Fuzzy Occultation By Mars

William Herschel was mentioned earlier; he was the German-British astronomer who discovered Uranus in 1781. His sister Caroline found many comets using her brother's telescopes, both from the city of Bath, where they had been living, and also from Slough, where they later moved. (Slough is near where London's main airport, Heathrow, was much later built.) The Herschel family had moved closer to the capital under the patronage of King George III. Nowadays the idea that major astronomical discoveries might be made from your rooftop or backyard in such locations seems bizarre, observatories being built on mountaintops in remote locations far from city lights, but two hundred years ago the skies were still relatively clear. The smoke of the Industrial Revolution was yet to have a crippling effect on sky translucency, and electrification causing light pollution (one of the main banes of modern-day astronomy) was an unimagined development.

If you ever visit London for the shopping, after the famous Oxford Street one of the best-known areas is Kensington High Street. Shoppers bustling along there might be surprised to learn that one of the world's largest telescopes was once situated nearby. Looking up a street directory, one may find Observatory Gardens (a road, despite the name), a few hundred yards off High Street. On that site, since built over, Sir James South established an observatory that stood for 40 years until his death in 1867. The blue plaque marking the spot is incorrect in stating that South's dome housed the largest telescope in the world. Actually it was the biggest refractor (lens telescope); Sir William Herschel, who had died in 1822, had previously constructed larger reflecting telescopes (using curved mirrors) out at Slough. South's telescope had a lens just below 12 inches in diameter. It is still in use today, at the Dunsink Observatory just outside of Dublin.

Although he has since been mostly forgotten, South was a very prominent astronomer in his day. He was one of the founders of the Astronomical Society of London in 1820 and, as the sitting President, pivotal in securing its royal patronage through contacts assembled by having the gentry come to Kensington to view comets and nebulae through his several telescopes. Thus the Charter of the Royal Astronomical Society, granted in 1831, begins with South's name. On the other hand the first Fellow of the Royal Astronomical Society could be claimed to be Charles Babbage, whom we met earlier, because he was listed first amongst the founders, due to his alphabetical advantage, being followed by Francis Baily (of Baily's beads fame).

In those days the scientific circle was limited. John Herschel, the son ofWilliam, often observed the heavens with South, and they jointly drew up catalogues of binary stars. The advent of electrification was mentioned above; this was in part due to the pioneering investigations of Michael Faraday, who frequented South's private observatory, as did Isambard Kingdom Brunel, the great engineer of the earlyVictorian age. Babbage was also a good friend, and it was ill-feeling fostered by a court case over the mounting of South's large telescope (which he claimed to be inadequate), that led to the opposing party recommending that the government cease all funding of Babbage's computing machines. Babbage made the political mistake of appearing as a witness on South's side in a trial that divided the scientific establishment. South was a fiery controversialist, never far from an argument with someone, and Babbage had a similarly bellicose temperament.

With his great telescope South made comparatively few useful observations, forever complaining that its pivot wobbled, blurring the objects he wished to monitor. In 1830, though, he did make a revolutionary discovery. While watching the planet Mars moving through the constellation Leo, he saw it pass in front of a bright star.

For all his faults and intellectual limitations, South was an experienced visual observer, and he recognized that this Martian occultation was not like the numerous lunar occultations he had seen previously. Instead of suddenly blinking off (perhaps with the "projected image" effect mentioned earlier: South was one of those who had noticed this visual phenomenon and debated its origin), as Mars crept up on the star he noticed that the starlight reaching his eye slowly wavered and attenuated.

How could this be? South made the correct deduction: Mars has a substantial atmosphere. Rather than the knife-edge provided by an airless body like the Moon, Mars has a fuzzy border. This produces effects like those we depicted in Figure 2-8, when we were considering how the Moon still receives sunlight, mostly from the red end of the spectrum, during a total lunar eclipse. When watching Mars as it cut across the star in question, South saw that the starlight was gradually absorbed by the ever-thickening layer of Martian atmosphere needing to be negotiated for the light to reach his eye, glued to the ocular of his precious instrument.

Using the primitive equipment of the era, little was yet known about Mars. It presents merely a ruddy disk through a telescope, with a hint of pale colorless patches at top and bottom, the polar ice caps. The imagined canals of American millionaire Percival Lowell were still many decades in the future, along with ideas of Martians and H.G. Wells's War of the Worlds. From his private obser vatory near the heart of London, largely surrounded in those days by green fields, James South discovered that Mars has an atmosphere via his acute observations of that planet eclipsing a star. That's something to remember next time your underground train rumbles through Kensington and Notting Hill Gate, not half a mile from South's old observatory.

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