| definition |
The empirical rule relating the approximate distances of the solar system planets
from the Sun. The original formulation was: a = (n + 4) / 10, where a is the mean
distance of a planet from the Sun in astronomical units and n = 0, 3, 6, 12, 24, 48,
96, 192 (doubling for each successive planet). The planets were seen to fit this sequence
quite well, provided the asteroids between Mars and Jupiter are counted as one planet,
as did Uranus discovered in 1781. However, Neptune and the ex-planet Pluto do not
conform to the rule. The question of whether there is any physical significance to
the 'law,' i.e. some dynamical reason that will explain planetary orbit spacing has
led to much discussion during the past two centuries. Today, many astronomers are
very skeptical and consider this 'law' to be numerical coincidence.
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