1. Bellwork
Who is credited with the revolutionary model of a HELIOCENTRIC solar system?
A. Aristotle
B. Ptolemy
C. Galileo
D. Copernicus
The planets loop backwards in their orbits.
TRUE
FALSE
During which months is Earth closest to the Sun?

2. The story so far….
Ancient Greeks had a GEOCENTRIC model of the solar system with Earth in center and Sun and planets in perfect circles.
Ptolemy add many epicycles to explain the looping of planets during retrograde motion.
Galileo used a telescope to see phases of Venus, Jupiter’s moons, rotating sunspots
Copernicus made a HELIOCENTRIC model of the solar system with the Sun in the center so the planets were NOT looping backwards, Earth’s faster orbit was ‘passing’ the slower outer planets.

3. Show the animation of the Sun’s diameter changing throughout the year.

4. Earth’s Distance From the Sun
Either the Sun is changing SIZE or the Earth is changing distance as it orbits the Sun.
Earth’s Orbit around the Sun is NOT a Perfect circle!
None of the planets orbit in perfect circles!!

5. Kepler’s Model of the Solar System

6. Tycho Brahe (1575)
After seeing the total eclipse on August 21, 1560, and became a master of careful astronomical OBSERVATIONS.
Lost part of his nose in a duel and replaced it with a metal one.
Made DETAILED naked eye observations of the motions of the planets.
In the course of 30 years, he had amassed the most accurate astronomical DATA of the day

7. Johannes Kepler’s (1600)
Kepler ‘inherited’ years of astronomical observation data after the sudden death of Tycho Brahe
Mathmatician - Kelper's mathematical skills were extraordinary.
He could not get Tycho's very careful observations to fit Copernicus’ model.
He constructed 3 Laws about planets orbits:
SHAPE SPEED TIME

8. 1st Law: Ellipses!
The SHAPE of the orbital paths of each planet is an ellipse (NOT a perfect circle)
(with the Sun not exactly in the center, but at one focus)
If the Sun was exactly in the center then the orbit would be a perfect circle, but the Sun appears to change size when the Earth is a little closer/farther in its orbit.

9. Eccentricity = (how flattened – not perfect circle the orbit)
= Distance between foci / major axis
Major axis

10. Eccentricity of outer planets, Pluto, comets orbits

11. Speed (appears to be constant or varying)
Object
Eccentricity:
Countdown Time
(same or varies)
Area swept out
Distance from Sun
((same or varies)
Speed (appears to be constant or varying)
Earth
Venus
Pluto
Comet

12. 2nd Law: SPEED of the planet
An imaginary line connecting the Sun to any planet sweeps out equal areas in equal time.
Area 1
90 days
Area 2
90 days
Planets vary in their orbital speed
and distance

13. The animation can be adjusted to show different elliptical orbits sweep out the same area in the same time.

14. If the orbit of the planet is NOT a perfect circle, but an ellipse,
Then is the SPEED (velocity) of the planet the same as it orbits?

16. Which planet has the shortest year (orbital period)?
Which has the longest? Why??
Mercury is shortest orbital period, Neptune is longest orbital period because it is farther distance from the Sun

17. P2 (years) = A3 (AU) Kepler’s 3rd Law:
a planet’s orbital TIME is proportional to its distance from the sun.
In other words……the farther away the planet, the longer its ‘year’
P2 (years) = A3 (AU)
1 Astronomical Unit = The Earth-Sun Distance (93 million miles)

18. P2 (years) = A3 (AU)
Object
P (year)
A (distance) P2 A3
Mars
1.88
1.52 =
Jupiter
11.9
5.20
a planet’s orbital TIME is always proportional to its distance.

19. Kepler’s Third Law Object a (AU) P (year) a3 P2 Mercury 0.387 0.241
0.058
Venus
0.723
0.615
0.378
Earth
1.00
Mars
1.52
1.88
3.51
3.53
Jupiter
5.20
11.9
141.
142.
Saturn
9.54
29.5
868.
870.
Uranus
19.2
84.0
7,080.
7,060.
Neptune
30.1
165.
27,300.
27,200.
Pluto
39.5
248.
61,600.
61,500. =

20. P2 (years) = A3 (AU) P2 = (5 AU)3 P2 = 125 P = 125
Use Kepler’s 3rd Law to find the orbital period (years) of a planet
If a planet is 5 AU’s distance from the Sun, HOW many years is it’s orbital period?
P2 (years) = A3 (AU)
P2 = (5 AU)3
Orbital period is years
P2 = 125 2
P = 125

21. P2 (years) = A3 (AU) (5)2 = AU3 25 = AU3 25 = AU
Use Kepler’s 3rd Law to find the distance of a planet
If a planet takes 5 years to orbit the Sun (orbital period),
What distance is it from the Sun?
P2 (years) = A3 (AU)
Distance = 2.92 AU’s
(5)2 = AU3
25 = AU3 3
25 = AU

22. Kepler’s Third Law Object a (AU) distance P (year) orbit a3 P2 Mercury
.387
.24
Venus
.72
.62
Earth
1.00
Mars
1.52
1.88
Jupiter
5.20
11.9
Saturn
9.54
29.5
Uranus
19.2
84.0
Neptune
30.1
165.
Pluto
39.5
248.