The speed of light was first measured in the 17th century by a Danish astronomer named Ole Rømer. He made his measurements using observations of the moons of Jupiter.
Rømer noticed that the timings of the eclipses of Jupiter's moon, Io, seemed to vary depending on the Earth's position in its orbit around the Sun. When the Earth and Jupiter were on the same side of the Sun, the eclipses appeared to occur earlier than expected. However, when the Earth was on the opposite side of the Sun from Jupiter, the eclipses appeared to occur later than expected.
Rømer correctly deduced that the observed variations were due to the finite speed of light. He realized that when the Earth was closer to Jupiter, the light from the eclipses had a shorter distance to travel to reach Earth. Conversely, when the Earth was farther away from Jupiter, the light had a longer distance to travel.
Rømer hypothesized that the observed time differences were due to the time it took for light to travel across the changing distance between the Earth and Jupiter. He estimated that light took about 22 minutes to traverse the diameter of Earth's orbit around the Sun.
Using this estimated value, Rømer calculated a speed of light of approximately 225,000 kilometers per second. This value was later refined by subsequent experiments and measurements.
Over the centuries, various methods and experiments have been conducted to measure the speed of light more accurately. Notable experiments include the Fizeau experiment in the 19th century, which used a rotating toothed wheel and a distant mirror to measure the speed of light, and the Michelson-Morley experiment, which was designed to detect the Earth's motion through the hypothetical luminiferous ether but indirectly contributed to measuring the speed of light with great precision.
Today, the speed of light is defined as exactly 299,792,458 meters per second (approximately 300,000 kilometers per second) in a vacuum, based on the International System of Units (SI) definition. Modern techniques, such as laser-based interferometry and highly precise timing methods, are used to measure the speed of light with extreme accuracy.