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This coming June a very special celestial event will occur, one that has had an indelible impact on human and astronomical history. On June 5th-6th, depending on your respective position across the International Date Line, the planet Venus will transit across the face of the Sun for the last time this century. And I can think of no finer reading companion to warm you up for this event than this week’s review, The Day the World Discovered the Sun by Mark Anderson. Out this year from Da Capo Press, this book reads like a fine historical adventure novel, tracing the exploits of three expeditions that raced across the globe to observe the transit of Venus in 1769. This was truly the first international effort of its kind and marked the beginning of journeys made with scientific discovery in mind.
Just what’s so special about the transit of Venus? First predicted by Kepler and first observed by Jeremiah Horrocks in 1639, transits of Venus occur in 8 year pairs separated by alternate spans roguely121 and 105 years apart. It was Edmund Halley who first realized that very accurate measurements of the transit of Venus across the disk of the Sun from widely separated stations could yield the value of the solar parallax and the distance of the Earth to the Sun. Splash in some Keplerian geometry, and viola! You have the scale of the solar system, all from one day’s worth of observations.
And the race was on. Expeditions fanned out across the globe for the two transits of the 18th century; one in 1761 and one in 1769. Failure would mean awaiting the next transit in 1874! Nicholas Delisle refined Halley’s technique with his own method, which only requires timings of ingress/egress from different locations. This is handy if the Sun rises or sets during the transit as it does here from Florida this June.
The Day the World Discovered the Sun tracks the expeditions Jean-Baptiste Chappe to Baja California, Father Maximilan Hell to Vardř Norway, and most famously, the journey of Captain Cook and the Endeavour to the island of Tahiti. Tales of adventure ensue, as parties combat disease, despair, welcome and suspicion in their race to be in place for the celestial show of the century. It’s always amazing for me to read just how those old observations were made and how they measured longitude by the angles of the Sun and Moon, and the book doesn’t back away from the “good stuff” that astronomical history buffs yearn. Such examples are the “marine chair” designed for viewing Jupiter and its moons at sea (which failed miserably) and methods of observing the Sun via “smoked glass” solar filters (NOT recommended, by the way!) A table is included for the mathematically curious, and tales of astronomical 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 expedition.
And just what good are transits of Venus today? The author points out that like in the 18th century, we had an earlier transit in 2004 to cut our teeth on to test a new technique; the hunt for exoplanet atmospheres. This June, all (properly protected) eyes will be on the Sun, and scientists will test a technique that may yet yield information in the hunt for atmospheres surrounding exo-Earths… don’t miss this summer’s transit of Venus, as the next one doesn’t occur until… 2117!