Ole Christensen Rømer

Danish astronomer

• Born: September 25, 1644; Aarhus (Århus), Denmark
• Died: September 19, 1710; Copenhagen, Denmark

Seventeenth-century Danish astronomer and mathematician Ole Christensen Rømer was the first scientist to demonstrate that light was not instantaneous and the first scientist to attempt to measure its speed. In addition to his work as a teacher, observatory builder, and instrument maker, he served two kings of Denmark in a number of important and influential posts.

Primary field: Astronomy

Specialties: Observational astronomy; theoretical astronomy

Early Life

Ole Christensen Rømer was born on September 25, 1644 in Aarhus (also known as Århus), Denmark, a large, ancient city and major port. He was the son of successful shipping merchant Christen Pedersen and his wife Anna Olufsdatter Storm Pedersen, the daughter of a local politician. Weary of being mistaken for other men in the area named Christen, Pedersen changed the family surname to Rømer to indicate his origin: Rømø, one of the Wadden Sea islands off the southwest coast of Denmark.

A bright and curious child, Rømer was educated through the equivalent of high school in Aarhus. At the age of eighteen, he was enrolled at the University of Copenhagen to study mathematics with a concentration on astronomy. He continued with his education following his father’s death in 1663.

At the university, Rømer took courses from brothers Thomas Bartholin, a mathematician and physician, and Rasmus Bartholin, professor of geometry and medicine. Rasmus observed and wrote about the great comet of 1664–65, visible for weeks, an object that was later blamed for the Great Plague of London (1665–66) and the Great Fire of London (1666). Rasmus, whom King Frederick III had selected to publish the papers of Danish astronomer Tycho Brahe, was much taken with Rømer’s keen mind, and in 1664 brought him into the Bartholin home to help with the task. The project consumed more than six years, during which time Rømer earned a bachelor’s degree at the university and fell in love with Rasmus’s daughter, Anne Marie.

Life’s Work

In 1671, while double-checking the calculations in Brahe’s manuscripts, Rømer and Bartholin traveled to Hven Island to visit the site of Brahe’s observatory, Uraniborg, which had been demolished in 1601. There they met and worked with Bartholin correspondent Jean-Felix Picard, a French priest and astronomer who was conducting his own research related to the late Dane’s observations.

Picard, impressed by Rømer’s enthusiasm and talent for astronomy, offered the young man a position at the brand-new Royal Observatory in Paris (opened in 1671). The opportunity was too good to pass up, so Rømer accepted. He was immediately inducted into the new French Academy of Sciences (founded by “the Sun King,” Louis XIV in 1666), and in 1672 he relocated to Paris.

In France, under the patronage of long-reigning Louis XIV, Rømer worked alongside Picard as an assistant to Italian-born astronomer Giovanni Cassini, designer and director of the Royal Observatory. Rømer was kept busy with a number of projects. He served as astronomy tutor to the king’s eldest son and heir apparent, eleven-year-old Louis (called the Dauphin, or dolphin, from an ancient coat-of-arms insignia). With Picard, Rømer worked to solve hydraulic problems associated with the renovation and expansion of the fountain systems in the garden of the Versailles palace. One of his major tasks, under the auspices of the French Academy of Sciences, was to conduct astronomical observations throughout the country. It was while making these observations in the mid-1670s that Rømer first measured the speed of light. After publishing his findings, Rømer came to the attention of King Christian V of Denmark, who reserved a professorship for Rømer.

Rømer did not leave France until 1681. Upon his arrival in Denmark, he was finally able to marry Anne Marie Bartholin. The same year, under the patronage of King Christian V, he was appointed professor of astronomy and mathematics at the University of Copenhagen. He was also named astronomer royal, in which capacity he became director of the Rundtarnen (Round Tower) in Copenhagen, one of the oldest state-supported observatories in Europe. Rømer refurbished the observatory and installed several new astronomical instruments at the facility, some of his own invention.

Christian V relied increasingly upon Rømer in technical matters. After Christian’s death in a hunting accident in 1699, his successor, Frederick IV, continued the astronomer’s sponsorship. Rømer, as a favored court official, served in a variety of advisory roles to both kings and was rewarded accordingly. He was named master of the mint, royal harbor surveyor, and inspector of naval architecture. He was also named the king’s authority on ballistics, and he was put in charge of a highway commission.

Rømer gained political power as well. In 1688, he was named a member of the Privy Council. Five years later, he served as a judicial magistrate. In 1694, he was appointed top tax assessor. By 1705, he had been elected as a senator and as mayor of Copenhagen, and in 1706 he held a post as head of the state council. A true dynamo as a public servant, Rømer was seen everywhere during Copenhagen’s rapid expansion in the late seventeenth and early eighteenth centuries. He plotted new paved streets and avenues, planned and installed city lighting (inventing an oil lamp in the process), improved sewage systems, and made provisions for more reliable water supplies and drainage. Rømer also served as a police prefect and fire marshal.

Despite all his civil duties, Rømer did not neglect his scientific work. He devised a number of new instruments and made improvements to other devices. In 1704, he built the Tusculum, or Tusculanum, observatory near Vridsløsemagle, his summer home, and filled it with the most accurate astronomical instruments of the day. Rømer continued to research particular problems—such as measuring the parallax of stars and the length of Earth’s orbit around the sun—until his death a week before his sixty-sixth birthday.

Impact

A multitalented scientist, Rømer delved into civil engineering, hydraulics, and cartography. He was also skilled at designing instruments related to his principal specialty, astronomy. Some of Rømer’s innovations still exist, while others have been lost.

Rømer’s solution to the problem of bringing water to the palace gardens of Versailles allows for the magnificent fountains to continue astonishing visitors to this day. As a Danish court official, he instituted the Gregorian calendar (introduced in 1582) on March 1, 1700, making Denmark one of the first non-Catholic countries to adopt the solar part of the new calendar (the lunar portion was not accepted until 1776). To simplify commercial trade, he reformed and standardized the Danish system of weights and measures, anticipating the metric system that would become commonplace in Europe in the early nineteenth century. Rømer created a new thermometer based on the fixed freezing and boiling points of water, which inspired his correspondent Daniel Gabriel Fahrenheit to devise the temperature scale that bears his name.

Models of some of Rømer’s astronomy-specific instruments can still be seen in museums throughout the world. Among these are a planetarium showing the planets orbiting the sun, and an eclipsarium to calculate the orbit and eclipses of the moon; both were first built in Paris in the late 1670s by royal clockmaker Isaac Thuret, who used Rømer’s designs. Another invention, the transit telescope, which measures angular distances from the zenith of stars passing the meridian, has long been adopted in astronomy. Rømer’s alt-azimuth mount, which rotates an instrument about vertical and horizontal axes, has been adapted for multiple modern purposes: telescopes, cameras, solar panels, and artillery.

The bulk of Rømer’s written work was destroyed in the Great Copenhagen Fire of 1728, which consumed much of the city and damaged the Round Tower observatory. Only a few pages of notes concerning his planetary observations were preserved. These papers assisted nineteenth-century German astronomer Johann Gottfried Galle in discovering the planet Neptune.

Bibliography

Christianson, John Robert. On Tycho’s Island: Tycho Brahe and his Assistants, 1570–1601. Cambridge: Cambridge UP, 2009. Print. Illustrated study of the work of the Danish astronomer and alchemist Tycho Brahe who inspired others, like his assistant Johannes Kepler and his compatriot Ole Rømer, to continue and expand upon his groundbreaking research.

Osborne, Richard. The Universe: Explained, Condensed, and Exploded. Edison, New Jersey: Chartwell, 2009. Print. Illustrated overview of the cosmos. Incorporates the history of observation and exploration while touching upon such astronomical concepts as the origin of the universe, the nature of time and the character of space.

Taton, Reni, and Curtis Wilson, eds. Planetary Astronomy from the Renaissance to the Rise of Astrophysics: Part A, Tycho Brahe to Newton. Cambridge: Cambridge UP, 2003. Print. Multivolume project that covers the entire scope of investigation of the solar system from earliest times to the present, with emphasis on individuals like Ole Rømer who helped advance the science of astronomy.