Johannes Kepler (1571
The 17th century German mathematician and
astronomer Johannes Kepler is best known for his three laws of planetary
motion. These served as a foundation for Isaac Newton's law of universal
gravitation first formulated later that century.
In the early 1600s, Kepler served as the imperial
mathematician under Holy Roman Emperor Rudolph II, filling a position left
vacant by another famous astronomer of the day, Tycho Brahe.
A supporter of the Copernican System (with the planets
orbiting the Sun and not Earth), Kepler took advantage of Brahe's extensive
observations of the planet Mars and tried to fit them to the Copernican model.
The changing position of the Mars in the night sky was best fit if Mars was
assumed to orbit the Sun following an elliptical orbit-not a circular orbit as
was the doctrine-with the Sun at one of the foci of the ellipse. Kepler assumed
that this also holds true for the rest of the planetary orbits:
1) All planets move
in ellipses, with the Sun at one focus.
Kepler also noticed that Mars moved the fastest on the side
of its orbit closer to the Sun (near perihelion) and slowest on the side
farther from the Sun (near aphelion). Expressed in terms of geometry:
2) Planets sweep out equal areas in equal times.
In 1610, Kepler corresponded with and endorsed the
observations of another brilliant astronomer of the day, Galileo Galilei.
Years later, while searching for harmony in the proportions
of the natural world, Kepler discovered the mathematical relation between the
planets' orbital period and their mean distance to the Sun:
3) The square of the
orbital period P (in years) equals
the cube of the mean distance to the Sun a
(in astronomical units), or P2
Kepler's 3rd law can be explained in terms the
inverse square law of universal gravitation-planets near the Sun feel a greater
pull and so orbit much faster than planets farther away.
The Kepler Mission
NASA named its Earth-like planet finder mission in honor of
Johannes Kepler. The Kepler Mission
is a space photometer that will observe the brightness of over 100,000 stars in
the Milky Way over 3.5 years to detect periodical transits (eclipses as seen by
the mission) of a star by its planets. It is expected to discover hundreds of
Earth-size and smaller planets in or near the habitable zone of those stars and
determine how many of the billions of stars in our galaxy have such planets.