Venus swinging past the sun…get ready

When I first heard about the upcoming transit of the planet Venus across the sun, I confess that my first thought was: ho-hum, kinda interesting but planets and stars move around all the time, so what if Venus swings across the face of the sun? Then I listened to the interview with Andrea Wulf on the Nature podcast (direct link to the interview is here; the podcast is also on iTunes, etc). How wrong I was in my initial judgment (which grew entirely from my ignorance — the world is only a ho-hum place when your head is buried in the sands of your own limitations).

The transit will certainly be less dramatic than lunar and solar eclipses, but unless any of us are planning to be around in 2125, this is our only shot. But rarity is not the main attraction. When the little dark disc of Venus hits the sun, we’ll be able to witness a natural phenomenon that changed the shape of the world of ideas. Now that is worth paying attention to.

The two transits in the 18th century — in 1761 and 1769 — allowed astronomers to estimate the distance of the sun from the Earth. This was a major achievement, made possible first by the heliocentric view of the solar system (unless you assume that the planets orbit around the sun, your trigonometry is very wrong) and, second, by the calculations of Edmond Halley (of comet fame) who correctly predicted in 1716 that Venus’ transit would offer an excellent alignment of celestial angles, allowing astronomers to get an accurate estimate of the distance to the sun from the Earth (this video has a great overview of how these estimates are made; in this case, we’re using parallax).

Halley did not just point out the scientific potential latent in Venus’ transit, he developed a detailed plan for how to take advantage of this opportunity, although he knew that he would be dead by the time the transit happened. The plan was necessary because an accurate estimate of the distance depended on timing the transit from multiple points on the Earth’s surface, preferably with these points being spread far apart (triangulation works best when the two people conducting the measurement are located at some distance from each other). And so this transit became the motivating force behind large, government-funded, collaborative international expeditions. Astronomers trekked to far-flung places with huge boxes of gear, then hoped for a clear day. Never before had such an enterprise been undertaken and science was forever changed by the success of the project. It is not too much of a stretch to say that the intellectual history of the West was nudged by this event: the measurement of the distance to the sun through the use of scientific principles and predictions was both a product of the Enlightenment and a spur to its further development. So let’s go out on June 5th and pay homage.

The website has much useful information about the upcoming event, including how to view the event without being blinded by Enlightenment (a useful skill, especially for those at liberal arts colleges). If you have binoculars, use them to project an image of the sun onto paper (as shown here), being careful not to look at the sun through the binocs — ouch.

I found the maps on the site particularly helpful:

Map showing where and when you can see the transit (from

For a more precise indication of when you should look, enter your location here. You’ll get a nice set of times and mock-ups of what the transit should look like. Here are the data for Sewanee, TN, where the transit will be visible from 5pm until the sun goes down:

And Santa Cruz, CA (where I’ll be — please, please let it be “sunny California”):

Last, for some musical accompaniment for the event, John Philip Sousa’s Transit of Venus March (who knew?) might be suitable (available in high school bandwidth here).

4 thoughts on “Venus swinging past the sun…get ready

  1. Jim Markowich

    The story of the understanding of Venus’ orbit, and predicting its transits also highlights the brilliance of the nearly forgotten young British astronomer Jeremiah Horrocks (1618-1641).

    I love the video describing parallax. It’s so well done! Thank you!

    Last week I made some simple illustrations about the transit, and about viscerally feeling the angle at which Venus and Earth orbit the sun:

    1. David George Haskell Post author

      Thank you for this, Jim. The picture that you posted with the motion of Venus is very helpful. Boy, I hope the skies are clear on June 5!

      And, yes, the Jeremiah Horrocks story is fascinating. I did not know of his work until I was reading for this blog post. Getting it right where Kepler fell short — not bad!

  2. Marie Wilson

    I am so happy that I finally subscribed to your blog! How do you come up with all of these insightful posts?? I promise to send you an update email soon. I hope all is well :-)

    1. David George Haskell Post author

      Hi Marie, Glad you like the blog! Please do send me an email when you get time — I’d love to hear how things are going. Doing OK here. I’m pretty much caught up from the semester and now doing triage on summer work… I saw Jordan yesterday — she is doing well — y’all were a GREAT team!


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