March 28, 2017

Review: The Transits of Venus by William Sheehan & John Westfall.

Out from Prometheus Books!
Out from Prometheus Books!

In less than a year’s time, an event will happen that none of us will live to see again; a transit across the face of the Sun by the planet Venus. In this regard, this week’s review of the book The Transits of Venus by William Sheehan and John Westfall looks at the history of this rare phenomenon. Published in 2004 by Prometheus Books, this work serves as essential reading covering the history, understanding, and what to expect when viewing a transit of Venus. These rare spectacles occur in eight year pairs spaced over longer 121.5 to 105.5 year intervals; the book was published in time to take advantage of the 2004 transit, but will still find use as a timeless as a resource on June 5-6, 2012. In 2004 half (my half!) of North America missed out; Next year, we’ll be treated to a sunset ingress. Transits traces back the history of chasing the Planet of Love in some of the greatest astronomical adventures in history. From the initial realization by Kepler that viewing transits of Mercury and Venus were in fact possible, the author tells the tale of English astronomer Jeremiah Horrocks and the first successful recording of a transit of Venus in 1639.

A transit as sketched by Nicholas Ypey in 1761. (Library of Congress/Ypey).

Sketch 0f the 1761 transit as seen by Nicholas Ypey. (Credit: Ypey/Library of Congress).

 The author also captivatingly treats each transit pair as an engaging snapshot of the state of humanity.  Each proceeding century except the 20th has seen pairings and has dispatched expeditions to measure them by ever more exacting methods. It soon became realized that accurate measurements of the transit of Venus could yield the figure of the solar parallax, or the amount of apparent shift of Venus and the Sun as seen from two different locations. This simple geometry could then be applied to determine the length of one astronomical unit, the gold standard upon which further cosmological distances are based to this day. For example, the term parsec is derived from the apparent shift of a star by one arc second as seen from alternate sides of the Earth’s orbit. In order to work this out, you need a baseline, or the radius of the orbit. This was what 18th and 19th century astronomers were willing to risk life and limb to achieve. Modern day value of the solar parallax is placed at about 8.8” arc seconds; to give you some idea of just how tiny this value is, the apparent angular diameter of Venus during inferior conjunction is about 66”! Add into the mix that astronomers also had to accurately determine their local time and position, and it’s remarkable that they nailed down the figure as well as they did.

Sheehan and Westfall also do an amazing job of relating how those golden age astronomers worked and observed. The timeframe of these expeditions also represented a turning point for science and exploration, seeing some of the first dedicated scientific missions such as Cook’s to Tahiti in 1769. Transits also captures a transition initiated by Kepler and Newton, as astronomical science moved from one with trappings of uncertain superstition to a field on endeavor with true predictive power. It still amazes me that I can look through an eyepiece and meet an event predicted 100+ years ago, right on time!  

Reading this also raises some interesting questions from our own recent astronomical research. One wonders what American astronomer Benjamin Banneker was doing during the transit of 1769… I’m also curious as to the role the Quito observatory in Ecuador might have played during the transit of 1882. Established only a few years prior, it seems to have been built specifically with this transit in mind. Could more historical astronomical sleuthing be in the works?


Photo from the 2004 transit capturing the elusive “black drop” effect. (Credit: Jan Arnold).  

The authors also describe phenomena and effects to be on the lookout for during the 2012 transits, such as the black drop and halo effects. Perhaps the scientific importance of planetary transits in our own solar system has faded, but that won’t stop us from gazing (and blogging) in wonder, as well as jockeying for clear skies of June 5-6th, 2012…just what will our world be like 105 years hence?

Do track down The Transits of Venus for a complete history of this event and a fascinating tale of how modern astronomy came to be. With its exhaustive tables and appendices, Transits will also serve as an indispensible companion as the final transit of Venus in our lifetimes draws near… where will you be next June?

Got Transit(s)? (Photo by Author.

Got transit(s)? (Photo by Author).


  1. [...] to be immutable, unchanging, and separate from the Earth. Like the books The Telescope and the Transits of Venus reviewed earlier this year, Seeing illustrates the evolution of our understanding by linking [...]

  2. [...] intrigue abound. Two other works I’d put on your reading list prior to the June 2012 transit are The Transits of Venus and the Age of Wonder, which also covers the Cook [...]

  3. [...] world for this century, and over the years we’ve reviewed two fine books on this unique event: The Transits of Venus by William Sheehan & John Westfall and The Day the World Discovered the Sun by Mark [...]

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