Ancient to Modern Ancient to Modern AstronomyAstronomy

Ptolemy 85 A.D. to Newton Ptolemy 85 A.D. to Newton 17271727

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Ptolemy (85 A.D.- 165 Ptolemy (85 A.D.- 165 A.D.)A.D.) PtolemyPtolemy modified the geocentric modified the geocentric

model to account for the model to account for the retrograderetrograde motion of planetsmotion of planets The "solution" to retrograde motion The "solution" to retrograde motion came in the form of a mad, but clever came in the form of a mad, but clever proposalproposal

Ptolemy (85 A.D.- 165 Ptolemy (85 A.D.- 165 A.D.)A.D.) ""EpicyclesEpicycles", and the concentric ", and the concentric

spheres to which they were attached spheres to which they were attached were termed the "were termed the "DeferentsDeferents”” Centers of the epicycles executed Centers of the epicycles executed uniform circular motion as they went uniform circular motion as they went around the deferent at uniform angular around the deferent at uniform angular velocity, and at the same time the velocity, and at the same time the epicyles (to which the planets were epicyles (to which the planets were attached) executed their own uniform attached) executed their own uniform circular motioncircular motion

Ptolemy (85 A.D.- 165 Ptolemy (85 A.D.- 165 A.D.)A.D.)

Claudius Ptolemaeus Claudius Ptolemaeus (Ptolemy) (85 A.D.- 165 A.D.)(Ptolemy) (85 A.D.- 165 A.D.)At first, not widely accepted At first, not widely accepted

because scientists thought the because scientists thought the model to be too complicatedmodel to be too complicated

Although the geocentric model Although the geocentric model with epicycles did explain the with epicycles did explain the motions of the planets better.motions of the planets better.

Important in terms of predicting Important in terms of predicting astrology and fate.astrology and fate.

Later refinements (2nd century B.C.) • Hipparchus: Placing the Earth away from the centers of the

“perfect spheres”

• Ptolemy: Further refinements, including epicycles

Epicycles

The Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

Introduced to explain retrograde (westward) motion of planets

Epicycles

(SLIDESHOW MODE ONLY)

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The Copernican Revolution

Nicolaus Copernicus (1473 – 1543):

Father of the Heliocentric Universe (Sun in the Center)

Nicolas Copernicus (1473-1543) • Polish cleric first proposed the heliocentric

model• Wrote the book De Revolutionibus and stashed

it under his mattress• Proposed all planets including Earth revolve

around the Sun• Circular orbits at constant velocities• Copernicus’ model was the best to date but still had problems accurately pinpointing the position of the planets

Copernicus’ new (and correct) explanation for retrograde motion of the planets

This made Ptolemy’s epicycles unnecessary.

Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

Explanation of Retrograde Motion

Explain the difference between the geo- and helio-centric

models of the solar system.

Earth-centered

Sun-centered

Galileo Galilei (1564 – 1642)• Invented the modern view of science: Transition from a faith-based “science” to an observation-based science. • Greatly improved on the newly invented telescope technology. (But Galileo did NOT invent the telescope!)

• Was the first to meticulously report telescope observations of the sky to support the Copernican Model of the Universe.

Major Discoveries of Galileo (2)• Surface structures on the moon; first estimates

of the height of mountains on the moon

Major Discoveries of Galileo

• Moons of Jupiter

(4 Galilean moons)

• Rings of Saturn

(What he really saw)

(What he really saw)

Major Discoveries of Galileo (3)

• Sun spots (proving that the sun is not perfect!)

Major Discoveries of Galileo (4)

• Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not the Earth!