ROEMER became the pupil and amanuensis of Erasmus Bartholinus at Copenhagen, who was impressed enough with his work to entrust to him the editing of Tycho Brahes manuscripts. He assisted J. Picard to determine the geographical position of Tycho Brahes observatory (Uraniborg on the island of Hveen. He accompanied Picard to Paris, where he remained nine years, occupied with observations at the new royal observatory and hydraulic works at Versailles and Manly. ). It is generally assumed that he was a member of the Academy. Louis XIV appointed him to tutor the Dauphin in astronomy and Roemer traveled around France making observations at the behest of the Academy.

He read a paper before the Academy on the successive propagation of light as revealed by a certain inequality in the motion of the first of Jupiters satellites. A scientific mission to England in made him acquainted with Newton, Halley and Flamsteed. On the summons of Christian V, king of Denmark, he returned to Copenhagen as royal mathematician and professor of astronomy in the university. In addition, he served in a number of advisory roles to the King, as master of the mint, harbor surveyor, inspector of naval architecture, ballistics expert, and head of a highway commission.

In Rømer’s position as royal mathematician, he introduced the first national system for weights and measures in Denmark. Initially based on the Rhine foot, a more accurate national standard was adopted in 1698. Later measurements of the standards fabricated for length and volume show an excellent degree of accuracy. His goal was to achieve a definition based on astronomical constants, using a pendulum. Notable is also his definition of the new Danish mile. It was 24,000 Danish feet, which corresponds to 4 minutes of arc latitude, thus making navigation easier. In Norway and Sweden, this 4 minute geographical mile was mainly used at sea, up to the beginning of the 20th century. In 1700, Rømer managed to get the king to introduce the Gregorian calendar in Denmark-Norway. Rømer also developed one of the first temperature scales. Fahrenheit visited him in 1708 and improved on the Rømer scale, the result being the familiar Fahrenheit temperature scale still in use today in a few countries. Rømer also established several schools for marine navigation in many Danish cities.

Roemer will always be remembered for his discovery of the finite velocity of light as well as for wonderful ingenuity in the improvement of astronomical apparatus. The first transit instrument worthy the name was in 1690 erected in his house. In the same year he set up in the university observatory an instrument with altitude and azimuth circles and an equatorial telescope. Aat his own cost he built the so-called "Tusculan" observatory at Vridlosemagle and equipped it with a meridian circle (the transit instrument and vertical circle combined) and a transit moving in the prime vertical. He held many important administrative functions, including those of mayor (1705), chief of police and privy councillor. He died at Copenhagen on the 23rd of September 1710.

Author: SABINA TAGORE IMMANUEL