision Halley had expected; and in which a hero makes his first entrance, observes the same problems as Mason, declares Act III a success, holds a cast party, after which most of the crew catch a nasty disease and die, and the hero, it is rumored, is eaten by cannibals.
There is little left to tell. The hero is James Cook, famous circumnavigator, European discoverer of Hawaii, and his was the crew of the good ship Endeavour. He enjoys a clear view of the entire transit from a small isle near Tahiti, and makes measurements with sophisticated instruments – fortuitously returned by the natives who had earlier stolen them. Venus and aura enter, robe again drags behind. Other than the plague and the cannibalism, there are no further eventualities.
America enters and will not be upstaged by the Old World fuddy-duddies. Philadelphian David Rittenhouse prepares to time the Venus crossing from his home, no doubt with a timepiece of his own making. Evidently he lives in a big house on a high hill. Rittenhouse would go on to be a Revolutionary War booster, but at this time he is not exactly battle-hardened. At the first sight of Venus he faints and drops his pocket watch.
And what of Gentil? Hapless Gentil, with a choice of half of the southern hemisphere for an observation post, chooses Manila; however, with just enough time to make it elsewhere, he receives instructions to go to India, and this time he does not ad lib; he sails. The final score: India zero, for “vexatious” clouds; Manila one, for being clear as a bell the entire day. Gentil tucks tail and returns to France to find, after his 12-year absence, that the family has had him declared legally dead, and they are in the process of dividing the flatware. The government forces him into retirement for “malingering;” however, when his alibis and excuses are finally confirmed by other parties, his reputation is to be restored, and he will die happy.
The 1769 efforts are somewhat more successful. The estimates of the distance to the sun now disagree by only 4%, but that, of course, is four million miles. In 2004, other technologies will give us this distance to the nearest mile, and our own moon’s distance to the nearest foot!
Act IV. 1874 and 1882. Play It Again, Sam. The nineteenth century. In which men who are looking to get out of the house for a few months continue to beg for more funds from ever more skeptical politicians; in which the U.S. Navy tries to solve the precision problem that all observers have had by holding training sessions with a “transit simulator” (oh yeah); and in which the distance discrepancy is reduced to a million and a half miles. Nothing else to say.
Prelude to Act V. 2004. She’ll be... Rustic music is heard offstage. “She’ll be comin’ ‘round the mountain... she’ll be drivin’ six white horses, when she comes.” During this last intermission, we have measured the size of the Milky Way, Einstein has told us that everything is relative; Heisenberg has told us that we can’t measure anything with perfect precision; and even the Queen of Sciences, air-tight-seeming mathematics, has been shown to be necessarily incomplete. In spite of these difficulties, men have returned safely from the moon, and we know the approximate size of the entire universe, and because of that, we know that the astronomical unit of 93 million miles is much too puny to be the “astronomical” unit.
What use will the June 8 transit be then? Perhaps none in science, but the sky is a resource which has been enjoyed by people in all walks of life. We've watched bright Venus coming round that burning mountain all spring . Very soon she will ride in front of Old Sol and turn jet black for a few hours. Many of us will see the transit start to finish, if not firsthand, then over the internet. You too should be a witness to this awesome sight. There are many who have lived and died since 1882, never having had the opportunity to witness one. Something about this event caused a flutter in the hearts of more folks than just Rittenhouse.
Act V then, will be written by us.
[My two primary sources for the “unfettered” information are Eli Maor’s June 8, 2004: Venus in Transit, Princeton University Press, 2000 and Sky & Telescope magazine, February, 2004, pp.46-55]