American Scientist

Nearly fifty years ago my wife and I joined a handful of passengers aboard a freighter out of New York bound for Cape Town, South Africa. Our first landfall was a small island, only seven or eight miles at its widest, that lies about eight degrees south of the equator and halfway between South America and Africa. The island of Ascension was a welcome sight—but not a pretty one. Even its official Web site describes the island's surface, covered by old basalt flows and cinder cones, as "rugged, dry, barren, and inhospitable."

Our ship anchored off Georgetown, the tiny, lone settlement, to offload cargo. We passengers didn't go ashore, as the only way to gain the headland was to leap from the gunwales of a small tender onto the steep, slippery steps cut into the rock. When the swell was heavy it became too dangerous. Later we sailed down the southwest coast and gaped at the forbidding cliffs of ancient lava. Suddenly there appeared between the gray-black rocks a small beach with brilliant white sand, a sight that caused a stir among the passengers at the deck rails. "That's Mars Bay," announced a nearby ship's officer, and suddenly I realized: This was the place where a young astronomer made the best 19th-century estimate of the solar system's size!

Surveying the Solar System

The man who did it was Scottish astronomer David Gill, who was only 34 when he achieved this remarkable result in 1877. However, the achievement was not his alone. Gill conceived the idea, made the observations and performed the calculations, but he likely would have failed without his feisty young wife, Isobel, who was instrumental in her husband's success. She later wrote a book, Six Months in Ascension, describing their adventures.

The expedition was based on an idea hundreds of years old. Johannes Kepler's laws of planetary motion, the first of which was announced in 1605,  made it possible to construct a map of the solar system that showed the orbits of all the planets relative to one another. Only the scale was missing. However, astronomers could calculate the absolute distances between any of the planets if ever they determined how far apart in miles or kilometers any two planets were at a given time. In particular, putting a scale to Kepler's plan would tell the distance between Sun and Earth in actual miles.

In theory, it's simple to find the distance between Earth and another nearby planet: Separate two observers on Earth by a known distance and have them simultaneously record the position of the planet against the invariant backdrop of stars or the face of the Sun. Each observer will see the planet at a slightly different position on the backdrop; they can then calculate the distance of the planet from Earth by simple triangulation.

A popular, if difficult, method of performing this calculation in the 18th and 19th centuries used the transit of Venus across the Sun's face (American Scientist 85:120-122). The tricky part was determining the exact moment the transit began and—hours later—ended. Gill was familiar with the method, having helped record the transit of 1874, and had noted several drawbacks. First, such transits are very rare. If something went wrong or clouds interfered it was either 8 or more than 100 years before the next transit. Second, to make the estimate as precise as possible, teams of observers had to go to widely separated regions across the globe. Dispatching multiple teams to different destinations was also prudent, if not absolutely required, to minimize the chance of being clouded out. However, travel was expensive and, at times, hazardous. Finally, after all the observations were made, the astronomer had to compare directly the data collected by different people using different types of telescopes and clocks of differing accuracy.

Gill realized there was a better way. His method only needed one observer, at one site, using one piece of equipment. Furthermore, by making the necessary observations over many nights, the data would be sufficient even if a few evenings were cloudy.

His idea was to view Mars when that planet (the first one past us from the Sun) was closest to Earth. Under these conditions Mars is opposite the Sun in the sky and therefore visible for much of the night. Gill planned to record the precise position of Mars among the stars that appear nearby, soon after it rose in the evening. He could repeat these measurements hours later before Mars dipped below the horizon. In the time between the two observations, the rotation of the Earth would have carried the observer thousands of miles, thereby providing a baseline of unprecedented width for triangulation. Moreover, Gill noted that the elliptical orbits of Mars and Earth would bring the red planet unusually close during the opposition of 1877, a coincidence that improved the chances of getting accurate results. He determined to make the necessary observations.

A Tropical Vacation?

Because of Earth's rotation, an observer near the equator travels a longer 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 being no established astronomical observatories in the tropics, Gill resolved to mount an expedition to erect a temporary 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 Astronomer Royal intervened on his behalf and persuaded another agency, the Royal Astronomical Society, to make a grant of £250; the rest Gill had to supply himself. As for transportation, no southbound English ships called at Ascension, so the Gills had first to travel to the island of St. Helena, 800 miles to the southeast, and there wait for a northbound ship. Finally, on July 13, 1877, they found themselves anchored off Georgetown. It was not a prepossessing sight. "Stones, stones, everywhere stones, that have been tried in the fire and are now heaped in dire confusion, or beaten into dust which we see dancing in pillars before the wind. Dust, Sunshine, and cinders, and low yellow houses frizzling in it all!" was Isobel's description. Their 20 tons of delicate equipment was lifted ashore by crane, but they themselves had to face jumping for the steep steps cut into the rock, very likely the same ones I saw 80 years later. Passengers do not often envy the privileged handling of freight, but in this case "… I certainly wished myself a chronometer … when I saw, rising up behind us, a long wall of threatening water, and before us, the steep, dark rock, wet with spray," wrote Isobel.

Once ashore, the Gills found they would be living in a tiny cottage on the outskirts of town and that Captain Phillimore, the senior officer of the garrison (as Georgetown was known), had courteously volunteered the garrison's croquet ground as a base for the observatory. This site proved to be a sweltering stretch of concrete, but little alternative existed. Although they could buy food from the naval canteen, the provender was usually less than desirable. Fresh meat and vegetables were rare, a quart of milk a week was considered a good ration, and fresh water was so limited that they often cooked with seawater. Fish, however, was plentiful. On Isobel's first visit to the bakery she found "a pallid baker [standing] at the threshold wiping the perspiration from his forehead. Evidently he made his bread by the sweat of his brow!" In any case, his bread was so hard it nearly required an axe to break.

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.