American Scientist

To the Editors:

I enjoyed William and Merri Sue Carter’s column “The British Longitude Act Reconsidered” (March–April). During the 1700s, many timekeeping methods were submitted to the Board of Longitude before John Harrison designed his successful chronometer. As the Carters describe, Captain James Cook appreciated a copy of Harrison’s clock, made by Larcum Kendall and dubbed K1, “as our never failing guide” during his second voyage of exploration.

The Longitude Act was not solely responsible for such advances. The transits of Venus across the Sun in 1761 and 1769 also stimulated progress in astronomical observation and timekeeping. Eighteenth-century astronomers saw the opportunity to calculate the distance between the Earth and Sun by timing the transits of Venus, observed from different locations on Earth. The imprecise results of 1761 encouraged the British Royal Society, with financial backing from the Admiralty and Royalty, to send out teams to try again in 1769. This sensitive, international scientific project required high-precision telescopes, timekeepers and pendulum clocks—which, together with Newtonian mathematics, meant better measurement results.

Incidentally, while Cook was sailing into the Pacific in 1768, a two-man team was sent to observe the transit from Prince of Wales Fort, a fur-trading post on Canada’s Hudson Bay. The senior astronomer of that team, William Wales, kept such meticulous meteorological records that he was later appointed as the astronomer and keeper of the K1 clock on Cook’s second voyage.

In June 2012, Venus will transit again, for the second time this century, not to return until December 2117.

Rita Griffin-Short
Hamilton, Canada

Drs. Carter and Carter respond:

Rita Griffin-Short is correct in pointing out that expeditions to observe the transit of Venus contributed to determining time at remote locations around the globe. Edmond Halley’s 1716 call for the international astronomical community to determine the distance from the Earth to the Sun resulted in major efforts to collect data on the transit of Venus. Unfortunately, visually observing the times that Venus and the Sun appeared to make contact turned out to be more difficult than Halley ever imagined. In 1769 Captain James Cook not only built an observatory at Point Venus, Tahiti, but also sent observing parties to nearby York Island and the islet of Puaru. Cook was disappointed in the results, reporting that:

We saw an atmosphere or dusky cloud round the body of the planet, which very much disturbed the times of contact, especially of the internal ones; and we differed from each other in our accounts of the times of the contacts much more than might have been expected.

By the time of the next pair of transits, in 1874 and 1882, the Canadian-born American astronomer Simon Newcomb developed a much-improved photographic approach for recording the times of contacts. But Newcomb and Albert Michelson then proceeded to accurately measure the velocity of light, and Newcomb combined this information with the latest value for the aberration of light, to determine the velocity and distance from Earth to the Sun far more accurately than could be derived from observations of the transit of Venus.