1. Solar System Overview

2. Early Ideas
It was assumed that the Sun, planets, and stars orbited a stationary universe
This is known as a “geocentric” model, which means Earth centered
Problem with this model though: doesn’t explain all aspects of planetary motion
EX: normal direction for motion of planets is toward the East as observed from Earth; sometimes though a planet will appear to move the opposite direction across the sky in what is called retrograde motion

3. Example of retrograde motion
Mars retrograde motion
The search for an explanation for this retrograde motion motivated early astronomers to keep searching for a better model of our solar system

4. Heliocentric Model
1543 Polish scientist Nicolaus Copernicus suggested Sun was center of solar system
First time a sun-centered or “heliocentric” model was proposed
In a heliocentric model, the inner planets move faster in their orbits than the outer planets do; as Earth bypasses a slower moving outer planet it appears the outer planet temporarily moves backward in the sky

5. Retrograde Motion in Heliocentric Model
Heliocentric retrograde motion

6. Kepler’s First Law
Ideas of Copernicus not originally accepted by scientific community, but within a century other astronomers found evidence to support the heliocentric model
Using accurate data of planetary observations, Kepler demonstrated each planet orbits the Sun in a shape called an ellipse instead of a circle
This is Kepler’s First Law

7. Kepler
The orbits of planets are not circles but oval-shaped curves called ellipses!

8. Kepler
Ellipse is oval shape centered on two points instead of a single point, like a circle
The two points are called the foci (singular focus)
Each planet’s ellipse is a different shape and size, and the Sun is always at one focus
The major axis is the line that runs through both foci and is the maximum diameter of the ellipse
Half the length of a major axis is called the semi-major axis

9. Kepler
The semi-major axis is the average distance between the Sun and the planet
For the Sun and the Earth, it is x 108 km, or 1 astronomical unit (AU)
Average distance between Sun and each planet are measured in astronomical units

10. Kepler’s Laws
A planet in a elliptical orbit is not at a constant distance from the Sun
When planet is at closest distance to the Sun in its orbit, it is at perihelion
When a planet is at farthest distance away from the Sun in its orbit, it is at aphelion
The shape of a planet’s elliptical orbit is defined by eccentricity (ratio of distance between foci to the length of the major axis)

11. Kepler’s Laws Eccentricity values range from 0-1 0 is a perfect circle
Nearly 1 is a very elongated oval
1 is equal to a parabola
Most planets have orbits not very eccentric and are close to being circles
Orbital period – time required for a planet or body to travel a complete elliptical orbit around the Sun

12. Kepler’s Laws
Planetary Motion

13. Gravity and Orbits
Newton realized any two bodies attract each other with a force that depends on their masses and the distance between the two bodies
This relationship is called the law of universal gravitation
The greater the distance between two bodies, the less the force between them is
The smaller the distance between two bodies, the greater the force between them is

14. Barycenter
Newton also determined each planet orbits a point between it and the Sun called the barycenter
The barycenter is the balance point between two orbiting bodies (where all the mass of an object is concentrated)
This is similar to the pivot point on a see-saw
If one of two bodies orbiting each other is more massive than the other, the center of mass is closer to the more massive body
If two bodies are similar in mass, their center of mass is near the middle position between them
For any planet and the Sun, the center of mass is just above the surface of the Sun (or within the Sun) because the Sun is more massive than any planet

15. Barycenter Barycenter is closer to the end of the broom
The Earth does not revolve around the Sun,
but rather the barycenter!