Rømer was employed by the French government: King Louis XIV made him teacher for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.
In 1681, he returned to Denmark and was appointed professor of Astronomy at Copenhagen University. He was active also as an observer, both at the University Observatory at the Rundetårn and in his home, using improved instruments of his own construction. Unfortunately, his observations have not survived: they were lost in the great fire of Copenhagen in 1728. However, a former assistant (and later an astronomer in his own right), Peder Horrebow, loyally described and wrote about Rømer's observations.
King Olaf was sailing home after an expedition to Wendland (Pomerania), when he was ambushed by an alliance of Svein Forkbeard, King of Denmark, Olaf Eiríksson, King of Sweden, and Eirik Hákonarson, Jarl of Lade. Olaf had only 11 warships in the battle against a fleet of at least 70. His ships were cleared one by one, last of all the Long Serpent, which Jarl Eirik captured as Olaf threw himself into the sea. After the battle, Norway was ruled by the Jarls of Lade as a fief of Denmark and Sweden.
The most detailed sources on the battle, the kings' sagas, were written approximately two centuries after it took place. Historically unreliable, they offer an extended literary account describing the battle and the events leading up to it in vivid detail. The sagas ascribe the causes of the battle to Olaf Tryggvason's ill-fated marriage proposal to Sigrid the Haughty and his problematic marriage to Thyri, sister of Svein Forkbeard. As the battle starts Olaf is shown dismissing the Danish and Swedish fleets with ethnic insults and bravado while admitting that Eirik Hákonarson and his men are dangerous because "they are Norwegians like us". The best known episode in the battle is the breaking of Einarr Þambarskelfir's bow, which heralds Olaf's defeat.
With a population of 89,071 (1 January 2010), the Kolding municipality is the seventh largest in Denmark. The city itself has a population of 57,197 (1 January 2011) and is also the seventh largest city in Denmark.
Kolding is well known as the location of the former royal castle of Koldinghus which was built in the 13th century. The castle is now a museum and tourist attraction.