Jupiter changed depending on the time of the year. He said that the time changed due to the speed light traveled from Jupiter to Earth. In 1849, French man Fizeay used a rotating wheel with notches cut in it. A light source behind the wheel created a flickering light that traveled over 8 kilometers to a mirror and was reflected back. In the late 1800 s, American physicist Michelson improved upon this idea by using rotating mirrors and found a speed of 299,800 kilometers per a second.
Modern methods use the fact that light is a form of electromagnetic wave.
The speed of any wave is the product of its frequency and its wavelength. Using pure laser light, with a single known frequency, the wavelength of the light is measured and the speed of light is calculated to be 299,792.458 kilometers per a second.