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rise in the east without being seen if it
was always the same sun. It was a great
step to suppose the earth to be spherical,
and to ascribe the diurnal motions to its
rotation. Probably the greatest step ever
made in astronomical theory was the plac-
ing of the sun, moon, and planets at differ-
ent distances from the earth instead of hav-
ing them stuck on the vault of heaven. It
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