Ole Rømer


Ole Rømer : biography

5 October 1644 – 19 September 1710
The eclipses of Io recorded by Rømer in 1676 Time is normalized (hours since midnight rather than since noon); values on even rows are calculated from the original data.
Month Day Time Tide orbits average (hours)
June 13 2:49:42 C
2,750,789s 18 42.45
13 22:56:11 C
4,747,719s 31 42.54
Aug 7 21:44:50 D
612,065s 4 42.50
Aug 14 23:45:55 D
764,718s 5 42.48
Aug 23 20:11:13 D
6,906,272s 45 42.63
Nov 9 17:35:45 D

By trial and error, during eight years of observations Rømer worked out how to account for the retardation of light when reckoning the ephemeris of Io. He calculated the delay as a proportion of the angle corresponding to a given Earth’s position with respect to Jupiter, Δt = 22·()[minutes]. When the angle α is 180° the delay becomes 22 minutes, which may be interpreted as the time necessary for the light to cross a distance equal to the diameter of the Earth’s orbit, H to E. (Actually, Jupiter is not visible from the conjunction point E.) That interpretation makes it possible to calculate the strict result of Rømer’s observations: The ratio of the speed of light to the speed with which Earth orbits the sun, which is the ratio of the duration of a year divided by pi as compared to the 22 minutes

≈ 7,600. 

In comparison the modern value is circa ≈ 10,100.

Rømer neither calculated this ratio, nor did he give a value for the speed of light. However, many others calculated a speed from his data, the first being Christiaan Huygens; after corresponding with Rømer and eliciting more data, Huygens deduced that light travelled Earth diameters per second.Huygens, Christian (8 January 1690) . Translated into English by Silvanus P. Thompson, Project Gutenberg etext, . Retrieved on 2007-04-29.

Rømer’s view that the velocity of light was finite was not fully accepted until measurements of the so-called aberration of light were made by James Bradley in 1727.

In 1809, again making use of observations of Io, but this time with the benefit of more than a century of increasingly precise observations, the astronomer Jean Baptiste Joseph Delambre reported the time for light to travel from the Sun to the Earth as 8 minutes and 12 seconds. Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometres per second. The modern value is 8 minutes and 19 seconds, and a speed of 299,792.458 km/s.

A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet.

Ole Romer Medal

The Ole Romer Medal is given annually by the Danish Natural Science Research Council for outstanding research.


In addition to inventing the first street lights in Copenhagen, Rømer also invented the meridian circle, the altazimuth and the Passage Instrument.