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MARGINALIA

Six Months in Ascension

Thanks in part to his intrepid wife, David Gill made the observations that metered the 19th-century solar system

J. Donald Fernie

Malignant Fate, or an Ill-Placed Volcano

After a week of setting up the equipment, David prepared to start his campaign of observing Mars. He was in for a shock: Although every day was brilliantly sunny, every night was cloudy! Isobel wrote:

Oh! those weary weeks. Fearful of losing a single hour of star-light during the night, we watched alternately for moments of break in the cloud, sometimes with partial success, but more frequently with no result but utter disappointment, and the mental and physical strain … grew almost beyond our strength.

 The torrid heat and dust of the daylight hours only added to their sullen mood, and tempers began to flare. One night David remarked it was almost as though a malignant fate brought the cloud over them; he could almost see clear skies away on the horizon. Isobel immediately announced that she was going to take a long walk across the lava beds to see whether the sky was clearer elsewhere. David was aghast. It was madness to traverse the broken surface in the dark, and David couldn't leave the telescope in case the sky cleared. But Isobel was adamant. David woke up one of their helpers and told him to prepare a lantern, food and water, and accompany Isobel on her 2 a.m. walk. Regrettably, this worthy's thoughts that night have gone unrecorded.

Whatever the impetus for her walk, Isobel had found the solution. The skies were indeed clear just a mile or two away. The Gills had been the victims of what today would be called an orographic cloud—one caused by air forced upward by a topographical feature. Their site had been downwind from the main volcanic peak on the island, and changes in the nighttime air produced a cloud streaming out from the peak. David would have to move the observatory away from town. And therein lay the rub! No means existed for moving 20 tons of delicate instruments across the smashed terrain, or "clinker." Finally one local pointed out a small, clear beach, later to be named Mars Bay, on the southwest coast. Workers loaded the equipment onto a naval steamer and brought it ashore at the beach.

The Gills camped for the next several months in two tents pitched on the lava beds ("I am at a loss to convey . . . an idea of what sort of flooring clinker makes." wrote Isobel). Their two helpers fetched food from town each day and brought water by boat, swell permitting. There were setbacks. The assistants had pitched the tents incorrectly, and a passing rainstorm soaked people and equipment. Hordes of "musquitoes" soon followed. Later, David fell and injured his knee on the clinker and suffered mild sunstroke. But through it all, the determined team got the data they came for, David at the telescope, Isobel recording the numbers he called out.

Not until they returned home and analyzed the measurements did they learn that they had been successful. David's calculations produced a solar distance of 93.08 ± 0.16 million miles, the most accurate determination up to that time and one that compares well with today's value of 92.9558. The young Scotsman became famous and was subsequently appointed Her Majesty's Astronomer at the Admiralty's southern Royal Observatory in Cape Town, where he flourished and eventually became Sir David Gill.

David died in 1914, and Isobel in 1919. They are buried side by side in the ruins of the 14th-century Cathedral of St. Machar in their beloved Aberdeen.




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