Measure of the Earth by Larrie D. Ferreiro

Author:Larrie D. Ferreiro
Language: eng
Format: epub
Publisher: Basic Books
Published: 2011-04-18T16:00:00+00:00

IX

The Dance of the Stars

“I am as constant as the northern star,” wrote Shakespeare, a line Voltaire may well have recounted to his friend La Condamine in 1735 as he reworked Julius Caesar for the French stage. Of course, La Condamine knew that the stars were anything but constant; they moved around the sky in small but detectable patterns as a result of the Earth’s orbit around the sun, a fact that had only recently become apparent to eighteenth-century astronomers. When Picard and Cassini had conducted their surveys in France decades earlier, their zenith sectors were not accurate enough to detect these tiny variations in stellar motion, so to them the stars had appeared fixed in their places. The new sectors used by the Geodesic Mission were far more accurate, and it was this precision that, ironically, led to the errors the scientists were now trying desperately to correct.

If the quadrant was the reliable workhorse of the Geodesic Mission, the zenith sector was its thoroughbred—finely tuned but temperamental, and prone to complications. The instrument’s purpose was to determine the latitude of its current position, and its operation was simple in principle: A long telescope mounted to a movable frame, it hung from a rafter and was gimbaled at the top so that it swung through a small arc on a precise north-south axis along the meridian, aimed at a small patch of sky almost directly overhead. The astronomer would recline uncomfortably on the floor, craning his neck up to the eyepiece. He was looking for a particular star—in this case, Epsilon Orion—to reach its maximum altitude as the Earth slowly rotated underneath. As the star entered his field of view from left to right—inverted because of the convex lenses—the astronomer would minutely adjust the telescope’s position so that the horizontal crosshair aligned with the star’s path. As he called out the star’s crossing of the vertical (meridian) crosshair, his assistant would note the time on the wall-mounted pendulum clock. The star’s exact angle north or south from the vertical, which was determined by a plumb bob, would be read from the precisely graduated limb, thus enabling the scientists to calculate the latitude of their position. Under ideal conditions, the crossing time and angle should be the same on any given night, but in order to average out any errors in the telescope or mounting, the astronomer would make the same observations over many successive nights.

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