Observing the Solar System: A History Chapter 26.2 P577-580

Galileo’s Observations: Copernicus’s Idea Inertia: Greek Observations: Wandering Stars: Brahe: Kepler: Newton: Gravity:

Wandering Stars Greeks noticed 5 points of light that appeared to wander amongst the stars. They called them Planets Greek for “wandering star”

Greek Observations Saw that these objects appeared to move and they thought that the Earth remained stationary. They believed they were inside a rotating dome- called the celestial sphere. Aristotle believed that the universe was perfect and finite and that the Earth was stationary at the center. That theory is known as: Geocentric (Earth Centered) Model

Greek Observations Ptolemy: Puzzled by retrograde motion He created the Geocentric Model Planets orbit the Earth and move in smaller circles.

Copernicus's Idea Early 1500’s Polish Astronomer Thought the Sun was at the center. Created the Heliocentric Model Helios- Greek for Sun The Earth and all the other planets move around the sun. Included the six planets.

Galileo’s Observations Made two discoveries that supported the heliocentric model. Saw 4 moons revolving around Jupiter Showed that not everything in the sky revolves around the Earth. Venus goes through phases Venus could not go through phases if Earth was at the center.

Heliocentric Model

Tycho Brahe A Danish Astronomer Made very accurate observations of the positions of the planets for 20 years. He died before he could analyze his data.

Keppler 1600’s A German mathematician Analyzed Brahe’s data. Kepler discovered that the orbits of each planet was an ellipse. An elongated circle or oval shape. His calculations fit Brahe’s observations.

Issac Newton An English scientist. Provided the answer to the question of what keeps the planets in orbit. Concluded that gravity and inertia combine to keep the planets in orbit.

Inertia An object’s tendency to move in a straight line or to remain stationary. The more mass, the more inertia it has.

Gravity The force that pulls objects towards one another. The strength depends on the masses and the distances between them.