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Title page of Kepler's New Astronomy
(Astronomia Nova). The book was completed
in and published
in Linz in 1609. Buried deep in this at times very mystical
book are Kepler's
first formulation of his first and second
Laws of planetary motion, which in modern parlance can be stated
as follows:
- Planetary orbits are ellipses, with the Sun located at one focus;
- Planets move along their elliptical orbit with a speed such that
a line segment joining the planet to the Sun sweeps equal surface areas
in equal time intervals
Another important novelty to be found in the New Astronomy
is Kepler's attempt to justify the above two Laws on physical
grounds, by ascribing planetary motions to a form of magnetically-mediated
action-at-distance. This insight is what led him to formulate
planetary orbits in relation to the Sun, as opposed to the center
of the Earth's orbit (or "mean sun"), and in turn
to his first two Laws.
This idea, interestingly, also
led him to postulate a movement of axial rotation for
the Sun, two years before
telescopic observations of sunspots
would establish solar rotation as an observational fact.
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Bibliography:
Kepler, J. 1609, Astronomia Nova, trans. W.H. Donahue 1992,
Cambridge University Press.
Stephenson, B. 1987, Kepler's physical astronomy, [1994 reprint,
Princeton University Press].
Kepler
Copernicus
Tycho
Newton
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