Earth and the Moon

Orbit and Rotation

Who was Foucault and how did he come up with the idea for his pendulum?

Jean-Bernard-León Foucault was a leading scientific figure of his time. Aside from his famous pendulum, Foucault also invented the gyroscope, made the most accurate measurement of the speed of light up to that time, and instituted improvements in the design of telescopes. In addition, Foucault was a prolific writer, producing textbooks on arithmetic, geometry, and chemistry, as well as a science column for a newspaper.

Together with physicist Armand Fizeau (1819–1896), Foucault was the first person to use a camera to photograph the Sun. The camera they used was a daguerreotype, which took pictures on a light-sensitive, silver-coated glass plate. These early plates were barely sensitive to light, compared to the film or digital detectors being used today, so to take their photos, Fizeau and Foucault had to leave the camera focused on Earth for quite a while. It took so long that the Sun’s position relative to Earth would change considerably, and the pictures would be blurry. This problem inspired Foucault to invent a pendulum-driven device to keep the camera in line with the Sun.


This is a web preview of the "The Handy Astronomy Answer Book" app. Many features only work on your mobile device. If you like what you see, we hope you will consider buying. Get the App