Running the Line: The Astronomy of Mason and Dixon

Mercury, November/December 2006 Table of Contents

The line of Mason and Dixon. Illustration by T. Ford.

by Robert Mentzer

In 1543, Copernicus published his Sun-centered model of the Solar System. Using the observations of the planets that had been made over many centuries, he was able to calculate the time it took for each of the five known planets to circle the Sun. He was also able to calculate the relative distances of the planets from the Sun. Kepler would later slightly improve these numbers by putting the planets in elliptical orbits. However, neither Copernicus nor Kepler could determine the absolute size of the planetary radii.

Over the next 150 years, many values were published by various authors, but these values differed widely and no consensus could be reached. Then, in 1691 and 1716, Edmund Halley published two papers showing how a transit of Venus could be used to find the planetary radii. This method required observations of the transit from at least two widely separated locations on Earth.

As 1761 approached, both France and England prepared to send several teams to northern and southern locations to observe the Venus transit in June of that year. One of the English teams was to be headed by Charles Mason and assisted by Jeremiah Dixon. Mason, a talented mathematician and astronomer, had been an assistant to the Astronomer Royal, James Bradley, for four years. It is believed that Dixon, a surveyor and keen astronomer, had been recommended by the great instrument maker, John Bird.

In December of 1760, Mason and Dixon left on a Royal Navy warship for Sumatra in the south Pacific. However, due to delays caused by the ongoing Seven Years War, they only reached the Cape of Good Hope by April. They decided to observe from Capetown and not risk running out of time. They carefully measured the latitude and longitude of the Cape, as these values were essential for the transit calculations. Luckily, the sixth of June was a sunny day, and the team was able to observe the transit.

They returned to England in 1762, little suspecting that within a year they would begin another equally exciting adventure.

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