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Name: Vicki Martin
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Question:
We are 5th graders at Munhall School in St. Charles. We are studying Astronomy and want to know how long is Pluto inside Neptune's orbit (in days)?



Replies:
According to the book "The Planet Pluto" by A. J. Whyte (1980), Neptune and Pluto were equidistant from the Sun on January 23, 1979; from that date until March 15, 1999, Pluto will be closer to the Sun than N. (I will leave it to you to compute how many days that is.) Note: Pluto's orbit does not actually intersect Neptune's. Pluto's orbit has a large eccentricity compared with the other planets (that is, most of the planets have orbits that are nearly circular; Pluto's orbit is notably elliptic), and the plane of its orbit is significantly tilted with respect to the plane of Neptune's orbit. A number of books I have consulted give 1999 or 1998 for the year that Pluto again becomes the ninth planet (in terms of distance from the Sun). Why the discrepancy? I suspect that this might be because of uncertainty about Pluto's orbit. Remember, Pluto was discovered only a little over 60 years ago; since its "year" is about 248 Earth years, we have been able to observe Pluto's motion through only about a quarter of one orbit.

RC Winther


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