Central forces, therefore, at once constitute and
maintain the system.
Our Sun may be considered as at rest when compared to all the large and
small, dense and almost vaporous cosmical bodies tht appertain to and
revolve around it; but it actually rotates around the common center of
gravity of the whole system, which occasionally falls within itself, that is
to say, remains within the material circumference of the Sun, whatever
changes may be assumed by the position of the planets. A very different
phenomenon is that presented by the translatory motion of the Sun, that is,
the progressive motion of the center of gravity of the whole solar system in
universal space. Its velocity is such* that, according to Bessel, the
relative motion of the Sun, and that of 61 Cygni, is not less in one day
than 3,336,000 geographical miles.
[footnote] *Bessel, in Schum., 'Jahrb. f??r' 1839, s. 51; probably four
millions of miles daily, in a relative velocity of at the least 3,336,000
miles, or more than couble the velocity of revolution of the Earth in her
orbit round the Sun.
This change of the entire solar system would remain unknown to us, if the
admirable exactness of our astronomical instruments of measurement, and the
advancement recently made in the art of observing, did not cause our advance
toward remote stars to be perceptible, like an approximation to the objects
of a distant shore in apparent motion.
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