deck 4: kepler’s laws
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20 Terms
Kepler's Laws
Three laws that describe how planets move around the Sun.
Kepler's First Law
Planets move in elliptical orbits with the Sun at one focus.
What is an ellipse?
An oval-shaped orbit, not a perfect circle.
Where is the Sun located in an elliptical orbit?
At one focus, not the centre.
What shape are planetary orbits?
Ellipses.
Why does the distance between a planet and the Sun change?
Because the Sun is at one focus of the ellipse, not the centre.
Kepler's Second Law
Equal areas are swept out in equal times.
What does Kepler's Second Law mean?
A line joining the Sun and a planet sweeps out equal areas in equal amounts of time.
How does a planet move when it is closer to the Sun?
It moves faster.
How does a planet move when it is farther from the Sun?
It moves slower.
Why do planets move faster when closer to the Sun?
Gravity is stronger when they are closer to the Sun.
Kepler's Third Law
Larger orbits have longer orbital periods.
Orbital period
The time taken for a planet to complete one orbit around the Sun.
Relationship between orbital period and orbit size
Orbital period² ∝ orbit size³.
What happens to a planet's year as orbit size increases?
It becomes longer.
What happens to a planet's year as orbit size decreases?
It becomes shorter.
Mercury's orbital period
About 88 Earth days.
Neptune's orbital period
About 165 Earth years.
What is the key idea of Kepler's Third Law?
Planets farther from the Sun take longer to complete one orbit.
Summary of Kepler's Laws
1st: Elliptical orbits with the Sun at one focus. 2nd: Equal areas in equal times, so planets move faster when closer to the Sun and slower when farther away. 3rd: Larger orbits have longer orbital periods.