Brahe, Tycho

Brahe, Tycho (1546-1601), Danish astronomer, who made precise, comprehensive astronomical measurements of the solar system and more than 700 stars. The data Brahe accumulated was superior to all other astronomical measurements made until the invention of the telescope in the early 17th century.

Brahe was born in Knudstrup in southern Sweden (then part of Denmark). He studied law and philosophy at the universities of Copenhagen and Leipzig; at night, however, Brahe busied himself with observing the stars. With no instruments other than a globe and a pair of compasses, he succeeded in detecting grave errors in the standard astronomical tables, and set about correcting them. In 1572 he discovered a supernova in the constellation Cassiopeia. After Brahe had spent some time traveling and lecturing, Frederick II, king of Denmark and Norway, offered to provide Brahe with funds to construct and equip an astronomical observatory on the island of Hven (now Ven). Brahe accepted the proposal, and in 1576 construction began on the castle of Uranienborg (“fortress of the heavens”), where for 20 years the astronomer pursued his observations.

Brahe never fully accepted the Copernican theory of the universe and sought a compromise by combining it with the old Ptolemaic system (see Copernican System; Ptolemaic System). In Brahe's system, the five known planets were supposed to revolve around the sun, which, with the planets, circled the earth each year. The sphere of the stars revolved around the immobile earth once a day.

Although Brahe's theory of planetary motion was flawed, the data he accumulated during his life played a crucial role in developing the correct description of planetary motion. Johannes Kepler, who was Brahe's assistant from 1600 until Brahe's death in 1601, used Brahe's data to help him formulate his three laws of planetary motion (see Kepler's Laws).