Johannes Kepler is considered one of the pioneers of astronomy. He came up with what are now known as Kepler's Three Laws of Planetary Motion. Kepler was born on December 27, 1571 in Weil der Stadt, Germany. He was one of seven children, three of which died at infancy. He was described as a very sickly child, and constantly suffered from one ailment or another. Kepler's grandfather was believed to be a nobleman and the Mayor of Weil. Kepler's father, however, became a mercenary and barely avoided being hanged. Kepler's mother, raised by an aunt who was later burned for being a witch, nearly met the same fate as well.
Kepler graduated from the University
of Tuebingen with a large focus on theology.
It was here that Kepler was introduced to Copernicus' heliocentric
model of the universe. This fascinated Kepler, but he believed Copernicus'
data was in error. Kepler made his own calculations and published his findings
in 1596. He was still, however, going on the assumption that the planets had
perfectly circular orbits.
After being pressured to leave the University of Graz because of his Lutheran faith, Kepler began working as Tycho Brahe's assistant until Brahe's death in 1601. Kepler used the data collected by Brahe to calculate the orbit of Mars
It was during these calculations
that Kepler came up with his first two laws. While investigating the orbits
of the other planets, Kepler came up with his third law. It was later determined
that Kepler's laws not only applied to the motion of the planets, but to that
of comets as well. His three laws were as follows:
1) Planets move in elliptical orbits with the sun at on of the focus points
2) Planets sweep out equal areas in equal times
3) Ta^2/Tb^2=Ra^3/Rb^3 So what does all that mean?
Kepler's first law states that
the planets do not move in perfect circles (despite what many people believed),
but rather ellipses. His second law is a tad more complicated.
We usually use Earth
as one of the planets because we know both its period
and its radius.
Earth's
period is one year and its
radius (or average radius since this is an ellipse)
is one astronomical unit. Kepler's
third law then simplifies to:
T^2=R^3
What this tells us is that if we square the period (measured in years) of a planet (which we can determine by its motion in the sky) and then take the cube root of that number, we will know the average distance from that planet to the sun (in astronomical units.)
