ABSTRACT
Pluto, the outermost known planet of the Solar System, was discovered in 1930 by Clyde Tombaugh, from the Lowell Observatory at Flagstaff in Arizona. Pluto has a curiously eccentric orbit, which can bring it closer to the Sun than Neptune can ever be; the aphelion distance of Neptune is 4537 million km, the perihelion distance of Pluto 4446 million km–a difference of over 90 million km. Pluto can depart from the official Zodiac, and for the 1999–2005 period it lies in Ophiuchus. In 1978 Leif Andersson, of Sweden, pointed out that for a period occurring twice every Plutonian year–that is to say, every 124 Earth years–a situation arises when the orbits are positioned in a way which allows for mutual transits and occultations. The first orbit for Pluto issued from Flagstaff gave an eccentricity of 0.909 and a period of 3000 years, but a few months later new observations yielded a better orbit.
