Kepler’s Laws EARTH SCIENCE 

Early Astronomy The Birth of Modern Astronomy  Johannes Kepler • Kepler discovered three laws of planetary motion: 1. Orbits of the planets are elliptical. 2. Planets revolve around the sun at varying speed. 3. There is a proportional relationship between a planet’s orbital period and its distance to the sun.

22.1 Early Astronomy The Birth of Modern Astronomy  Johannes Kepler • An ellipse is an oval-shaped path. • An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers.

Planet Revolution Makes no sense without caption in book

The Solar System Model Evolves Makes no sense without caption in book

Gravity’s Influence on Orbits Makes no sense without caption in book

Note: The seasons are NOT caused by the distance from the sun.

Let’s see some ellipses

Not drawn to scale! The shape is more like a circle Perihelion Jan 3 93,000,000 miles Aphelion July 4 95,000,000 miles Not drawn to scale! The shape is more like a circle

Kepler’s Laws 1) All planets travel in elliptical orbits with the sun at one focus. Note: The Earth's orbit is so close to a perfect circle that it can't be distinguished with the human eye.

An ellipse: the sun as one foci

2) Each planet travels in such a way that a line joining the planet and the sun sweeps equal areas in equal times.

March April February May June January July August September December October November

3) Relationship between distance and period: P2 = D3 Where P is the Period in Earth Years And D is the distance in “AU’s” An “AU” is an astronomical unit, which is the average distance from the sun to the Earth.

Kepler’s Third Law In this equation P represents the period of revolution for a planet and R represents the length of its semi-major axis. Kepler's Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we find that Mercury, the innermost planet, takes only 88 days to orbit the Sun but the outermost object (Pluto) requires 248 years to do the same.

http://www.physics.sjsu.edu/tomley/Kepler12.html