1. Solar Eclipses

2. Solar Eclipses
The Moon is almost exactly the same angular size as the Sun
The Sun is farther away and bigger than the Moon

3. Angular Size Versus Actual Size
The angular size of an object is how big across it looks l  ½ d
The bigger an object is, the larger its angular size
The more distant an object, the smaller its angular size
Angular size is always easy to measure
If you know the distance d, you can get the size l
If you know the size l, you can get the distance d

4. Lunar Eclipses Sun Earth Moon The Earth is round
The Moon is significantly smaller than the Earth
Sun
Earth
Moon

5. Eclipse Geometry

6. Why Aren’t There Eclipses Every Month?
The Moon’s orbit is tilted 5o compared to the Earth’s
It lines up properly only twice a year – eclipse season
Currently January and July
Tilt of orbit gradually changes over time – eclipse season drifts through the year
January 20-21, 2019, total Lunar Eclipse

7. The Motion of the Planets
As viewed from Earth:
The motion of the planets is much more complicated than the Sun or Moon
Combined motion of Earth and planets around the Sun
Like everything else, they rise in the East and set in the West
Rotation of the Earth
They move slowly against the background of the stars
Planets compared to the stars:
Compared to the stars, the planets usually move west to east (prograde), but sometimes east to west (retrograde)
Planets get brighter and dimmer
Planets compared to the Sun
The planets usually follow the ecliptic – roughly
Mercury and Venus never stray too far from the Sun

8. Copernicus - Early 16th century
History
Copernicus - Early 16th century
Inspired by Greek models (Aristarchus)
Sun Centered Models
His models were not really right, and no more accurate than older models
But they had conceptually right idea

9. Mercury and Venus Near Sun: Copernicus aV aE
Earth
Sun
Q. 5: What’s different about Mercury and Venus?

10. The Astronomical Unit 1 AU = Earth-Sun distance = 150,000,000 km
Copernicus (and later astronomers) didn’t know how far away the Earth was from the Sun
1 AU = Earth-Sun distance
= 150,000,000 km
We now know this distance
It is still used to measure the distance to the planets

11. Retrograde Motion - Copernicus

12. Tycho Brahe - Late 16th century
Proved the heavens changed
Best pre-telescope observations
Thought Earth was center of universe
Data was inherited/stolen by Kepler

13. Johannes Kepler - Early 17th Century
Sun-Centered models
Cause-based explanations of motion
Three Laws of Planetary Motion:
1. Ellipses, focus on the Sun
2. Equal areas in equal times
3. Period and size of orbit related by:
P2 = a3

14. Kepler’s First and Second Laws
1. Planets move on ellipses with one focus at the Sun a b d1 d2
2. A line joining the Sun and the planet sweeps out equal area in equal time

15. Kepler’s Third Law P2 = a3 Q. 6: Applying Kepler’s Third Law
3. The square of the period (in years) equals the cube of the semi-major axis (in AU).
P2 = a3
Far away planets move slower
Q. 6: Applying Kepler’s Third Law

16. Galileo Galilei - Early 17th Century
Used telescope to observe the heavens
Discovered:
Phases of Venus
Mountains and Craters on the Moon
Moons of Jupiter
Rings of Saturn
Argued strongly for heliocentric view
Challenged authority of the church

17. Galileo Galilei - Early 17th Century
Moons of Jupiter
Phases of Venus
Craters/Mountains on the Moon