Celsius, the son of a mathematician, became professor of astronomy at the university in his native city of Uppsala, where he opened an observatory in 1740. In 1742 he devised a temperature scale in which the temperature of melting ice was taken as 100° and the temperature of boiling water was taken to be 0°. The modern Celsius (or centigrade) scale has the opposite fixed points (ice point 0°; steam point 100°C).