PLUTO is the outermost known member of the planetary system, ninth in order of distance from the sun. It was discov ered by C. W. Tombaugh of the Lowell Observatory on January 23, 1930 by examination of photographic plates of the region of the sky near the star 6 Geminorum. The discovery was the result of a systematic search, both theoretical and observational, instituted by the late Percival Lowell because of his belief that the motion of Uranus gave evidence of the existence of a trans Neptunian planet. It is believed among those most conversant with this field of astronomy, however, that the finding of Pluto was a happy accident of the search and that the discordance be tween theoretical and observed motion of Uranus is evidence of inaccuracy of the older observations and incompleteness of theory for that planet rather than of gravitational attraction by the newly-discovered one.
Pluto moves around the sun in an ellipse with major axis 79 astronomical units in length and minor axis three per cent shorter, passing within less than 3o units of the sun at perihelion (as in 1989 for instance) and receding to an aphelion distance of almost 5o units during the century and a quarter following. The orbital
plane is inclined about 17 degrees to the plane of the ecliptic and is so oriented in space that the apparent path of the planet on the celestial sphere intersects the ecliptic near 6 Geminorum and near Sagittarii. The planet was near its northward crossing (or ascending node) when discovered and requires 248 years to make one complete circuit of its path. It should be noticed that it is only this inclination of the orbit of the outer planet which pre vents the orbits of Neptune and Pluto intersecting in space.
Pluto appears of the fifteenth magnitude on the astronomical scale, that is of the brilliance of an ordinary candle four hundred miles from the observer; it is half as bright photographically and must therefore have a surface of slightly yellowish cast. The evidence so far adduced suggests a diameter about one-half and a mass about one-tenth that of the earth. Solar radiation pro vides illumination on Pluto about equal to that produced by a 75-watt mazda lamp at a distance of one yard, but furnishes so little heat that the surface temperature is probably below Centigrade.