How is the distance between the Earth and the sun measured?

Weather Blog

Dear Tom,
How is the distance between the Earth and the sun measured?
Thanks, Tom Strayher

Dear Tom:
Astronomer Dan Joyce tells us, “Nowadays, it’s rather simple: Just bounce radar off it (the sun).” The original determination by researchers in the late 1700s was a value of 87.5 million miles.
Joyce explains that physicists inferred the Earth-sun distance over 200 years ago by measuring the transit time of Venus on one of its rare passages across the sun’s face, then comparing those data as measured from different locations on Earth. Applying Kepler’s “Harmonic Law” (the square of the orbit time of a planet about the sun equals the cube of its orbiting distance), they arrived at an Earth-sun distance of 87.5 million miles, close to the correct distance of about 93 million miles.

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