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Building on the Work of Others
Building on the
Work of Others
During the 1500s, a Danish scientist named Tycho Brahe (Tooko Bra-ha) carefully observed the planets. For more than 25
years, Brahe studied how the planets moved through the sky.
Based on his observations, Brahe concluded that both the sun
and the moon orbit Earth. Of course, Brahe was wrong about
the sun orbiting Earth.
However, Brahe was correct about one thing. He concluded that
other planets in the solar system orbit the sun. This was an
amazing discovery when you consider that Brahe died before the
invention of the telescope. Even without a telescope, Brahe was
able to make observations that opened the universe to scientists.
One of these scientists was a German scientist named Johannes
Kepler. Kepler had been Brahe’s assistant. However, the two
men did not get along very well. Brahe was concerned that
Kepler might one day become more famous. So Brahe did not
share most of his observations with Kepler.
Tycho Brahe was the scientist who first
concluded that the planets orbit the
sun.
At that time, scientists thought that the orbits of the planets were
circles. Brahe’s observations on Mars did not fit with this idea.
Rather than struggle to make sense out of his observations,
Brahe decided to give this problem to Kepler to keep him
busy. This was one of the few pieces of information that
Brahe shared with Kepler. However, this was all Kepler
needed.
Like Brahe, Kepler also thought that the orbits of planets
were perfect circles. Yet, Brahe’s observations on Mars also
made no sense to Kepler. Rather than ignore them, Kepler
kept working on the problem.
Kepler discovered that Mars did not orbit the sun in a circle.
Rather, its orbit was more like an ellipse, or stretched out
circle. Kepler was the first to discover that the orbits of
planets are ellipses as they travel around the sun. He went on
to make more discoveries. Just as Brahe feared, Kepler did
become more famous.
Johannes Kepler discovered the elliptical
orbits of the planets.
Discovery Education Science
© 2007 Discovery Communications, LLC
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Building on the Work of Others
However, Kepler could not explain what holds the planets in
their orbits. An English scientist provided the answer.
His name was Isaac Newton. Newton showed that the same
force that controls the motion of objects on Earth also
controls the motion of objects in space. This force is gravity.
All the planets are moving through space. Some move faster
than others. Because of their motion, you would think that
planets would fly off into space in a straight line rather than
orbiting the sun. Newton realized that gravity is the reason
why planets do not fly off into space. Gravity is a force of
attraction. The sun has a gravitational pull on the planets.
The sun’s gravitational pull attracts planets to the sun.
So why don’t the planets fall into the sun? While gravity is
pulling the planets down, the planets are also moving
sideways. They move so fast sideways that they miss the sun.
If they moved slower, they might crash into the sun. If they
moved faster they would fly off into space. It is a delicate
balance!
Sir Isaac Newton found that the planets
revolve around the sun due to gravity.
Kepler explained what Brahe had first observed. Newton
explained what Kepler had first discovered. As you can see,
scientists often build upon the work of others.
Discovery Education Science
© 2007 Discovery Communications, LLC
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