Dixieland Blues

All will be familiar with the southern parts of the United States— the old Confederacy—being termed Dixieland or simply Dixie. There are several theories concerning the origin of this moniker. A leading idea is that it derives from the name of an English astronomer, Jeremiah Dixon. With his compatriot, Charles Mason, Dixon surveyed the border between Maryland and Pennsylvania from 1763 to 1767, defining the famous Mason—Dixon line. Until the Civil War in the 1860s this was considered to be the demarcation between the free states and the areas of black slavery below.

In 1760, however, the pair was looking at heading east towards Asia rather than west towards the Americas. The British wanted to send a transit observing team to Bengkulu in Sumatra, four degrees south of the equator. In those days nearby Jakarta was named Batavia, the capital of the Dutch East Indies. Mason and Dixon were engaged for the task, and in March 1760 they set sail from Portsmouth on a Royal Navy ship. Before slipping out of the Channel, their vessel was attacked by a French frigate. Eleven sailors were killed, and 37 wounded, the British limping back into Plymouth.

Not surprisingly Mason and Dixon had lost much of their enthusiasm for the adventure, and despite the navy offering to provide a mighty escort out of the Channel after their vessel was repaired, they wrote to the Royal Society petitioning for their destination to be switched to the Black Sea. One might suggest that this would involve an even more dangerous voyage through the Mediterranean, but it seems that it was not only the French guns that worried Mason and Dixon. On their abbreviated venture the landlubbers had been stricken with seasickness, and they felt they could not stomach a voyage down through the Atlantic and across the Indian Ocean. This they were charged to do, though, in a forceful rejoinder from London. Nevertheless, a postscript to their instructions allowed them some discretion, and in the event they decided to halt at the Cape, from where they observed the egress. This was just as well: in the interim the French had seized Bengkulu, so that Mason and Dixon would hardly have been afforded a welcome there.

The timings our heroes made at the Cape were useful in the analysis of the transit carried out by mathematician James Short back in London, but there was a problem. A French expedition had gone to Rodrigues, a little island just east of Mauritius, where astronomer Alexandre Pingré watched the egress. Jacques Cassini, Director of the Paris Observatory like his father, grandfather, and great-grandfather before him, supplied that egress timing to Short. (Just because their countries are perpetually at war does not mean that scientists will not collaborate.)

Unfortunately the value obtained from the Rodrigues recording was discrepant when compared to the relatively nearby Cape, and Short thought that Mason and Dixon, who were already subject to some opprobrium, had mistimed the event by precisely one minute. By making this "correction" Short derived a distance to the Sun that was more than 10 percent lower than the real length. In fact the error was due to the longitude quoted for Rodrigues being out by a quarter of a degree. In the late nineteenth century the American astronomer Simon Newcomb, with the advantage of valid geographical coordinates for the observation sites, reanalyzed all the 1761 transit timings and showed that they were consistent with the true solar distance, which by then had been determined by other means.

Back in the 1760s this was not known. It seemed that the transit ofVenus in 1761 had passed by without the necessary timings having been made with a sufficiently wide geographical spread. There was a determination that the opportunity in 1769 would not be similarly wasted.

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