A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Marginalia Six Months in Ascension J. Donald Fernie early fifty years ago my Venus across the Sun’s face (American Nwife and I joined a handful Thanks in part to Scientist 85:120–122). The tricky part of passengers aboard a freighter out was determining the exact moment the of New York bound for Cape Town, transit began and—hours later—end- South Africa. Our first landfall was a his intrepid wife, ed. Gill was familiar with the meth- small island, only seven or eight miles od, having helped record the transit at its widest, that lies about eight de- David Gill made the of 1874, and had noted several draw- grees south of the equator and half- backs. First, such transits are very rare. way between South America and Af- observations that If something went wrong or clouds rica. The island of Ascension was a interfered it was either 8 or more than welcome sight—but not a pretty one. metered the 19th- 100 years before the next transit. Sec- Even its official Web site describes the ond, to make the estimate as precise island’s surface, covered by old basalt century solar system as possible, teams of observers had to flows and cinder cones, as “rugged, go to widely separated regions across dry, barren, and inhospitable.” the globe. Dispatching multiple teams Our ship anchored off Georgetown, feisty young wife, Isobel, who was in- to different destinations was also pru- the tiny, lone settlement, to offload car- strumental in her husband’s success. dent, if not absolutely required, to go. We passengers didn’t go ashore, as She later wrote a book, Six Months in minimize the chance of being clouded the only way to gain the headland was Ascension, describing their adventures. out. However, travel was expensive to leap from the gunwales of a small The expedition was based on an and, at times, hazardous. Finally, after tender onto the steep, slippery steps idea hundreds of years old. Johannes all the observations were made, the cut into the rock. When the swell was Kepler’s laws of planetary motion, the astronomer had to compare directly heavy it became too dangerous. Later first of which was announced in 1605, the data collected by different people we sailed down the southwest coast made it possible to construct a map of using different types of telescopes and and gaped at the forbidding cliffs of the solar system that showed the orbits clocks of differing accuracy. ancient lava. Suddenly there appeared of all the planets relative to one anoth- Gill realized there was a better way. between the gray-black rocks a small er. Only the scale was missing. How- His method only needed one observer, beach with brilliant white sand, a sight ever, astronomers could calculate the at one site, using one piece of equip- that caused a stir among the passengers absolute distances between any of the ment. Furthermore, by making the nec- at the deck rails. “That’s Mars Bay,” an- planets if ever they determined how essary observations over many nights, nounced a nearby ship’s officer, and far apart in miles or kilometers any the data would be sufficient even if a suddenly I realized: This was the place two planets were at a given time. In few evenings were cloudy. where a young astronomer made the particular, putting a scale to Kepler’s His idea was to view Mars when best 19th-century estimate of the solar plan would tell the distance between that planet (the first one past us from system’s size! Sun and Earth in actual miles. the Sun) was closest to Earth. Under In theory, it’s simple to find the dis- these conditions Mars is opposite the Surveying the Solar System tance between Earth and another near- Sun in the sky and therefore visible The man who did it was Scottish as- by planet: Separate two observers on for much of the night. Gill planned tronomer David Gill, who was only Earth by a known distance and have to record the precise position of Mars 34 when he achieved this remarkable them simultaneously record the posi- among the stars that appear nearby, result in 1877. However, the achieve- tion of the planet against the invari- soon after it rose in the evening. He ment was not his alone. Gill conceived ant backdrop of stars or the face of the could repeat these measurements the idea, made the observations and Sun. Each observer will see the planet hours later before Mars dipped below performed the calculations, but he at a slightly different position on the the horizon. In the time between the likely would have failed without his backdrop; they can then calculate the two observations, the rotation of the distance of the planet from Earth by Earth would have carried the observer J. Donald Fernie is professor emeritus of astronomy simple triangulation. thousands of miles, thereby provid- at the University of Toronto. Address: David Dunlap A popular, if difficult, method of ing a baseline of unprecedented width Observatory, Box 360, Richmond Hill, Ontario L4C performing this calculation in the 18th for triangulation. Moreover, Gill noted 4Y6 Canada. Internet: [email protected] and 19th centuries used the transit of that the elliptical orbits of Mars and © 2006 Sigma Xi, The Scientific Research Society. Reproduction 308 American Scientist, Volume 94 with permission only. Contact [email protected]. Earth would bring the red planet un- usually close during the opposition of 1877, a coincidence that improved the chances of getting accurate results. Office Post Island Bureau/Ascension Stamp Agents Gibbons/Crown Stanley He determined to make the necessary observations. A Tropical Vacation? Because of Earth’s rotation, an ob- server near the equator travels a lon- ger distance in a given time than does an observer at higher latitudes. Thus, Gill expected the best results from a field outpost in the tropics. There be- ing no established astronomical obser- vatories in the tropics, Gill resolved to mount an expedition to erect a tem- porary one. He decided Ascension would do nicely. Getting there was another matter. Gill applied to the Royal Society for a grant of £500 to finance the expedition. He was refused. However, the Astron- omer Royal intervened on his behalf The Post Office and Philatelic Bureau of Ascension Island issued this set of four stamps in 1977 to mark the centenary of David and Isobel Gill’s fruitful stay there. The three-pence (3p) and and persuaded another agency, the eight-pence (8p) stamps depict general views of their camp at the small, sandy cove that was Royal Astronomical Society, to make later named Mars Bay after their astronomical observations. The 12p stamp shows the Gills a grant of £250; the rest Gill had to relaxing outside their tent, and the 25p stamp indicates the location of Mars Bay on the island. supply himself. As for transportation, no southbound English ships called a sweltering stretch of concrete, but The torrid heat and dust of the day- at Ascension, so the Gills had first to little alternative existed. Although they light hours only added to their sullen travel to the island of St. Helena, 800 could buy food from the naval canteen, mood, and tempers began to flare. One miles to the southeast, and there wait the provender was usually less than night David remarked it was almost as for a northbound ship. Finally, on July desirable. Fresh meat and vegetables though a malignant fate brought the 13, 1877, they found themselves an- were rare, a quart of milk a week was cloud over them; he could almost see chored off Georgetown. It was not a considered a good ration, and fresh clear skies away on the horizon. Isobel prepossessing sight. “Stones, stones, water was so limited that they often immediately announced that she was everywhere stones, that have been cooked with seawater. Fish, however, going to take a long walk across the lava tried in the fire and are now heaped was plentiful. On Isobel’s first visit to beds to see whether the sky was clearer in dire confusion, or beaten into dust the bakery she found “a pallid baker elsewhere. David was aghast. It was which we see dancing in pillars before [standing] at the threshold wiping the madness to traverse the broken surface the wind. Dust, Sunshine, and cinders, perspiration from his forehead. Evi- in the dark, and David couldn’t leave and low yellow houses frizzling in it dently he made his bread by the sweat the telescope in case the sky cleared. all!” was Isobel’s description. Their 20 of his brow!” In any case, his bread But Isobel was adamant. David woke tons of delicate equipment was lifted was so hard it nearly required an axe up one of their helpers and told him to ashore by crane, but they themselves to break. prepare a lantern, food and water, and had to face jumping for the steep steps accompany Isobel on her 2 a.m. walk. cut into the rock, very likely the same Malignant Fate, or an Ill-Placed Volcano Regrettably, this worthy’s thoughts that ones I saw 80 years later.