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Kepler’s Laws
1. Definitions: Law, Theory, etc.
2. Johannes Kepler
3. Patterns of Orbits
4. Bonus: Tycho Brahe
Fact
• Data, measurements.– “This winter Boston got 108.6 inches of
snow.”
• Sometimes, obvious conclusions from data.– “Boston’s winter of 2014-2015 was the
snowiest winter on record.”
Law
• A simple relationship or formula that is found through observations or experiment, and describes what happens, and may or may not address why it happens.
• Laws can later be explained (and turn into a theory), or can remain unexplained and thus just be coincidences (sometimes called an empirical law).
Bode’s Law
Well, it’s also possible we just don’t
understand planet formation well enough yet…
Theory
• A hypothesis (reason why) that has been tested many times and is accepted by the scientific community.
Example Theories
• Gravity
• Big Bang
• Nuclear fusion and fission
• Germs cause disease
• Dinosaurs were killed by an asteroid
• Evolution
• Global warming / climate change
Kepler’s Laws don’t say WHY things happen, they only
describe WHAT happens.
Johannes Kepler
Germany1571-1630
http://en.wikipedia.org/wiki/Image:Johannes_Kepler_1610.jpg
Kepler = Heliocentric
Tycho Brahe’s student, used his data to support heliocentric
Kepler’s Laws
1. Ellipses
2. Equal Areas
3. P2=a3
1. Planets orbit the Sun in ellipses, with the Sun at
one focus
http://www.astro.virginia.edu/class/oconnell/astr121/im/ellipse-geom-NS.gif
http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/kep5.gif
Kepler’s Laws
1. Ellipses
2. Equal Areas
3. P2=a3
2. Planets’ orbits sweep out equal areas in equal times.
http://physics.uwyo.edu/~stark/outreach/Kepler/oldareaellipse.gif
http://upload.wikimedia.org/wikipedia/commons/7/75/Kepler2.gif
2. Planets’ orbits sweep out equal areas in equal times.
When one planet is closer to the Sun, it moves faster than when the same planet is farther from
the Sun.
Kepler’s Laws
1. Ellipses
2. Equal Areas
3. P2=a3
3. P2=a3
• Period - in Earth years
• Semi-major axis (average distance from the Sun) - in AU
• One planet close to the Sun orbits in a shorter time than a different planet farther from the Sun.
Kepler’s Law of Periods Multiple Versions
1. P2=a3 – around the Sun, years and AUs
2. P2=ka3 – any units, but k is different for each unit set and for each central object
3. (Below) – true for all units, all central objects
2 3
A A
B B
P a
P a
Newton’s Laws (including gravity) explain all of Kepler’s Laws.
Newton’s Theories explain Kepler’s Laws
1. Ellipses • Gravity (F=GMm/r2) makes objects move
in this sort of shape.
2. Equal Areas• Conservation of Angular Momentum,
which follows from Newton’s Laws.
3. P2=a3
• Gravity (F=GMm/r2) causes this too.
Kepler’s Laws
1. Ellipses
2. Equal Areas
3. P2=a3
Tycho BraheGermany
1546-1601
http://en.wikipedia.org/wiki/Image:Tycho_Brahe.JPG
Tycho Brahe = Geocentric
Best measurements of planets
Post-Test
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