Models of the Solar System

*Early Models of the Solar System

*Kepler’s Laws

Aristotle

Early Models of the Solar System

• Aristotle-Greek Philosopher– Geocentric Model: Earth-centered

model of the solar system– The sun, the stars, and the planets

revolve around the Earth– Retrograde Motion-Model could not

explain why some planets sometimes appeared to move backward in the sky relative to the stars.

Claudius Ptolemy

Early Models of the Solar System

• Claudius Ptolemy-Greek Astronomer–Thought planets moved in small

circles, called epicycles as they revolved in larger circles around Earth.

–Epicycles seemed to explain why planets sometimes appeared to move backward.

Nicolaus Copernicus

Early Models of the Solar System

– Nicolaus Copernicus-Polish Astronomer• Proposed model that was sun-centered• The planets revolved around the sun• The planets revolved in the same

direction as the sun, but at different speeds and distances from the sun

Copernicus’ Model and Ptolemy’s Model

Galileo Galilei

Early Models of the Universe• Galileo Galilei-Italian

Scientist–Discovered four moons traveling around Jupiter

–Observation showed him that objects can revolve around objects other than Earth.

Tycho Brahe- Danish Astronomer

Tycho Brahe

• Made detailed observations of the solar system

• After his death, his assistant, Johannes Kepler discovers a pattern and develops three laws to explain planetary motion.

Kepler’s Laws

Kepler’s First Law• Law of Ellipses

– Each planet orbits the sun in a path called an ellipse not a circle

– Eccentricity is the degree of elongation of an elliptical orbit

– Formula: divide the distance of the foci of the ellipse by the length of the major axis

– (e=0 for circular orbit, e=1 for an elongated orbit)

Kepler’s Second Law• Law of Equal

Areas• Speed of which

objects travel at different points in their obits. Whether slow or fast, the area covered is the same.

Kepler’s Third Law

• Law of Periods• Relationship

between average distance of a planet from the sun and the orbital period of the planet.

Kepler’s Third Law Cont’d

Mathematical Formula that describes this relationship:See page 693 in your book

Scientists can find out how far away the planets are from the sun using this law. They measure the orbital periods by observing the planets.

Isaac Newton

Newton’s Model of Orbits

• Inertia: Newton hypothesized that a moving body will stay in motion and resist change in speed or direction until an outside force acts upon it.

• Example: a ball rolling on a smooth surface will continue to move unless a force stops it or makes the ball change direction.

Newton• Named the force that causes the

planets orbital path to curve• Named it gravity• Mass and Distance determines

gravity’s force-the farther away an object is, the weaker the pull (gravitational force) by the larger object.