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Lecture

Kepler's laws, orbits, light


Department
Astronomy & Astrophysics
Course Code
AST201H1
Professor
Stefan Mochnacki

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LECTURE 4
KEPLERS 3 LAWS
1. planets move in elliptical orbits, the sun at one focus (there are foci)
2. as a planet moves around its orbit, it sweeps out equal areas in equal times
(time around perihelion= time around aphelion)
closest to sun: perihelion, further: aphelion
speed x radius= constant (mass stays constant anyway)
result of [orbital] conservation of angular momentum
(mass) x speed x radius= constant
thus, perihelion (closest) moves faster than the aphelion
3. further away planets orbit sun slower (average speed) than closer planets
P^2=A^3: Period= average distance from sun
Period: years, distance: AU
Relationship between orbital period and average orbital distance tells us the
total mass of the system
**first 2 laws apply to all orbiting objects not just planets
Universal law of gravitation
every mass attracts every other mass
attraction is directly proportional to the product of their masses
attraction is inversely proportional to the square of the distance between their
centres
Force= G(M1)(M2)/ (d^2)
Orbits
Bound (ellipses)
Unbound
Parabola
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