Transcript History of Astronomy pt.2 - Jessamine County Schools

History of Astronomy
History of Astronomy pt.1
• 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 cycles.
• 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).
• Planets, Sun, Moon, and Stars move in perfectly
circular orbits.
• The speed of the planets, sun, moon, and stars is
constant and does not change
• 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 away.
• 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 pt.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 (ovals).
• A circular orbit revolves around one point,
called a focus.
• An ellipse has two focal points or foci
Kepler’s 1st Law
• 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.