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to a test in the case of the moon, and to cal-
culate the sun’s effect, from its mass com-
pared with that of the earth, and from its
distance. He proved that the average ef-
fect upon the plane of the orbit would be
to cause the line in which it cuts the plane
of the ecliptic (i.e., the line of nodes) to re-
volve in the ecliptic once in about nineteen
years. This had been a known fact from the
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