Models of the Universe: Ancient Astronomy

• Models of the universe have evolved over time with contributions from various astronomers and mathematicians.

Ancient Astronomy

• As early as 500 B.C, Greeks believed that the Earth's shape was round, not flat.
• Pythagoras first proposed the spherical Earth.
• The true shape of the Earth is an oblate spheroid, bulging at the equator and squeezed at the poles.
• Anaxagoras supported Pythagoras' idea of a spherical Earth.
• Anaxagoras observed that the shadow Earth cast on the moon during a lunar eclipse was circular.

Eudoxus Of Cnidus

• Eudoxus of Cnidus, one of the best mathematicians and astronomers of the Plato era, devised a system of concentric spheres around 400 B.C.
• Eudoxus provided the first systematic explanation of the movements of the sun, moon, and planets.
• Eudoxus emphasized the ancient Greek's insistence on spherical perfection.

Aristotle

• Aristotle patterned his model after Eudoxus' model.
• Aristotle's model showed that the universe was spherical and finite.
• Like Eudoxus, Aristotle believed the Earth was stationary at the center of the universe.
• Aristotle believed Earth was too big to move and could not rotate.
• Aristotle believed that Earth consists of four elements: Earth, Water, Air, and Fire.
• The models of both Eudoxus and Aristotle were geocentric, placing Earth at the center of the universe.

Aristarchus

• Aristarchus (BCE-230 BCE) initially attempted to create a heliocentric model,placing the sun at the universe's center.
• Aristarchus proposed that the sun and fixed stars were at rest, while Earth revolved around the sun in a circular path.
• Aristarchus work, "On The Sizes And Distances Of The Sun And The Moon", describes the calculation sizes of the sun and moon using relative angles from Earth.
• Aristarchus had three assumptions, the Earth was spherical, further from the sun, and the moon passes through the Earth's shadows when they align.

Claudius Ptolemaeus (Ptolemy)

• Claudius Ptolemaeus (Ptolemy) assumed that Earth was at the center of the universe, and other celestial bodies revolved around it in perfect circles with constant velocity.
• Ptolemy's geocentric model, which included epicycles, could accurately explain the motion of celestial bodies.
• Ptolemy (100-AD-178 AD) assumed planets revolved on epicycles or small spheres which moved around the deferent (larger sphere).
• The stars belonged to the celestial sphere located beyond the planetary spheres.
• The center of the deferent is termed the eccentric.
• Ptolemy shifted Earth's position, placing it below the system's center but still inside the deferent.

Nicolaus Copernicus

• In the 16th century, Nicolaus Copernicus (1473-1543), a Polish astronomer, presented a heliocentric, sun-centered model that was published a year before death.
• Copernicus believed the sun was at the center of the universe.
• The order of the planets from the sun are Mercury, Venus, Earth (and moon), Mars, Venus, Saturn.
• Beyond the planets were the fixed stars.
• Heavenly bodies exhibited constant circular and perpetual motion along their epicycles.
• Copernicus strongly believed in the heliocentric model because there were loopholes in Ptolemaic Model in terms of predicting the positions of the planets.

Tycho Brahe

• Tycho Brahe (1546-1601), a Danish astronomer, made planetary observations.
• Unlike Copernicus, Brahe believed in a geocentric universe, but his idea was slightly different from Ptolemy's.
• In Brahe's Tychonic System, the Earth was at the center with the sun and moon revolving around it, while all other planets orbited the sun - a geoheliocentric system.

Explore the evolution of universe models, starting from ancient Greek ideas. Learn about Pythagoras and Anaxagoras who proposed a spherical Earth. Discover Eudoxus's concentric spheres system and Aristotle's adaptations.