What are Transits?

When one celestial body passes in front of another, the result is either an eclipse, an occultation, or a transit. We say it’s an eclipse if the nearer object blocks out much or all of the more distant one, or if the Moon goes into Earth’s shadow. It’s an occultation if a nearer large object totally obscures a much littler-looking one, for example when the Moon covers up smaller-looking Saturn. But if the nearer planet – in this case Venus – appears much smaller than the distant one – in this case, the Sun – then it’s a transit. In a Venus transit, Venus appears as a little black disk slowly crossing the face of the Sun, over the course of several hours.

Why are Transits so Special

Close one eye and hold up a finger and notice what part of your distant wall it’s in front of. Then close that eye and open the other – in other words alternatively blink each eye -- and the finger’s position jumps. (You may have already performed this experiment in school, as an alternative to whatever was happening on the blackboard.) This is parallax – seeing things from different angles. If you measure how much the finger shifts you can trigonometrically calculate how far it must be from your eyes. By the 18th century, this method was used to find the distance to the moon. Observers at two cities a known distance apart would note what background stars were next to the Moon at the same pre-arranged time, and compare notes. The amount the Moon differed from one observer’s location to the other let them figure the Moon’s distance.

A Venus transit allows us to do the same thing, and determine with great accuracy the distance to Venus, and the distance to the Sun.

How Rare Are Transits?

Extremely rare. Once a transit happens, another will follow in exactly eight years minus two days. But then 105 ½ years must pass, followed by another pair of transits eight years apart. Then 121 ½ years elapse and then eight. So that’s the curious pattern: 8, 105 ½ , 8, 121 ½ , 8, 105 ½ , 8, 121 ½ , and on and on forever. And they’re always in December or June. No one in the entire 20th century could see a transit. When the event of June 8, 2004 occurred, it was the first transit in 121 ½ years. Thus, we are now in that 8-year interval. When the transit of June 5, 2012 is over, there won’t be another until the 22nd century! Put another way, few if any people, even young children who see this transit, will be alive for the next. Indeed, even now, only six transits of Venus have ever been observed. They happened in 1639, 1761, 1769, 1874, 1882 – and now finally in 2004.

History of Transits

By the early 18th century, astronomers realized that Venus transits offered them their only way to determine the distance to the Sun, and to all the planets. Pinning down the size of our solar system then became all the rage – and really got important much later, on the eve of space travel.

You don’t even need a telescope. Just a solar filter so you won’t blind yourself. Of course, a good telescope will show the event much better – and this is where SLOOH comes in, to present the entire transit, since most of the event, including the all-important start or finish or both, will not be visible from your location.

At one time in history, only a single human being had ever deliberately observed a transit. This had been a British cleric named Jeremiah Horrocks. He was only 20 at the time, the age of a college junior. He had been born into a poor non-noble family but was brilliant in math, and managed to get into Cambridge. Without the aid of any of our modern computing aids or tables, or even the calculus which had not yet been invented, Horrocks figured that Venus should cross the sun’s face on November 24, 1639 (using the old, pre-Gregorian calendar). His math was detailed enough for him to even know the transit would be visible from England. He observed the event and recorded its details for posterity. He even had a witness, having convinced his friend William Crabtree to look for it too.

After the transit, Jeremiah offered an estimate of the Sun’s distance. Sadly, Jeremiah died suddenly and without any obvious cause at the age of 22, in January, 1641. William Crabtree, now and for the rest of his life the only person on Earth to have witnessed a transit, said of his best friend, "What an incalculable loss!" Today, essentially unknown, he is immortalized by the lunar crater Horrocks.

Nearly one lifetime later, astronomers and governments raised impressive funds for expeditions to be dispatched with the best clocks and the best telescopes for the long-awaited transit of June 6, 1761. It was a matter of national pride and prestige.

Eight European nations sent expeditions. At least 120 observers, including the top astronomers of the period, sailed, hiked, horse-backed and trudged to 62 locations as far as India, South Africa, Constantinople, Peking and Siberia. It was a costly international effort, the likes of which the world had never seen.

These observations were so precious, so rare, that the French and British, though in the midst of the fierce and deadly Seven Years War fought even in their possessions around the globe, officially provided each other’s astronomers safe passage.

The adventures, trials, and even deaths among the far-flung expeditions, and the problems using equipment in steamy humid environs, not to mention overcast skies in some, nonetheless yielded numerous published papers, along with some intriguing if weird reports. Several astronomers saw a ring of light around Venus when it was almost in front of the Sun and concluded -- absolutely correctly -- that the sun, from behind, must be illuminating a thick Venusian atmosphere extending 50 miles into space. But many teams had trouble pinpointing the all-important exact moment Venus was within and tangent to the sun’s disk because of a bizarre effect called The Black Drop.

Instead of an inky dark Venus in cameo, sitting at the zero-moment with its curved edge just perfectly and momentarily meeting the sun’s inner edge, a black protrusion like a ligament strangely joined the two for several long seconds. It resembled what one might expect if the blackness of space just beyond the sun’s edge was a kind of ink that leaked inward to meet the ebony Venus disk as it approached the solar limb. No one could figure it out, but it threw off the timing. Nowadays we know it’s a diffraction effect caused by the interference between light waves. (We are looking forward to seeing this in the upcoming transit, too).

The same nations then pulled up their sleeves for the next transit opportunity, which would come in just eight years. Preparations were even grander for the transit of June 3, 1769. Now instead of eight European nations there were 11. Over 150 astronomers and assistants, and a small army of support staff, set sail to 77 locations around the world. It was all in the name of science with no practical benefit whatsoever, no hope of it yielding some improved medicine, technology, or riches.

Nearly half the ships had been sent by Britain alone, one of them led by Captain James Cook. Actually, for his ship, the transit was a cover. His was an intriguing spy mission. He was given the secret assignment of trying to learn whether Australia (“The new southern continent”) actually existed or was merely a myth. While he was at it, he was to explore the little-known islands of the South Pacific.

On August 26, 1768, the Endeavor set sail from Plymouth with a crew of 94, and with 18 months’ worth of supplies. This was Cook’s very first major voyage and he was not fooling around. After rounding the always-perilous Cape Horn with its characteristically wild seas, they reached and anchored at Tahiti at the beginning of June, just in time.

The expeditions had been better prepared, telescopes were bigger and finer, and the clocks had in the intervening eight years improved as well. The Venus parallax results coughed up a Sun-Earth distance of somewhere between 93 and a little more than 97 million miles. Astronomers decided that the best average was 95 million – and this indeed is the AU told to school children for the next century.

No one alive for those transits was around for the next pair in 1874 and 1882, and a new litter of astronomers was now raring to try their own luck. Again, 80 expeditions were dispatched around the world. Again it was a big hit in the popular press. John Philip Sousa even wrote the Transit of Venus March in 1883 to celebrate the transit the year before, although this was not one of his big hits. Now, the science results were even better, and the Sun-Earth distance was refined to 92,880,000 miles, a mere 76,000 miles from the truth.

In short, transits used to be spectacularly important, and the focus of global observations and world-wide cooperation. Thus the upcoming event is not only fascinating, and will not be seen again by anyone now living, but takes place as a historical event with a venerable history.

SLOOH's 2012 Live Coverage

Much of the world, but not all of it, will witness at least part of this transit, the final one until December 11, 2117. From the US, no one will see its final several hours. However, SLOOH’s remote telescopes will be positioned at traditionally clear and dry sites around the world, and bring live telescopic coverage along with real-time commentary by experts, starting at 6 PM EDT.