dipper@utastro.UUCP (Debbie Byrd) (01/21/86)
This is the anniversary of the day that Neptune became the outermost known planet. We'll tell you about it -- when we come back. January 21 The Planets Change Places On today's date in the year 1979, the ninth planet from the sun, Pluto, became the eighth planet out. And the former eighth planet, Neptune, took over as the sun's outermost known planet. This strange state-of-affairs stems from the peculiar shape of Pluto's orbit. None of the planets orbit in perfect circles -- but Pluto has the least circular orbit of all the planets. When Pluto draws in toward its perihelion -- the closest point to the sun in its 250-year orbit -- it gets closer to the sun than Neptune. Pluto will reach perihelion in 1989. At that time, it'll be only 2.75 billion miles from the sun -- as compared to Neptune's closest distance of about 2.77 billion miles. When Pluto is farthest from the sun, it's more than four billion miles away. All of this makes now a better-than-average time for astronomers to observe and study Pluto. This fascinating frozen world is tiny -- and has a moon nearly half its own size. These two worlds are gravitationally locked face to face in a sort of celestial staring contest. They're now eclipsing as seen from our vantagepoint, and the eclipses will continue every few days for the next several years. What's more, Pluto has frozen methane on its surface, which may sublime into gaseous methane when the planet is closest to the sun -- giving Pluto a thin methane atmosphere. It'll be interesting to see what else we learn about Pluto in this decade -- as the planet edges ever-nearer to perihelion, the closest point in its orbit to the sun. Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin