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Seth Barnes Nicholson was an American astronomer.
Nicholson was born in Springfield, Illinois and was raised in rural Illinois. He was educated at Drake University where he became interested in astronomy.
In 1914, at the University of California's Lick Observatory, while observing the recently discovered Jupiter moon Pasiphaë, he discovered a new one: Sinope, whose orbit he computed for his Ph.D. thesis in 1915.
He spent his entire career at Mount Wilson Observatory, where he discovered three more Jovian moons: Lysithea and Carme in 1938 and Ananke in 1951, as well as a Trojan asteroid 1647 Menelaus, and computed orbits of several comets and of Pluto.
Sinope, Lysithea, Carme and Ananke were simply designated as "Jupiter IX", "Jupiter X", "Jupiter XI" and "Jupiter XII". They were not given their present names until 1975. Nicholson himself declined to propose names.
At Mt. Wilson, his main assignment concerned solar activity and he produced for decades annual reports on sunspot activity. He also made a number of eclipse expeditions to measure the brightness and temperature of the Sun's corona.
In the early 1920s, he and Edison Pettit made the first systematic infrared observations of celestial objects. They used a vacuum thermocouple to measure the infrared radiation and thus the temperature of the Moon which led to the theory that the Moon was covered with a thin layer of dust acting as an insulator, and also of the planets, sunspots and stars. Their temperatures measurements of nearby giant stars led to some of the first determinations of stellar diameters.
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