Illustration of Roemer's determination of the speed of light. In 1676 he observed a series of eclipses of the innermost satellites of Jupiter, which completes a revolution around the planet in 42 1/2 hours, and is eclipsed in it's shadow-cone once in each

- Image ID: G15N98
Illustration of Roemer's determination of the speed of light. In 1676 he observed a series of eclipses of the innermost satellites of Jupiter, which completes a revolution around the planet in 42 1/2 hours, and is eclipsed in it's shadow-cone once in each
Science History Images / Alamy Stock Photo
Image ID: G15N98
Illustration of Roemer's determination of the speed of light. In 1676 he observed a series of eclipses of the innermost satellites of Jupiter, which completes a revolution around the planet in 42 1/2 hours, and is eclipsed in it's shadow-cone once in each revolution. Let A be the Sun, BCDE the Earth's orbit, F Jupiter, and GN the orbit of the satellite. Then he noticed that the satellite's period seemed longer, and eclipses succeeded one another more slowly, when the Earth was receding from the planet in the portion of BC in its orbit, than when approaching it along DE. He explained this effect by supposing that light has a finite velocity. His theory was controversial at the time he announced it, but it quickly gained support among other natural philosophers of the period, such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades after his death, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley.