Roemer, Olaus (1644-1710), Danish astronomer, born in Århus, Jutland (now Denmark). After studying at the University of Copenhagen, Roemer spent nine years at the Royal Observatory in Paris, France. Roemer discovered that the eclipses of Jupiter's satellites seemed to take place at different times, depending on the time of year at which observations were made. He concluded that this discrepancy arose from the difference in the distance that the light from Jupiter had to travel when the earth was nearest to Jupiter and when it was farthest from it, six months later. This discovery of the finite velocity of light probably ranks second only to the law of gravity in scientific import. Roemer's inventions and discoveries were also of immense scientific value: he invented the transit instrument, the altazimuth (mounting for astronomical telescopes permitting both horizontal and vertical rotation), and the equatorial telescope. In 1681 Roemer was recalled to the University of Copenhagen as royal professor of mathematics and astronomy. He set up his inventions at his observatory on the outskirts of Copenhagen. A devastating fire in 1728 destroyed most of his records, yet his improved observation techniques and inventions advanced astronomy immensely.
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Rømer, Ole (Christensen)
b. Sept. 25, 1644, Århus, Jutland
d. Sept. 23, 1710, Copenhagen
Rømer also spelled RÖMER, OR ROEMER, OLE also spelled OLAUS OR OLAF, astronomer who demonstrated conclusively that light travels at a finite speed.
Rømer went to Paris in 1672, where he spent nine years working at the Royal Observatory. While there he noticed that the time elapsed between eclipses of Jupiter's moons by Jupiter became shorter as Earth moved closer to Jupiter and became longer as Earth and Jupiter drew farther apart. He correctly deduced that this phenomenon was caused by the time needed for light to cross the increased distance between the two planets and in 1676 announced that, according to his observations, the speed of light was 140,000 miles (225,000 km) per second. (Modern measurements have established a figure of 186,282 miles, or 299,792 km, per second.)
In 1679 Rømer went on a scientific mission to England, where he met Sir Isaac Newton and the astronomers John Flamsteed and Edmond Halley. Upon his return to Denmark in 1681, he was appointed royal mathematician and professor of astronomy at the University of Copenhagen. At the university observatory he set up an instrument with altitude and azimuth circles and a telescope, which accurately measured the position of celestial objects. He also held several public offices, including that of mayor of Copenhagen in 1705.
These small but extremely important telescopes play a vital role in mapping the celestial sphere. Without the transit instrument's very accurate determination of stellar and planetary positions, the larger deep-space telescopes would not be able to find their desired celestial object.
Astronomical transit instruments are usually refractors with apertures of 15 to 20 centimetres. (Ole Rømer, a Danish astronomer, is credited with having invented this type of telescope system.) The main optical axis of the instrument is aligned on a north-south line such that its motion is restricted to the plane of the meridian of the observer. The observer's meridian is a great circle on the celestial sphere that passes through the north and south points of the horizon as well as through the zenith of the observer. Restricting the telescope to motion only in the meridian provides an added degree of stability, but it requires the observer to wait for the celestial object to rotate across his meridian. The latter process is referred to as transiting the meridian, from which the name of the telescope is derived. There are various types of transit instruments, as, for example, the transit circle telescope, the vertical circle telescope, and the horizontal meridian circle telescope. The transit circle determines the right ascension of celestial objects, while the vertical circle measures only their declinations. Transit circles and horizontal meridian circles measure both right ascension and declination at the same time. The final output data of all transit instruments are included in star or planetary catalogs.