3 Wandering StarsGreeks noticed 5 points of light that appeared to wander amongst the stars.They called them PlanetsGreek for “wandering star”
4 Greek ObservationsSaw 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
5 Greek Observations Ptolemy: Puzzled by retrograde motion He created the Geocentric ModelPlanets orbit the Earth and move in smaller circles.
6 Copernicus's Idea Early 1500’s Polish Astronomer Thought the Sun was at the center.Created the Heliocentric ModelHelios- Greek for SunThe Earth and all the other planets move around the sun.Included the six planets.
7 Galileo’s Observations Made two discoveries that supported the heliocentric model.Saw 4 moons revolving around JupiterShowed that not everything in the sky revolves around the Earth.Venus goes through phasesVenus could not go through phases if Earth was at the center.
9 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.
10 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.
11 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.
12 InertiaAn object’s tendency to move in a straight line or to remain stationary.The more mass, the more inertia it has.
13 Gravity The force that pulls objects towards one another. The strength depends on the masses and the distances between them.