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