Our View of the Universe How did early humans view their universe and create models to explain what he saw? (click here) How can we explain the real motions of celestial objects? (click here) How can we prove Earth rotates on its axis? (click here) How was the Heliocentric model improved? (click here)

1. First Model of the Universe Our original view of the universe is based on just looking up and making observations. Originally all thought everything revolved around Earth. A Greek scientist, Ptolemy (150 A.D.), designed a model which puts Earth at the center and all other celestial objects orbiting around it. This model is known as the Geocentric Model of the universe.

Ptolemy’s model had to explain the motion that were being observed Ptolemy’s model had to explain the motion that were being observed. Ptolemy put Earth at the center of the Universe and then put the known planets at that time (Mercury, Venus, and Mars) and the Moon and Sun orbiting around Earth in circular orbits. Because of the motions of the Planets, the model had smaller orbits called epicycles. The model was very complicated. The model was able to help predict where the planets would be but it was not very accurate.

Geocentric Model

The model was able to explain why Mercury and Venus were only seen during Sunrise and Sunset. It also explain the Retrograde Motion of Mars Retrograde Motion is the apparent motion that a planet seems to back up in its orbit.

As Mars orbited Earth it also moved in its epicycle which produce the retrograde motion.

Problems with Geocentric Model It is not that accurate and had troubled predicting where celestial objects would be. When outer planets were discovered and put into the model, many epicycles needed to be added to the planets orbits to show their motions. The model became extremely complicated. (click here)

2. Heliocentric Model Of Solar Systems Model created by Nicolaus Copernicus (1500 A.D.) It put the Sun as the center of a solar system with the planets including Earth orbiting around the Sun in circular orbits. It also placed the moons orbiting the planets. To prove it, the model had to explain why Mercury and Venus are only seen during Sunrise and Sunset, the retrograde motion seen of the planets, and Earth rotates on it’s axis.

Heliocentric Model

Mercury and Venus Why we only see Mercury and Venus around the time of Sunrise and Sunset is because Mercury and Venus are closer to the Sun than we are. We have to look towards the Sun to see these planets.

Retrograde Motion Of Outer Planets Retrograde Motion is the apparent motion that a planet seems to back up in its orbit. Ptolmey used epicylce to explain this motion. Copernicus’ model explains this motion, because Earth is passing or being passed by other planets in their orbits and this creates this apparent motion of the planets going backwards for a brief time.

Retrograde motion of Mars (click here)

3. Evidence of Earth Rotates on it’s Axis Coriolis Effect A free flying object will deflect due to the rotation of the Earth. Foucault Pendulum A free swinging pendulum will seem to rotate. Actually the Earth rotates under the pendulum.

4. Tweaking the Heliocentric model After Tyco Brahe passing, Kepler inherited Tyco’s work. From this Kepler was able to create 3 Laws to describe planetary motion. These laws are still used today. First law Second law Third Law Newton’s Law of Gravity

Kepler’s First Law All planets revolve around the sun in elliptical orbits. The Sun exists at one of the focal points of the ellipse. At the other focal point is empty space. Therefore the distance between the planet and the Sun changes as the planet revolves around the Sun.

Eccentricity of an ellipse Focal Points Distance between foci Length of major axis Length of major axis Eccentricity = 1 eccentricity (click here) At the end

Kepler’s Second Law Equal Area: As a planet revolves around the sun it will sweep out an area between two points equal to the area of another two points providing it occurs during the same time period. The planet will change velocity (speed) as it moves around the Sun. (click here) At the end The closer to the Sun the faster the planet will travel in it’s orbit.

Kepler’s Third Law States that the cube of the mean distance from a planet to the Sun is proportional to the square of the planets time to orbit the Sun. Example: Neptune Time of revolution is 164.8 earth years 164.82 = 27159.04 ³√27159.04=30.058 AU 30.058 AU x 149.6 million km = 4496.79 million km. The distance must be in Astronomical Units (AUs)1AU =149.6 million km Time must be in Earth years. (click here) at the end Actual distance of Neptune to Sun d3≈t2 Mean distance 4,496 million km or 30.05 AU

Newton’s Law of Gravity It is based on the mass of two objects, the universal gravity constant (G), and the square of the distance between the two objects. Gravity= Mass1 x Mass2 distance2 X G G = 6.6726x10-11 Increase either mass and you will increase the gravity If you change the distance it will change the gravity by its square. Meaning if you double your distance gravity will be 4 times less.