In 1676, the Danish astronomer Ole Rømer[note 1] demonstrated that light has a measurable speed and so does not travel instantaneously.

By timing the eclipses of Jupiter's moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun.

In his calculations Rømer used the idea and observations that the apparent time between eclipses would be greater while the Earth is moving further from Jupiter and lesser while moving closer.