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History of Astronomy
People have been looking up at the sky trying to figure it out for as long
as we have been around. Even earliest man noticed that things were
happening up in the sky and that certain events were happening in
Over the last 3000 years our view of what is happening to the stars and
planets has changed many times.
These first astronomers followed a Geocentric belief that Earth was the
center of the universe.
Pythagoras (475 B.C.) Geocentric: he was the first person to take all of
the most popular ideas of what is happing in space and put them into a
set of brief rules. This is called the Pythagorean Paradigm (means,
common sense).
1. Planets, Sun, Moon, and Stars move in perfectly circular orbits.
2. The speed of the planets, sun, moon, and stars is constant and
does not change
3. Earth is at the center of the universe
Aristotle (322 B.C.) Geocentric: He thought that each of the planets,
moon, and the sun were stuck on perfect crystalline spheres.
He also thought that all of the planets, stars and the sun were “perfect”
and unchanging.
Aristotle noticed that throughout the year the planets seemed to move
backwards in their movement and as they did this they got brighter.
This is called retrograde motion.
Ptolemy (165 A.D.) Geocentric
Ptolemy explained retrograde motion with epicycles. Epicycles
are small circles stuck to the larger circles, called deferents that
represent the orbit.
Ptolemy’s epicycles worked to explain
retrograde motion, he even built in a
wobble to explain why the deferent
was not always a constant distance
One problem was that if the planets
were on crystalline spheres that the
epicycle would crash into the sphere!
But, despite that huge flaw that he could not explain his theory stood
for almost 1500 years. And many epicycles were added to make the
model work
History of Astronomy 2
These next scientists created their theories nearly 1500 years after
Ptolemy. The big difference was that they believed in a Heliocentric
universe where the Sun is the center of the universe.
Nicolaus Copernicus (1543 A.D.) Heliocentric: he found that Ptolemy’s
model was too complicated and was not elegant.
He was the first to create an acceptable model where the sun was the
center of the solar system. And he put all of the planets in the correct
order up to Saturn.
Even though he had the sun as the center of the solar system he still
believed that the planets traveled in perfect circles and so he still had
to use small epicycles to explain the planets motion.
He used trigonometry to calculate the distances from each planet to
the sun relative to earth. This gave us the ASTRONOMICAL UNIT or AU.
Which is how many times a planet is farther from the sun as compared
to Earth.
He also discovered that retrograde motion is just an optical illusion,
caused by the projected position of a planet on the background of stars,
as earth passes it.
Galileo Galilei (1642 A.D.) Heliocentric: He was the first person
recorded to use a telescope to observe the planets, sun, or moon.
He discovered that in fact the planets are not perfect, the moon had
huge craters, the sun had spots and Jupiter even had its own moons!
Johanness Kepler (1630 A.D.) Heliocentric: tried to refine Copernicus’
model and after years of failure discovered that the problem is that
planets DO NOT ORBIT IN CIRCLES. Resulted in the creation of three
laws about planetary motion
Kepler’s 1st law: planets do not orbit in circles they orbit in ellipses
A circular orbit revolves around one point, called a focus.
An ellipse has two focal points or foci
As the foci get farther apart the ellipse gets more eccentric (flatter).
Eccentricity is the ratio of the distance between the foci and the length
of the major axis. A perfect circle has an eccentricity of 0 and a flat
ellipse has an eccentricity of 1.
Kepler’s 2nd law: That planets move around the sun at changing speed.
He also found that if you draw a line from the planet to the sun, that it
will sweep across an equal area in an equal amount of time.
Kepler’s 3rd law: the amount of time it takes for a planet to orbit the
sun is related to how far away from the sun it is.
The equation is p2=r3, where “p” is the period, amount of time it takes
for a planet to orbit. And “r” is the average distance from the sun.