Transit Science
Astronomers no longer need to travel halfway around the world risking encounters
with hostile natives and dysentery to measure the distance from Earth to the
Sun. In fact, the transit of Venus hasn't been of much interest to modern
astronomers.
Observations of the transit of Venus from various latitudes on Earth (points A and B) gave 18th century astronomers a fairly accurate measure of the Earth-Sun distance.
Edmond Halley first proposed using observations of the transit of Venus to
measure the Earth-Sun distance in 1716. His method required that two or more
observers at different latitudes (see diagram) note the paths Venus describes
across the surface of the Sun. The angular separation of the paths yields Venus' distance
from the Sun. Given the distance from Earth to Venus, which had been known
through careful observations of their orbits since the time of Copernicus,
Earth's distance to the Sun is a matter of simple addition.
Expeditions to observe the transit of Venus from various points around the
globe were undertaken in 1761 and 1769. Though the first attempt at making
coordinated observations yielded few scientific results, Cook and Green led
one of 76 observing parties for the latter. The combined observations led to
calculations of the Earth-Sun distance of between 92 and 96 million miles.
Observations of the transit of Venus were still important to nineteenth-century
astronomers when the most recent pair occurred in 1874 and 1882. Captain Ralph
Chandler of the U.S. Navy escorted five observing parties to the southern hemisphere
aboard the USS Swatara to record the 1874 event.
Today, astronomers are still trying to get more accurate measurements of Earth's
distance from the Sun in order to better characterize the behavior of Earth's
orbit. Their methods no longer involve observations of the transit of Venus
nor simple tools like Cook's quadrant and basic mathematics.
According to Art Whipple, an astronomer at the University of Texas McDonald
Observatory, Earth's orbit is measured today using laser ranging to the
Moon and artificial satellites, radio communications to space probes like Galileo
and Voyager, and very long baseline interferometry. The data from these observations
are put into complicated computer models that correct for observational errors.
A better understanding of Earth's orbit helps scientists study the mass
of the Sun, Moon and planets; test Einstein's Theory of General Relativity;
and study Earth's interior.
Astronomers certainly won't ignore the next transit, due on June 8,
2004, though its value to our understanding of the solar system has been diminished
by more accurate modern methods. Other observations may be planned, however,
perhaps to gain new insight into the content of Venus' upper atmosphere
by watching it against the enhancing backdrop of the solar disk.
-- Doug Addison
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