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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

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.

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