History of Astronomy

Ancient Cultures to Isaac Newton

Early Civilizations

• A practical approach– Predicting seasons– Navigation

Aristotle and the Greek View: Geocentric Model

• Geo- Earth• Centric- Centered• The earth is a motionless sphere at the center

of the Universe. • Explained the apparent motion of the sun,

moon and stars.

Planets

• Greek astronomers observed that certain celestial objects do not follow a predictable path like the moon, sun, and stars.– Called these objects Planets (Greek word planetes

means wanderer)– The observable motion of Planets is that they

change speed and even loop back and forth relative to the stars.

Retrograde Motion

• Motion of planets in “backwards” or westward loops is known as Retrograde Motion.

Ptolemy (140 A.D.)

• A more complex model of the Universe/Solar System was needed to explain Retrograde Motion.

• Ptolemy suggests that planets orbit the Earth in a large circular orbits but also follow a small circular orbit around an imaginary point.

• These small orbits were known as Epicycles

The Copernican Revolution: Heliocentric Model

• Ptolemaic Model survived for 13 centuries• Copernicus suggested that the Earth is a planet and

spins on its axis and orbits the Sun.• Heliocentric- Sun Centered– This accounts for the apparent motion of the sun and

stars.• The realization that Earth is not at the center of the

universe is now know as the Copernican Revolution.– What about Retrograde motion of the planets?

Just like when you pass another car on the highway.

Galileo Galilei

• Used the telescope to view objects in the sky (1609).

• Observations of celestial objects supported the Heliocentric Model of the solar system.

Galileo’s Observations

• The terrain of the Moon, Sunspots, the moons of Jupiter

• Phases of Venus

Johannes Kepler

• A student of Tycho Brahe, Kepler used the precise data of his mentor to develop three simple laws that describe the motion of planets.

Kepler’s Laws of Planetary Motion

1. The orbital paths of planets are elliptical with the Sun at one focus.

2. An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time.

3. The square of a planet’s orbital period is proportional to the cube of its semi-major axis.

• Kepler’s Laws certainly describe the motion of planets around the Sun but they tell us nothing about WHY planets orbit the Sun.

• What prevents the planets from flying off into space or from falling into the Sun?

Isaac Newton

• Described the basic laws of motion in what is now known as Newtonian mechanics. – Newton’s 3 Laws of

Motion. – The Law of Universal

Gravitation

Newton’s Laws

1. Inertia: An object in motion stays in motion, an object at rest remains at rest, unless acted upon by a force.

2. F = ma3. To every action there is an equal and

opposite reaction.

Universal Gravitation

• Every particle of matter in the universe attracts every other particle with a a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.

Gravity: Why we orbit the Sun.