Date: December 5, 2009

Title: The Celestial Alignment of 2012


Podcaster: Chuck Bueter


Description: An astronomical alignment in 2012 heralds the second coming (in the 21st century) of a significant global event. Share the enthusiasm of adventurous astronomers from the past as you witness the heavens in motion, the means by which we learned the size of our solar system—a transit of Venus.

Bio: Chuck Bueter is an amateur astronomer who appreciates a good story but pronounces names poorly. His interests include dark sky advocacy, the transit of Venus, paper plate education, and his supportive family.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research” at


The Celestial Alignment of 2012

Yes, a significant celestial alignment occurs in 2012. Mark your calendar—June 5-6, depending on where you live—and witness a phenomenon that changed mankind’s understanding of the solar system. Behold, the second coming (in the 21st century) of the transit of Venus.

Because the orbits of Venus and earth are inclined to each other, Venus is usually either a little above or a little below the sun at inferior conjunction. A transit of Venus occurs when Venus, on the inside orbit, passes directly between the earth and the sun. A magnified view of the sun from earth reveals a tiny black dot—a mere arcminute across—gliding across the sun over several hours. Of all the visible highlights in the firmament, Edmond Halley said, “This sight…is by far the noblest astronomy affords.”

Transits occur in pairs that are over a century apart, with the pair of events themselves being 8 years apart. Since the era of the telescope, there have been four pairs of transits, and that history is loaded with great stories. Let’s look at how some of our predecessors prepared for, observed, and reacted to the transit of Venus.

Consider Jeremiah Horrocks of England. A fan of Johannes Kepler and his laws of motion, Horrocks had shown by the age of 20 that the moon moved in an ellipse, with the earth at one focal point. He was keen to show that Kepler’s laws of motion applied universally to the planets as well.

Horrocks had noted a discrepancy between existing astronomical tables of his day—including an omission by his hero, the reliable Kepler—so, with less than a month before the 1639 transit, Horrocks decided to do the math himself and correctly predicted the transit circumstances for his location.

On that Sunday in November, Horrocks projected an image of the sun onto a 6-inch piece of paper and awaited the appearance of Venus. He kept vigil from sunrise until shortly after 3:00 p.m., taking time off for a few 1-hour blocks to tend to his Sunday duties. When Horrocks returned to his viewing, he “beheld a most agreeable spectacle.” Venus had just appeared on the limb of the sun. For 35 minutes, Horrocks tracked the small black circle until the sun set. The prey had been captured.

In 1677 Edmond Halley suggested that if observers around the world timed—to the second—how long it took for Venus to move across the sun from edge to edge, one could mathematically calculate the distance to Venus and ultimately yield a sun-earth distance, or the Astronomical Unit. Using Kepler’s laws, one could then easily calculate the distance to all of the planets. As with his namesake comet, Halley knew he would not live to see the transit itself. For his successive colleagues, however, quantifying the scale of the solar system was an irresistible challenge.

When the 18th century pair of transits rolled around, many nations prepared global expeditions to time Venus cutting a swath across the sun. One noteworthy trip was by the Frenchman Le Gentil , who headed to India to time the 1761 transit. Unfortunately the British-French war prohibited his ship from making landfall, and he was on a rolling and pitching ship when the transit occurred–useless.

So he waited for the next transit, eight years later, during which time he visited—and had harrowing experiences at—several places in the Pacific and Indian Oceans. Finally, in 1769, with the war over, he set up his observatory in India at a spot famous for its clear skies. Unfortunately, on the morning of the transit the weather was unseasonable cloudy—he missed it again.

Dejected, he headed home to France. After having suffered from fever and dysentery, from nearly being shipwrecked twice, and away from home for almost 12 years total, Le Gentil found that he had been given up for dead and his estate had been looted and divided by his relatives.

Another intrepid observer of the 1761 transit of Venus was Father Maximilian Hell, who went to the arctic coast of Lapland. The Jesuit priest obtained excellent results, but was later accused —convincingly—of falsifying his results. Many years later astronomer Simon Newcomb realized Father Hell’s respected accuser was color blind and had misinterpreted the ink marks on Father Hell’s papers. Father Hell had been wrongly accused. He eventually got back his good name.

