The History of Astronomy

brought to you by:

Mr. Youngberg

The Geocentric View

• All motion in the heavens is uniform circular motion.

• The objects in the heavens are made from perfect material, and cannot change their intrinsic properties (e.g., their brightness).

• The Earth is at the center of the Universe.

• Prograde motion: forward motion

• Retrograde motion: backward motion

Aristotle’s Theory384-322 BC

• Geocentric view dominated thinking for 1800 years.

• Earth is made up of only four elements: earth, water, air, and fire.

• The celestial bodies were perfect and divine, and made of a fifth element called Aether.

Aristotle’s Cosmological System

• Universe is made up of 55 celestial spheres.

• Each sphere rotates• Outside the spheres

is the prime mover that caused the rotation.

Claudius Ptolemy87 – 150 A.D.

• Geocentric view of the Universe

• Explained retrograde motion of the celestial bodies.

• Used over 80 epicycles to explain the motions of the Sun, the Moon, and the five planets known in his time

Ptolemaic System

• The planets’ move on large circles around the Earth- the deferent

• The planets travel on small circles called epicycles that move on the larger circles.

• Explained retrograde motion and brightness variation.

Nicolaus Copernicus1473-1543

• Heliocentric theory of Universe: Sun Centered

• Earth is not stationary• Earth turns on its axis

once a day!• Still used a few epicycles

to explain backward motion

The Copernican Universe

Retrograde Motion in the Copernican System

Tycho Brahe1546-1601

• Most precise observations with the best instruments available, prior to telescope

• Observations of planetary motion, lead to our current model of the solar system.

• Observations of a Super-nova in 1572. No parallax so therefore a star, and a change in the heavens!

• Observed a comet in 1577. Used parallax to prove distance. Conflict!

Johannes Kepler1571-1630

• Heliocentric View• Worked under Brahe and

used his observations to devise his three laws of planetary motion

• Planets orbit the Sun in ellipses with the Sun as one focus.

• The closer a planet is to the sun, the faster its speed.

Kepler’s 1st Law

I. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

                              

Kepler’s 2nd law

The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.

                                                            

Kepler’s 3rd LawThe ratio of the square of the period of revolution is

proportional to the cube of the semimajor axis of the ellipse.

Galileo Galilei1564-1642

• First person to successfully use a telescope

• The sun had dark patches, now called sunspots

• Four points of light (moons) orbit Jupiter

• Venus has phases• Observations paved the

way for Heliocentric

Phases of Venus

Isaac Newton1642-1727

• Three laws of motion described all motion whether on Earth or in the Heavens.

• Proved that Kepler’s three laws were special cases of Newton’s Laws

• Invented Calculus and the Newtonian telescope.

Gravity!

• Observed an apple accelerating toward the ground.

• Called this force gravity!• Extended this force from a

tree to the Moon. • Every object in the

Universe attracts every other objects (G)

• On Earth g = 9.8 m/s/s

Albert Einstein1879-1955

-Special Theory of Relativity: the speed of light- 300,000 km/s- is an important constant that cannot be exceeded.

-General Theory of Relativity: The presence of mass curves space.

Special Theory General Theory

Matter and Energy and Equivalent m=massc=speed of light

History Quiz

1. Which astronomers believed the Heliocentric view of the universe?

• Copernicus, Kepler, Galileo, Newton, Einstein

2. Which astronomers believed the Geocentric view of the universe?

• Aristotle, Ptolemy, Brahe

History Quiz

3. Which model believed that all heavenly bodies were perfect and had perfect motion?

Geocentric

4. Who introduced the elliptical orbit?

Kepler

5. Whose theories break down near the speed of light or near very massive bodies?

Newton

The EndReferences:• Astronomy 161 The Solar

System• Journey Through the Universe• And more…