Ancient Cosmological Beliefs

Ancient Cosmological Beliefs

• Ancient people had diverse beliefs and myths regarding the creation and appearance of the world and universe.

Mesopotamian Cosmology

• Mesopotamians believed in a flat, disc-shaped Earth surrounded by a vast, cosmic ocean.
• They also believed that the gods created the world.

Ancient Egyptian Cosmology

• Ancient Egyptians believed the world was created by the god Atum through self-generation.
• They envisioned Earth as a flat, rectangular structure surrounded by water, with the sky as goddess Nut.

Ancient Greek Cosmology

• Ancient Greeks developed myths and philosophical ideas about the cosmos.
• Hesiod's "Theogony" describes the universe's creation through primordial gods like Chaos, Gaia, and Uranus.
• Plato and Aristotle introduced the concept of a spherical Earth.

Geocentric Model

• The geocentric model places Earth at the center of the universe.
• Homer and Hesiod (8th century BC) proposed a flat or cylindrical Earth within a hemispherical cosmos.
• By Pythagoras' time (560-480 BC), the idea of a spherical Earth gained acceptance.
  • This was based on the theoretical perfection of circles/spheres & practical observations like ships disappearing hull first over the horizon.

Plato's View (427-347 BC)

• Plato believed the universe consists of five types of matter: earth, air, fire, water, and cosmos.
• Each element corresponded to a specific platonic shape, with earth represented by a cube.

Eudoxus of Knidos (395-390 BC)

• Eudoxus was a polymath with contributions to geography, metaphysics, and ethics.
• He presented a general, geometrical model of celestial motion.
• Eudoxus' model had five principles:
  • Earth is at the universe's center.
  • Celestial motion is circular and regular.
  • The center of any celestial motion's path is the same as its motion center.
  • The center of all celestial motion is the universe's center.

Claudius Ptolemais (Ptolemy - c. 150 AD)

• Ptolemy elaborated on Hipparchus' geocentric system.
• He added features to improve the model's ability to match observations.

Aristotle (384-322 BC)

• Aristotle explicitly stated Earth is the center and does not move.
• He thought the sun, moon, planets, and stars circled Earth.

Aristotle's Model

• Aristotle envisioned the heavens as 55 concentric, crystalline spheres.
• Celestial objects were attached to these spheres, rotating at different velocities with a constant angular velocity.
• Earth was at the center of these spheres.

Aristotle's Cosmos Views

• He observed the daily rising and setting of the sun.
• He noted lack of sensation of Earth's movement.
• He saw stars tracing semicircles each night

Geocentric Theory Elements

• Key components include:
  • Earth is the unmoving center of the cosmos (geocentrism).
  • The sun revolves around Earth ("heliodynamism").
  • Celestial motions are circular.

Geocentric Ideas

• Earth is still and at the center of the universe.
• Celestial bodies move in perfect circles at uniform speeds.
• Stars are fixed in a rotating sphere, moving east to west daily.
• Planets, the moon, and the sun move slower in separate spheres.

Problems & Solutions

• Retrograde motion of planets: Planets appeared to reverse direction.
• Varying brightness of planets: Explained by changing distances from Earth.
• Unequal solar seasons: A challenge to perfectly circular motion.
• The sun appeared to speed up and slow down in its orbit.

Heliocentric Model

• This model places the Sun at the center of the universe.

Aristarchus of Samos (310-230 BC)

• Aristarchus proposed a heliocentric system. He based this on the large size he calculated for the Sun.

Heliocentric Model Challenges

• The heliocentric view was not widely accepted.
• It was considered radical due to the prevailing belief in uniform circular motion and Earth's immobility.

Galileo Galilei

• Galileo was a key figure in developing modern astronomy.
• He proved the Copernican Hypothesis.

Galileo’s Astronomical Discoveries

• Galileo discovered:
  • Jupiter's four major moons.
  • The phases of Venue
  • Changes in apparent sizes of Venus and Mars.
  • Mountains on the moon.
  • Sunspots.
  • The small apparent sizes of stars.

Copernican Model

• The Earth-centered model of Aristotle and Ptolemy dominated for almost 2000 years.
• Nicolaus Copernicus (1473-1543) proposed a new idea in the 16th century.

The Heliocentric System

• Copernicus suggested the Sun, not Earth is at the solar system's center.
• This is known as a heliocentric system.

Copernican Revolution

• The Earth being the center of the Universe, was one assumption that was challenged.
• Another was the assumption of uniform circular motion in the heavens.
• Also, that objects in the heavens were of perfect, unchanging substance, was challenged.

Tycho Brahe

• Brahe developed precise instruments before the telescope's invention.
• He also attempted to merge the Copernican model with Ptolemaic benefits using his Tychonic System.

Brahe's Contributions

• He contrived the best pre-telescope instruments for observation.
• His planetary motion observations provided vital data for later models.
• Brahe observed a supernova in 1572 and a comet in 1577.
• He made the best measurements in the search for stellar parallax.
• Brahe proposed a hybrid Solar System model, with Earth at the center.

Johannes Kepler

• Kepler adopted Copernicus' heliocentric view and Tycho Brahe's data.
• He then developed three laws explaining the motion of planets around the Sun.

Kepler's Laws

• Law of Orbits: Planets move in elliptical orbits with the Sun at one focus.
• The planets move in elliptical orbits. Elliptical orbits have the sun as one of their foci
• Law of Areas: A line from a planet to the Sun sweeps out equal areas in equal times.
• Law of Periods: The square of a planet's orbital period is proportional to the cube of its semi-major axis