Review of “Transit of Venus” (Nick Lomb) by Simon O’Toole

In just days from now, on June 6, the planet Venus will pass between the Earth and the Sun for the last time this century; the next opportunity to observe this event will be December 11, 2117. In Transit of Venus: 1631 to the Present, Nick Lomb, of Sydney Observatory, presents the fascinating history of this celestial event and some of the characters who observed the seven transits in the 400 years since the invention of the telescope.

The book covers a simple idea: observe the start and end times of Venus’ passage across the solar disc (the “transit”) from different places around the globe, then use geometry to determine the distance from the Earth to the Sun. Without this distance, the distances to the other planets, such as Jupiter and Saturn, were only known in relative terms (Jupiter is 5.2 times further than Earth, for example).

In practice things are never straightforward of course. The Transit of Venus is therefore not just a book about measuring a distance; it is also the story of the trials and tribulations of early efforts to achieve this goal. It is a story of hardships and personal sacrifices, long arduous journeys across unforgiving seas, and too often of failure at the final hurdle.

Lomb’s narrative interweaves the contribution of well-known figures such as Lieutenant James Cook, Edmund Halley and Johannes Kepler, with unsung heroes including Jeremiah Horrocks and Henry Chamberlain Russell. It is well known that Cook came to the Southern Hemisphere with the primary goal of observing the transit of Venus from Tahiti in 1769; his search for the Great South Land, while arguably more successful, was more a side project.

The story is told from an Australian perspective – after all Australia often played a key role due to its geographical location – but my favourite story is that of the unfortunate Frenchman Guillaume Le Gentil in the 18th century. Imagine sailing across the globe to find your chosen site had fallen into enemy hands en route, staying away from home for the next 8 years to observe the following transit, then missing out again due to bad weather, and finally returning home to find your family believing you dead.

A key point that Lomb comes back to is on the accuracy of measurements: acquiring both the timing and longitude of the observation accurately was extremely difficult. The transit of 1874 was probably the most successful, with large numbers of people observing it all over the world. The accuracy of the observations was thought to be almost as disappointing as previous attempts however, and this caused enthusiasm for the project to wane.

In the end, there is a somewhat tragic air to Lomb’s tale. Despite the efforts of many talented people, other methods of determining the distance from the Earth to the Sun won the day, making the measurement with far higher precision; transit measurements only ever achieved an accuracy of about 1 million kilometres.

Nowadays, a transit of Venus is more a curiosity, albeit a rare one. In the modern age, Venus is observed scientifically to gain a better understanding of its formation and geology rather than for our understanding of distance. The book also includes many of the spectacular images taken from various space missions to the planet in the last 50 years.

It is the rarity that still makes the transit a major event though, similar to the passage of Halley’s Comet; this is the final chance for any of us to witness a transit. The final chapter of the book contains information on when, where and how best to observe the 2012 transit on June 6. In Australia we are well placed once again!

The Transit of Venus is beautifully presented and thoroughly researched, with many archival images covering the history of the quest to accurately measure Venus’ transit times. Nick Lomb is to be congratulated for putting together this very worthwhile and enjoyable read.

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