“Geocentric vs. Heliocentric Theory”

Claims and Evidence from the Ancient Astronomers

Cornell Notes pg. 61

Astronomy

•The study of celestial objects such as moons, planets, stars, and galaxies.

•One of the oldest branches of science.

Moving Back in time…

•Think back to over 2000 years ago. No high tech equipment for studying the far reaches of space… no idea of the law of gravity… scientists were stilling trying to develop basic explanations for the world.

Geocentric Theory

•CLAIM: Earth is the center of the universe–Sun, planets, and

stars all rotate around the earth.

•EVIDENCE: things appear to move around Earth; Earth doesn’t feel like its moving…

Basic Assumptions about the Universe…

•Everything outside of Earth is perfect–Planet shapes are

perfect spheres–Orbits of Planets

are perfect spheres

–Stars are fixed to an outer sphere

Aristotle 384-322 BC

•CLAIM: 5 elements each have a natural place in the universe- Earth at center (geocentric).

•EVIDENCE: earth sinks in water…gas bubbles rise, fire rises above air… etc.–Scientific teachings

were taught for over 1000 years, but never tested!

A Problem with the Geocentric Model…

•Retrograde Motion- planets temporarily appear to move backwards

Ptolemy 90-168 AD

•CLAIM: Celestial bodies revolve around the Earth (geocentric).

•EVIDENCE: created a model of space that could be used to predict future positions of planets AND explained retrograde motion

http://www.polaris.iastate.edu/EveningStar/Unit2/unit2_sub1.htm

Heliocentric Theory

•CLAIM: The sun is at the center with planets revolving around the sun.

•EVIDENCE: failure of Ptolemy’s model over time; retrograde motion can be explained simply if earth is moving, too.

http://www.polaris.iastate.edu/EveningStar/Unit2/unit2_sub2.htm

Copernicus1473-1543 AD

•Given credit as developing the Heliocentric Theory.

•Created a simpler, more accurate model that could be used to predict future locations of planets.–Other Claims: earth

is spinning on axis and revolving around the sun.

Brahe1546-1601 AD

•CLAIM: Moon’s orbit isn’t a perfect circle, it’s an ellipse.

•EVIDENCE: careful observations with diligent records.

– (Brahe actually supported the Geocentric Theory)

Kepler1571-1630 AD

•CLAIM: Planets do not orbit in perfect circles but instead orbit in ellipses.

•EVIDENCE: used records collected by Brahe

•This discovery was the last piece needed to perfect Copernicus’s model. (Still not widely accepted).

Galileo1564-1642 AD

•CLAIM: our moon has craters and the sun has spots (they’re not perfect!); and Jupiter has it’s own moons- they don’t orbit the earth!

•EVIDENCE: observations made through his carefully crafted telescope

Sir Isaac Newton1642-1727 AD

•Defined the Laws of Motion and the Law of Gravity.

•Now our solar system makes sense!