BY SCOTT DAHM
Special to NHN

On June 5, the planet Venus will transit the disk of the sun in one of the most spectacular and yet rare events of celestial mechanics visible from Earth. The transit will begin at 12:09 p.m. and will last approximately six hours and 40 minutes.

The orbit of Venus around the sun is interior to that of the Earth, but because its orbit is slightly inclined relative to that of the Earth, transits of the sun do not occur on a routine basis.

They do take place, however, in a repeatable pattern with pairs of transits separated by eight years followed by gaps of about 115 years on average. After the June 5 event, the next transit of Venus will not occur until 2117.

While many people will likely ignore this event altogether, we should be reminded of the extraordinary role the transit of Venus played in one of the greatest scientific achievements of the 18th century – the accurate determination of the absolute distance to the sun.

This measurement of the “astronomical unit” was the first rung in a distance scale ladder that now extends from the sun, to the nearest stars, our local group of galaxies, and ultimately, to the most distant quasars. Astronomers today owe a debt of gratitude to those who came before them and their all-but-forgotten efforts directed at measuring the astronomical unit.

In 1716, Edmund Halley suggested that the transits of Venus in 1761 and 1769 would provide the best opportunity to accurately determine the distance to the sun. Although Halley died in 1742, an international consortium was organized with expeditions being sent to various parts of Europe, North America, Siberia, South Africa, and India to observe the transits from multiple locations. Each group used the diameter of Earth as a baseline to triangulate the distance of Venus and the sun.

While such notables as Captain James Cook, Charles Mason and Jeremiah Dixon embarked or participated in these expeditions, it is the extraordinary tale of the French astronomer Guillaume Le Gentil that has attracted much attention. His adventure was even incorporated into a play as well as a modern opera.

After boarding a French frigate in 1761 bound for India, Le Gentil found his proposed India observing site was occupied by British forces, with the Seven Years War was at its peak. Still at sea on the day of the transit, Le Gentil watched the 1761 transit from the deck of the rolling ship with uncertainty in its position and time, which negated the observation altogether. Having come so far and reluctant to admit failure, Le Gentil vowed to stay eight years in the region and wait for the next transit event in 1769.

During much of the intervening time, Le Gentil stayed on Isle de France (Mauritius) and Madagascar. Finally, the day arrived and as he prepared to observe the transit from India, thick clouds came in and obscured the transit event completely. Having invested over a decade of his life in a failed attempt to observe the transit of Venus, Le Gentil returned to France utterly disheartened only to find that he had been declared dead, his wife had re-married, and his estate was in ruins. Ultimately, from the 1761 and 1769 observations of the other expeditions, the astronomical unit was determined to be 153 million kilometers (the modern value is 149.6 million km.)

Now, we have our own final opportunity to observe the transit of Venus on June 5. While the perils and stakes will not be nearly so great as those endured by Mason and Dixon or Le Gentil, we will undoubtedly remember their extraordinary efforts to give us forever our place in the universe.

S. E. Dahm earned his PhD in Astronomy from the Institute for Astronomy, University of Hawaii at Manoa in 2005. He now works as an astronomer at W. M. Keck Observatory.