Light travels instantaneously.

On human scales, light appears instantaneous. A lamp illuminates a room without any detectable delay, and seventeenth-century terrestrial experiments lacked both sufficiently long paths and clocks precise enough to measure the travel time.

Ole Rømer found the delay on an astronomical scale. He tracked eclipses of Jupiter’s moon Io and noticed that the predicted times drifted depending on Earth’s position in its orbit.

When Earth was farther from Jupiter, the eclipses appeared later; when it was closer, they appeared earlier. Rømer interpreted the changing delay as the extra time required for light to cross the changing distance.

The result showed that light had a finite speed, although Rømer did not determine today’s precise value.

Later measurements using stellar aberration, rotating wheels and mirrors, interferometry, and electronic timing steadily improved the number.

Today the vacuum speed of light is fixed by definition at exactly 299,792,458 metres per second and helps define the meter itself.

Light can propagate more slowly through materials because of interactions with matter; the exact constant refers to vacuum.