Module 5—Circular Motion
Lesson Summary
In this lesson you focused on the solution to this question:
- How did Kepler’s laws help calculate the position of the planets as they orbited the Sun?
Kepler’s three laws describe the motion of the planets as they orbit the Sun.
Kepler's First Law
The orbit of a planet around the Sun is an ellipse, with the Sun located at one of the two focal points of the ellipse.
Kepler's Second Law
In equal time intervals, the radius vector from the Sun to a planet sweeps across equal areas.
Kepler's Third Law
For all planet orbits, the square of the period is proportional to the cube of the semi-major axis (the mean distance from the Sun or mean orbital radius). Expressed as an equation, it is
Lesson Glossary
astronomical unit: the average distance between Earth and the Sun
axis of symmetry: a line that divides a shape into two identical parts
calendar year: 365 days or 366 days (leap year)
elliptical: having the shape of an ellipse or oval
epicycle: a circle that rolls along the circumference of another circle
focal point: special points used in describing an ellipse
sidereal year: the orbital period of Earth