1644-1710

Danish astronomer celebrated for demonstrating light's finite velocity. Römer noticed that the intervals between successive eclipses of Jupiter's satellites varied depending on Earth's positions—diminishing as Earth approached and increasing as it receded. He correctly attributed this to the time required by light to travel the Jupiter-Earth distance and in 1676 calculated light's velocity at 225,000 km/sec (139,808 mi/sec). Römer invented various scientific instruments including a micrometer, planetaria, and an alcohol thermometer that influenced Daniel Fahrenheit's thermometric researches.