How was the speed of light first measured?

The first measurement used astronomy. In the 1670s the Danish astronomer Ole Roemer noticed that eclipses of Jupiter's moon Io were timed differently depending on how far Earth was from Jupiter, and he concluded that light takes time to travel. The first measurement made entirely on Earth came in 1849, when Armand Fizeau sent light to a mirror and back through a spinning toothed wheel. [1] [2]

Galileo tried a simple method. He and an assistant stood on distant hilltops with covered lamps. Galileo uncovered his lamp, and the assistant uncovered his own as soon as he saw the light. The result was inconclusive, and Galileo concluded that light, if not instantaneous, is extraordinarily rapid. He did judge that it travels at least ten times faster than sound. Human reaction time and the short distances made the method too crude for the speed involved. [3] [4]

Roemer studied Io, which has an orbital period of about 42.5 hours. The time between its successive eclipses by Jupiter varied slightly with Earth's position in its orbit. When Earth was moving away from Jupiter, each eclipse's light had farther to travel than the last, so the eclipses appeared later. When Earth was approaching, they appeared earlier. Roemer correctly saw that this was caused by the finite speed of light and not by a change in the moon's orbit. According to [openstax.org](https://openstax.org/books/university-physics-volume-3/pages/1-1-the-propagation-of-light), this happened in 1675, while [aps.org](https://www.aps.org/apsnews/2010/07/fizeau-speed-of-light-experiment) and [math.ucr.edu](https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html) give 1676. [1] [3] [2]

The sources give different values for his result: 200,000 km/s (NOVA, which gives it as the low end of early results), 214,000 km/s, 240,000 km/s, and 2.0×10^8 m/s, which is 33% below the modern value. The results do not agree, and the differences probably reflect how the figure was reconstructed. One source notes that the result was very approximate because planetary distances were not accurately known at the time. [5] [2] [3] [1]

In 1728 James Bradley used stellar aberration, the apparent shift in star positions caused by Earth's motion. Knowing Earth's orbital speed, he obtained about 301,000 km/s. [2]

In 1849 Fizeau made the first terrestrial measurement. He aimed a light beam through the gaps of a rapidly spinning toothed wheel toward a mirror about 8 km away. He raised the wheel's speed until the returning light struck a tooth and was blocked. From the wheel's speed, the number of teeth and the distance, he found about 315,000 km/s, roughly 5% too high. Léon Foucault replaced the wheel with a rotating mirror and got about 298,000 km/s, within 0.6% of today's accepted value. OpenStax dates this to 1862, while another source says he improved the result a year after Fizeau. [1] [4] [2]