Jean-Baptiste Chappe d’ Auteroche went to Siberia in 1761, where angry locals blamed him and his equipment for the severe floods that year, for he was “messing with the sun.” Protected by armed Cossacks, he eventually recorded the transit. Eight years later Chappe trekked to Baja California to time the 1769 transit successfully. However, he and his party succumbed to an outbreak of fever, with only one member surviving to deliver his notes back to Paris. The dedicated Chappe wrote, “I know that I have only a little time left to live, but I have fulfilled my aim and I die content.”

In 1761, after his expedition to Rodrigues Island near Madagascar was thwarted by rain, the island was pummeled by British warships, and his own ship captured after a fierce battle, Alexandre Guy Pingré of France was transferred to a British ship as a prisoner-of-war. Tapping into the ship doctor’s medicinal supplies, Pingré concluded, “Liquor gives us the necessary strength for determining the distance of the Earth from the Sun.”

For the 1769 transit, American David Rittenhouse was so overcome by emotion that he allegedly passed out at the sight of Venus against the sun.

Meanwhile, the British navy had promoted a young lieutenant, James Cook, to command the HMS Endeavour, destined for Tahiti. The voyage’s express purpose was to time the 1769 transit of Venus, which set James Cook on his way to the first of three grand voyages of discovery.

The observations of Cook and others were often plagued, however, by a phenomenon known as the “black drop” effect. At internal contacts, when the disk of Venus just touches the edge of the sun, a meniscus sometimes appears between the planet and the sun. Circular Venus briefly elongates on one side.

To simulate the “black drop” effect, almost pinch your thumb and forefinger together against a bright background and observe with one eye shut. Near contact you’ll see the meniscus between them appears.

Because it reduces the accuracy of the timing, the “black drop” effect becomes the limiting factor in determining the Astronomical Unit via transit timings.

By the time the 19th century transits rolled around, astronomers such as Maria Mitchell, David Todd, and Jules Janssen employed new techniques like photography. Even though the value of using transits to determine the astronomical unit had essentially run its course, the US Naval Observatory in particular led multiple expeditions around the globe.

The first record of scientific astronomical observations being made from Hawai`i appears to be that of a British expedition in 1874. King David Kalakaua, who reigned over the Kingdom of Hawai`i at that time, established the first permanent observatory in Hawai`i, in 1883. Thus began a tradition of telescopic excellence from those Pacific islands.

American bandmaster John Philip Sousa composed The Transit of Venus March for an unveiling ceremony before the 1882 transit.

There are many more characters from many countries who were part of the transit of Venus story. A good resource is Steven van Roode’s website at or see links from

In the modern era, astronomers are using satellites to detect transits of new worlds around distant stars. For example, the Kepler mission is looking at thousands of stars in one swath of sky to find small yet periodic variations in the brightness of stars. Such a regular dip in brightness may one day indicate the presence of an earth-like planet—not too big, not too small; not too hot, not too cold. Perhaps just right—to harbor life.

The most recent transit of Venus occurred June 8, 2004. How well was it received worldwide? An imperfect indicator of public interest is Google’s Zeitgeist feature, which aggregates millions of search queries every day. According to Google, the 2004 Transit of Venus was the #1 Most Popular Event for June 2004—the #1 Most Popular Event for the entire month! That alone suggests why we ought to prepare well for the 2012 transit of Venus.

I invite you to peruse to launch your own transit of Venus adventure. Be safe—protect your eyes. After June 5-6, 2012, the next transit of Venus will occur December 5, 2117.

As Jeremiah Horrocks wrote, “Years must roll away, but then at length the splendid sight again shall greet our distant children’s eyes.”

I’m Chuck Bueter, wishing you clear skies as you witness the major astronomical alignment of 2012.

(Music: Transit of Venus March, composed by John Philip Sousa; performed by Loras Schissel and the Virginia Grand Military Band.)

End of podcast:

365 Days of Astronomy
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