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