Historical Perspective on Transits of Venus and Mercury

In 1716, Edmund Halley (1656-1742) suggested using timings of the passage (transit) of Venus across the face of the Sun taken from different locations on the Earth and applying simple geometry to determine the Earth-Sun distance (called the AU—astronomical unit). Consequently, expeditions were dispatched around the globe positioning observers to make precise measurements and timings to calculate the solar parallax. (This measurement is an apparent shift in the position of Venus’ transit across the disk of the Sun due to its being observed from different locations on the Earth’s surface.) Timing of the event was crucial, as was knowing one’s position to a great degree of accuracy. Unfortunately, bad weather plagued many of these expeditions (some things never change). Also, a phenomenon called the “black drop” effect made getting precise timings nearly impossible.

When Venus appears to touch the limb (edge) of the solar disk, this event is called first contact. It will appear as if a little notch or bite has been taken out of the Sun. As the planet moves farther onto the disk, just before Venus is fully in front of the Sun (called second contact), the “black drop” forms. Its appearance looks like a drip about to detach itself from a faucet, or like the shape of a teardrop. A piece of the planet seems to elongate outward toward the blackness of space along the Sun’s limb. If an observer was situated in a location to experience the beginning and the end of the transit, you had two opportunities to conduct timings. For just before third contact when Venus would begin to exit the solar disk, one could conduct a second timing. The “black drop” affected those timings as well.

The “black drop” effect can last for several seconds, depending upon atmospheric conditions, thereby preventing astronomers from obtaining precise timings of the beginning (ingress) and ending (egress) of a transit.  Observations differed greatly, thereby throwing calculations off by millions of miles. Using Mercury transits for timings was even more difficult to accomplish, since Mercury’s disk is about six times smaller than that of Venus. Accordingly more magnification is required to time the event accurately, but increased magnification also increased distortion caused by the Earth’s atmosphere. See this website for an example of and explanation for the “black drop” effect:  http://www.am.ub.edu/twiki/bin/view/ServiAstro/FaqTrme#What_is_the_black_drop_effect.

Unfortunately, for all intents and purposes, the use of transits centuries ago to determine the scale of the solar system proved fruitless. The expeditions to faraway lands did provide valuable scientific discoveries in other disciplines, not to mention the exploration of our world. For example, if you want to follow up on just one of these expeditions, read about Captain Cook’s voyage and his involvement with the Venus transit of 1769.

During the transits of 1874 and 1882, photography was the new preferred method of acquiring data to derive the solar parallax and to make other discoveries. However, simpler methods already had revised the value for the AU to unparalleled accuracy, and although scientific expeditions were still funded for the purpose of research, very little new information was forthcoming. The one thing that did arise from the 1882 transit was an increased interest and excitement by the general public.

The same is true today. No new scientific knowledge is expected from observing transits of Venus or Mercury. Despite their historical significance, transits have become mere curiosities for the average citizen. Regardless, amateur and professional astronomers have always eagerly awaited observing transits using properly filtered telescopes. Transits are unique events to experience when you know exactly what is occurring.

David Huestis