20 kroner DENMARK (2013) Scientists - Ole Rømer

in the coin centre head of Queen Margrethe II of Denmark facing right; along the top edge: MARGRETHE II DANMARKS DRONNING (Margrethe II Queen of Denmark); along the bottom edge year of issue: 2013

in the coin centre drawing by Ole Rømer showing a geometrical alignment of the Sun, Earth, Jupiter and Io in respect to each other, on the basis of which he calculated the speed of light; below a date: 1675; along the top edge: OLE RØMER • LYSETS HASTIGHED (speed of light); along the bottom edge face value: 20 KRONER

seven plain and seven reeded sections

7 X 2013

still circulating

Lis Nogel

Den Kongelige Mønt (The Royal Mint), Copenhagen (mint mark before DANMARKS on the obverse)

2013

300 000

Commemorative coin with face value of 20 kroner from 2013 commemorates world's first estimation of the speed of light by Ole Römer (1644-1710). This Danish astronomer after several years of observation of Jupiter's moon Io noted that despite his precise calculation of the orbit of Io, his calculations of the moment of Io's illumination by the Sun disagree with his observations. Rømer noticed that the error of observation time depended on the distance from Earth to Jupiter. During a lecture for the French Academy of Sciences in 1675 Rømer announced that this observation error results from a finite speed of light. He then challenged the existing theory of the infinite speed of light. He estimated that light from the Sun takes about 10 minutes to reach the Earth. Rømer was not able to calculate the speed of light, because the distance from Earth to the Sun was not known. His theory was published a year later, but confirmed only after 50 years. Today we know that light from the Sun reaches the Earth in average of 8 minutes and 19 seconds, and the light travels at speed of 299792.458 km/s.

Drawing by Ole Rømer from 1676 showing the Sun (A) and Earth in different movement phases (E,F,G,H,L,K) in respect to the moon Io (C,D) orbiting Jupiter (B).

The Danish series of thematic coins "Scientists" was launched to commemorate the centenary of the publication of the atomic model by Niels Bohr. Danes decided to show that Bohr was not the only Danish, who had part in the history of scientific progress. The thematic series includes:

	20 kroner - Scientists - Niels Bohr (2013)
	20 kroner - Scientists - Hans Christian Ørsted (2013)
	20 kroner - Scientists - Ole Rømer (2013)
	20 kroner - Scientists - Tycho Brahe (2013)