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(p.2) |
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| Finding
the AU: How the Transit of Venus Tells Us Our Distance from the Sun |
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| By
1619, German astronomer Johannes Kepler had figured out the relative distances
of all the planets from the Sun. For example, if the Earth’s distance
from the Sun is one astronomical unit (AU), then Venus’s distance
from the Sun is .72 AU, Mars’s is 1.5 AU, and so on. |
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| However,
no one knew the value of AU, so the absolute distances between the celestial
spheres was not known. |
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| In
1716, English astronomer Edmond Halley proposed a method for calculating
our distance from the Sun—the astronomical unit—using the
transit of Venus. |
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The
underlying principle behind Halley’s method is something called
parallax, the shift in position that comes from viewing an object from
two different points. (What
is parallax? Try this.)
Imagine two different people, one on each pole of the Earth, viewing the
transit of Venus. The person on the North pole sees Venus following one
path across the Sun. The person on the South pole sees Venus follow a
slightly higher path, one that’s shifted a little to the north. |
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Because we see the Sun as a circle, these two different paths will have
different lengths. Halley proposed that an easy way to measure the difference
between the lengths of these two paths would be to time the transits,
using the four phases of the transit—the first, second, third, and
fourth contacts—as indicators. |
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|
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| With
the two different paths known, the distance between the Earth and the
Sun can be pretty easily calculated using trigonometry and Kepler’s
third law of planetary motion. |
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