Transit Trivia: Mason and Dixon

The above image presents the surveyed line (in red) which separated Pennsylvania and Delaware from Maryland and West Virginia. The line is commonly known as the Mason-Dixon Line. Image Credit: The image is in the Public Domain.

Encouraged by the appeals of the late Edmund Halley, Many expeditions were made to various parts of the world so that precise observations could be made of the Venus transit of 1761. These observations could then be used to calculate a more precise solar parallax, and thus a more accurate distance between Earth and the sun — a measurement which we today describe as an Astronomical Unit.

Despite the fact that this transit took place during the latter half of the Seven Years' War (1756-1763), a worldwide conflict involving the major powers of the time and their colonies, the event was an early example of international scientific collaboration. Scientists and explorers from Britain, Austria and France traveled to destinations around the world, including Siberia, Norway, Newfoundland and Madagascar. Most of the teams managed to observe at least part of the transit, but successful observations were made in particular by English surveyor and astronomer Jeremiah Dixon (1733 – 1779) and English astronomer Charles Mason (1728 – 1786). The pair took their measurements from the Cape of Good Hope on the southern tip of Africa.

If the names Dixon and Mason sound otherwise familiar, it might be because of some other measurements they made together. From November 1763 to 1768, Mason and Dixon established the boundary line between the American provinces of Pennsylvania and Maryland. Colonial surveyors had been unable to accurately establish the boundary due to their poor training and inadequate scientific instruments. Mason and Dixon, accompanied by a large party of assistants, established three important boundaries: (1) the south boundary line of Pennsylvania separating it from Maryland and Virginia; (2) the west boundary of the three lower counties of Pennsylvania (now Delaware) separating it from Maryland; and (3) the south boundary of the three lower counties. The two men also conducted a number of experiments for the Royal Society such as measuring a degree of longitude.

The second boundary referenced, which would separate Pennsylvania and Maryland, became commonly known as Mason and Dixon’s Line, or the Mason-Dixon Line. When projected westward, the line came to symbolize a cultural boundary between the Northeastern United States and the Southern United States — which was later given the nickname “Dixie.”

The next transit of Venus will occur over June 5-6. To learn more, visit these links.


2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html


Transit of Venus, Sun-Earth Day 2012, NASA: sunearthday.nasa.gov/transitofvenus/


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