Transcript Planetary Motions - LathamWHS13-14

Planetary Motions
The motions of planets in space and the reasons behind them
Revolution
 Path of the planet around
the sun
 How long does this
take?
 360º / year = ~ 1º / day
 Technically 365.25 days
-- how do we account
for this?
 Path is an ellipse
 Perhelion:
closest (147
6
*10 km) on January 3
 Aphelion:
farthest (152
6
*10 km) on July 4
Rotation
 Why do we have day light and night time?
 What is the difference in Earth’s position relative to the
sun?
 The Earth spins around its axis!
 An aside on Earth’s axis……
 http://highered.mcgrawhill.com/sites/007299181x/student_view0/chapter2/sea
sons_interactive.html
 How long does one rotation take?
 How would we measure this……..?
Solar Days
 Solar day
 Measures time interval from noon on one day to noon on
the next day
 Noon occurs when the sun is at the highest point in the sky
 24 hours
Sidereal Day
 Time for Earth to make one complete rotation
w/ respect to a star OTHER THAN the sun
(same spot in sky)
 23 hours, 56 minutes, 4 seconds
 Direction relative to the star doesn’t significantly change
because it is farther away
 BUT the sun is closer, so our direction changes
~ 1º / day
 It takes 4 more minutes to compensate for this, so
solar day is 24 hours
Earth Wobbles
 Precession
 Like a spinning top
 http://www.youtube.com/watch?v=hwY8zS4zGsg
 Earth does the same thing.
 Axis varies from 21.5º to 24.5º every 41,000 yrs
 Direction of axis pointing changes every 13,000 years (cycle
is 26,000)
 Polaris vs. Vega
Varying Orbit
 Earth’s elliptical orbit becomes stretched and
condensed (more / less of an ellipse) over 100,000 to
400,000 years
 Why?
 Gravitational pull of other planets slightly pulls Earth
What Governs this?
 Newton’s Law of Universal Gravitation
M1M 2
F =G
r2
Kepler’s Laws of Planetary
Motion
 1st Law: “The path of each planet about the sun is an
ellipse with the sun at one focus”
 2nd Law: “Each planet moves so that an imaginary
line drawn from the sun to the planet sweeps out equal
areas in equal periods of time”
 3rd Law:
T1 2 r1 3
( ) =( )
T2
r2
In English, please…..
 Planets travel in an ellipse with the Sun at one
focus; the other is symmetrically located at the
opposite end of the ellipse
 Each planet revolves so that an imaginary line
connecting it to the Sun sweeps equal areas in
equal time intervals. This means it moves faster
when it is nearer to the Sun.
 The length of time it takes a planet to orbit the
Sun is related to the average distance from the
Sun so that if you know its orbital period you can
calculate its distance and vice versa.
Consequences
 What must Kepler’s 2nd law mean in terms of a
planet’s speed?
Data
 Earth’s closest distance: (147 *106km)
 Earth’s farthest distance: (152 *106 km)
 Design / perform an experiment that proves Kepler’s
2nd Law to be true.