Celsius, Anders (1701–1744), Swedish astronomer. Celsius's major contribution was his thermometer scale. Although not the first thermometer with a 100-degree scale, his scale, described in 1742, used the freezing and boiling points of water as its two fixed points, with 0° for the boiling point and 100° for the freezing point. Five years later the system was reversed so that 0° represented the freezing point and 100° represented the boiling point and then it gradually gained acceptance. Since about 1800 Celsius has been used as an adjective to designate a thermometer based on the revised scale and more recently to indicate the scale itself.