1. Aim: How can we explain the laws that control the planets orbits?

2. Planet Orbits Are NOT circular
They are elliptical (a flattened circle)
The Sun at one ‘focus’ – not at the center
The other focus is an empty point in space

5. Eccentricity how flattened an ellipse is
Eccentricity = distance between foci (d)
length of major axis (L)

6. Example
d = 2 cm
L = 6 cm
e = d L
= 2 = 0.33 6

7. Example
d = 0 cm
A circle is an ellipse with the 2 foci on top of each other (e = 0)
L = 2 cm
e = d L
= 0 = 0 2

8. Example A line is an ellipse with d = L (e =1) e = d = 6 = 1 L 6
d = 6 cm
A line is an ellipse with d = L (e =1)
L = 6 cm
e = d L
= 6 = 1 6

9. Gravity
The force of attraction holding planets, etc. in the solar system
Depends on
masses of objects
distance between objects

10. Gravity and Position
the time it takes a planet to orbit the sun is related to its distance from the sun
As distance increases, gravitational attraction decreases

11. Gravitational Attraction
Distance

12. x Position X moving fastest Winter in N. Hemp. Position Y
moving slowest
Summer in N. Hemp. x Y

13. How fast do planets move around the sun?
Planets travel the fastest when they are closest to the sun.

14. In the Winter:
Earth moves fastest
Gravitational attraction is the strongest
In the Summer:
Earth moves the slowest
Gravitational attraction is the weakest