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stellar motion

proper motion, in astronomy, the apparent motion of a star across the celestial sphere at right angles to the observer’s line of sight; any radial motion (toward or away from the Sun) is not included. It is observed with respect to a framework of very distant background stars or galaxies. Proper motion is generally measured in seconds of arc per year; the largest known is that of Barnard’s star in the constellation Ophiuchus, about 10″ yearly. The English astronomer Edmond Halley, in 1718, was the first to detect proper motions—those of Arcturus and Sirius. The symbol for proper motion is the Greek letter μ (mu).

This article was most recently revised and updated by Erik Gregersen.

escape velocity, in astronomy and space exploration, the velocity needed for a body to escape from a gravitational centre of attraction without undergoing any further acceleration. The escape velocity vesc is expressed as vesc = Square root of2GM/r,where G is the gravitational constant, M is the mass of the attracting mass, and r is the distance from the centre of that mass. Escape velocity decreases with altitude and is equal to the square root of 2 (or about 1.414) times the velocity necessary to maintain a circular orbit at the same altitude. At Earth’s surface, if atmospheric resistance could be disregarded, escape velocity would be about 11.2 km (6.96 miles) per second. The velocity of escape from the less massive Moon is about 2.4 km (1.5 miles) per second at its surface. A planet or moon cannot long retain an atmosphere if its escape velocity is low enough to be near the average velocity of the gas molecules making up the atmosphere. Inside the event horizon of a black hole, the escape velocity exceeds the speed of light, so not even rays of light can escape into space.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.