Ancient Greek Models of the Universe

• The Greek's view of motion and the universe are explored
• The models of the universe differ in terms of their center and how heavenly bodies orbit around it

Thales

• He was the first known person to use natural explanations for natural phenomena instead of supernatural ones
• Thales believed that everything comes out of water, and that Earth floats on water

Anaximander

• Refined the ideas of Thales
• Proposed a cylindrical Earth at the center of the universe, surrounded by air, and by one or more spherical shells with holes

Anaximenes

• Suggested that the stars were fixed onto a solid transparent crystalline sphere that rotated about the Earth

Anaxagoras

• Stated that the moon shines by reflected sunlight, has mountains, and is inhabited
• Stated that the sun was not a god but a large fiery stone much larger than Greece and a large distance from Earth

Empedocles`

• Mentioned that light travels fast, but not at infinite speed

Democritus

• Proposed that the Milky Way was composed of thousands of unresolved stars

Pythagoras

• One of the first to use mathematics to challenge and support observations of the stars
• Believed in a well-ordered harmonious universe based on geometry rather than experiments

Plato

• Viewed the universe as perfect and unchanging
• Reasoned that the most perfect orbit of a planet would be circular, with constant motion like the stars
• Described the universe as a large spherical ball with stars at the edge and Earth in the center

Heavenly Objects

• Change their position in the sky over a period of time

Diurnal Motion

• The apparent daily motion of the sky from east to west, where celestial objects rise and set

Annual Motion

• Carries the Sun eastward in the sky over the course of an entire year, bringing in new constellations

Wandering Stars and Planets

• They move against the backdrop of the celestial sphere

Retrograde Motion of Planets

• Planets change direction, generally moving from west to east

Circular Motion

• The motion of heavenly objects must be circular motion at a constant speed, remaining the same

Hipparchus

• Noted that the position of the stars were shifting in a systematic way, indicating that it was not the stars moving, but the Earth

Precession

• Motion of Earth consisting of a cyclic wobbling in the Earth's axis of rotation, over a period of 25,772 years

Equinoxes

• The ecliptic and the celestial equator intersect at two points

Aristotle

• Believed motion had properties and rest is the natural state, as most things are not moving

Celestial Motion

• Known as an unchanging, endless circular motion of heavenly objects in a sphere

Terrestrial Motion

• Refers to the movement of matter, classified as alternation (alternate motion)

Models of The Universe

• Differences lie in their center and how heavenly bodies orbit

Eudoxus (408-355 BC)

• Homocentric model used three concentric spheres arranged such that a planet attached revolves around a common center, creating retrograde motion

Aristotle (384-322 BC)

• Geocentric Model with a spherical Earth at the center of the universe where the Sun, Moon, and planets revolve around it.

Aristarchus (240 BC)

• Heliocentric model where the Sun is at the center, much bigger than the Earth
• All the planets orbit the sun, and the Moon orbits the Earth, which spins on its axis

Ptolemy (AD 140)

• Epicycle and deferent with an eccentric model of planetary orbits
• All celestial objects, including planets, the sun, the moon and stars, orbited in epicycles around a stationary Earth at the center

Nicolaus Copernicus (1433-1542)

• Heliocentric model with the Sun is at the center, and all the other heavenly bodies revolve around it in circular paths

Tycho Brahe (1546-1601)

• Earth at the center, with the Sun and Moon orbiting it
• The other planets are orbiting the sun in this system

Mural Gradient

• Brahe used the mural gradient and observed with the naked eye and recorded the positions of hundreds of stars and the motion of planets over decades

Galileo Galilei (1564-1642)

• First to use a telescope to study heavens

Galileo Galilei's observations

• The heliocentric model was correct and the Ptolemaic model was incorrect

Galileo Galilei

• The sun had dark patches on it that are now called sunspots.

Galileo Galilei

• The surface of the moon was rough.

Galileo Galilei

• There are the brightest moons orbit around jupiter which are now commonly called Galilean Moons

Galileo Galilei

• Venus showed complete phase, consisten with the Copernican System

Galileo Galilei

• Supernova of 1604 was observed and this disproves that new stars could not appear in heavens

Johannes Kepler (1571-1630)

• Used Tycho Brahe's data to build his heliocentric model
• Developed three laws of planetary motion

Kepler's First Law

• The orbits of planets are ellipses with the sun at one focus

Kepler's Second Law

• The line joining a planet to the sun sweeps out equal areas during equal time intervals
• Known as the law of equal areas

Kepler's Third Law

• The squares of the periods of planets are proportional to the cubes of their average distance from the sun

Summary of Greek's motion

• Motion can be either celestial or terrestrial
• Clock calendar consists of diurnal and annual motion
• Other characteristics include wandering stars, perfect circular orbits, Earth as the center, and a spherical Earth

Summary of Kepler's law

• The planets move in elliptical orbits
• The planets move faster when closer to the Sun, and slower when further away
• A planet's orbital period increases faster than the radius of its orbit