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 Theory 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 Ptolemy 87 – 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 Copernicus 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 Brahe 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 No parallax so therefore a star, and a change in the heavens! Observed a comet in Used parallax to prove distance. Conflict!

Johannes Kepler 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 1 st Law I. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

Kepler’s 2 nd 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 3 rd Law The ratio of the square of the period of revolution is proportional to the cube of the semimajor axis of the ellipse.

Galileo Galilei 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 Newton 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 Einstein 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=mass c=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 End References: Astronomy 161 The Solar System Journey Through the Universe And more…