In conformity with the Copernican view of our system, we must learn to
look upon the sun as the comparatively motionless centre about which
the earth performs an annual elliptic orbit of the dimensions and
excentricity, and with a velocity, regulated according to a certain
assigned law; the sun occupying one of the foci of the ellipse, and
from that station quietly disseminating on all sides its light and
heat; while the earth travelling round it, and presenting itself
differently to it at different times of the year and day, passes
through the varieties of day and night, summer and winter, which we
enjoy.--_Sir John Herschel’s Outlines of Astronomy._
Laplace has shown that the length of the day has not varied the
hundredth part of a second since the observations of Hipparchus, 2000
years ago.