June 22, 2009
Dutch astronomer Christiaan Huygens made the first calculation of light's speed. The estimate was based on observations of when Jupiter's shadow eclipsed its tiny moon Io.
Øle Christensen Rømer figured out that variations in eclipses of Jupiter’s moon Io meant that light must travel at a set speed. The eclipses happened earlier when Earth was closer to Jupiter because light from Io had less distance to travel; they happened later when Earth was farther away. Combining Rømer’s observations of Io and an estimate of the diameter of Earth’s orbit, Dutch astronomer Christiaan Huygens made the first calculation of light’s speed in 1678.
Photo by Astronomy: Roen Kelly
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