Observing the Solar System:A History

Chapter 26.2

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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.