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Pluto Becoming the Most Distant Planet From the Sun
E. M. Standish
January 28, 1999
At about 10:08 UT on February 11, 1999, Pluto will no longer be closer
to the Sun than Neptune. There have been many different numbers
quoted in the media for this date and time, mostly because of
different choices of the following two considerations:
- which set of planetary positions ("ephemeris")
is used, and
- which distance is being calculated:
- the distance from the Sun to the center of mass of just the planet
itself, or
- the distance from the Sun to the barycenter
of the full system of the planet (center of mass of planet plus satellites).
Table 1 (below) gives the times using the different choices.
The often-quoted date of February 9, 1999 results from using JPL's
20-year old planetary ephemeris DE200, the standard,
(which is still used in the national almanacs) and from using simply the
barycenters of Neptune and Pluto.
However, if one uses the most recent planetary ephemeris,
DE405, and if one is able to calculate the position of the
planets themselves (which requires using both the planet and the satellite
ephemerides), then the calculated time is
10:08 February 11, 1999 UT.
One can see the uncertainties in these times by comparing similar calculations
done using different ephemerides.
Presumably, the newer the ephemeris, the
more accurate the positions. As such, the change from DE403 to DE405 is
attributed mostly to errors in the DE403 positions. If so, then the
errors coming from DE405 should be no more than about an hour.
The errors contributed by inaccuracies of the
satellite ephemerides are
expected to be only a few minutes.
The table also shows when, in the 2200's, Pluto again becomes closer to the Sun
than Neptune.
Table I.
The times (UT) during which Pluto is closer to the Sun than Neptune
DE 200
1979 FEB 08 05:09 - 1999 FEB 09 11:45 Planet Centers
1979 FEB 08 05:53 - 1999 FEB 09 11:30 System Barycenters
DE 403
1979 FEB 07 06:30 - 1999 FEB 11 07:41 Planet Centers
1979 FEB 07 06:36 - 1999 FEB 11 07:10 System Barycenters
DE 405
1979 FEB 07 05:39 - 1999 FEB 11 10:08 Planet Centers
1979 FEB 07 05:44 - 1999 FEB 11 09:39 System Barycenters
2231 APR 05 13 - 2245 OCT 31 17 Planet Centers
2231 APR 05 13 - 2245 OCT 31 17 System Barycenters
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Questions regarding this page should be addressed to the author,
E. M. Standish
(E.M.Standish@jpl.nasa.gov)
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