### Terms

**Astronomia Nova** - · The *Astronomia Nova,* or the *New
Astronomy* was Kepler's masterpiece. Published in 1609,
it was the result of over eight years of work. Kepler spent those
years trying to work out the shape of the orbit of Mars. Using
Tycho's data about the motion of the planets, Kepler was finally
able to determine the shape of the orbit more accurately than anyone
who had come before him. This resulted in the formation of his
first two laws, which were published in the *Astronomia
Nova.*

**Geocentric** - · A geocentric system is one in which the earth is at
the center of the universe. For thousands of years, scientists,
philosophers, and theologians believed that the universe was geocentric.
They were unwilling to believe Copernicus when he challenged that assumption.

**Harmonice Mundi** - · The *Harmonice Mundi,* or *Harmony
of the World* was the culmination of Kepler's life-long
study of the structure of the universe. Published in 1618, it described
a system in which the spacing between the planets was determined
by universal harmonies. The theory was wrong, but the book is nonetheless
important, as it marks the first appearance of Kepler's third law.

**Heliocentric ** - · A heliocentric system is one in which the sun is at
the center of the universe. The system that Copernicus introduced
was a heliocentric system. This was not a completely original idea
– some of the philosophers of ancient Greece had imagined that the
universe might be constructed in this way. However, the dominant
view had always been that the universe was geocentric, so Copernicus's
claims were a shock to the European system.

**
Kepler's Three Laws ** - · Kepler is best known today for his contribution of
the

three planetary
laws, which were instrumental in

Newton's later development
of his theory of universal gravitation. They are as follows: 1.
The planets travel around the sun in elliptical orbits with the
sun located at one focus. 2. As the planets travel around their
orbits, they sweep out the same amount of area per unit of time,
no matter where they are on the orbit. 3. The distance a planet's
orbit is from the sun, cubed, is directly proportional to the time
it takes the planet to travel around the orbit, squared. Mathematically,
this can be stated as

*a*3/

*p*2 =
K where "

*a*" is the distance a planet's orbit is
from the sun, "

*p*" is the period, the time it takes
for a planet to revolve around the sun once, and "K" is a constant.

**Mysterium Cosmographicum** - · Published in 1597, the *Mysterium Cosmographicum,* or *Mysteries
of the Cosmos,* was Kepler's first major work. It described
his theory of the perfect solids, which, although he never fully
admitted it, was completely wrong. More importantly, the *Mysterium* was
Kepler's first step to rejoining physics and astronomy, as he grasped
for physical explanation for the structure of the universe. He
was the first astronomer in centuries to do so. It is in the *Mysterium* that
Kepler first proposes that the sun be moved to the exact, physical
center of the universe, and that a force from the sun is responsible
for moving the planets around their orbits. The *Mysterium* was
also the major work in fifty years to support the Copernican system.

** Perfect solid** - · A perfect solid a three dimensional figure, such as
a cube, whose sides are all identical. There are only five perfect
solids: the tetrahedron (which has four triangular sides), cube
(six square sides), octahedron (eight triangular sides), dodecahedron (twelve
pentagonal sides), and icosahedron (twenty triangular sides). Each
perfect solid can be inscribed in and circumscribed around a sphere.
In the beginning of his career, Kepler believed that the planetary
orbits could all be inscribed in one of the perfect solids.