Changing Our View of the Cosmos
The history of astronomy is a story of progressively understanding our place in the universe.
The Geocentric Model (Ptolemy)
For over 1500 years, the dominant model of the universe was the geocentric model, formalized by the Greek astronomer Ptolemy.
Core Idea: The Earth is stationary and at the center of the universe.
The Sun, Moon, and planets all revolve around the Earth in perfect circles.
To explain the observed retrograde motion (the apparent backward looping of planets like Mars), Ptolemy's model required a complex system of epicycles—small circles upon which the planets moved, which in turn moved along larger circles (deferents) around the Earth.
The Heliocentric Revolution (Copernicus)
In 1543, Nicolaus Copernicus published his work proposing a heliocentric model.
Core Idea: The Sun is at the center of the universe, and the Earth is just one of the planets that revolve around it.
This model provided a much simpler and more elegant explanation for retrograde motion: it is an illusion caused by the Earth, in its faster inner orbit, overtaking a slower outer planet like Mars.
However, Copernicus still insisted on perfect circular orbits, which meant his model didn't perfectly predict planetary positions.
The Bridge: Tycho and Kepler
Tycho Brahe was a Danish astronomer who, before the invention of the telescope, made the most accurate and comprehensive naked-eye observations of planetary positions ever recorded. He did not fully accept the Copernican model but his data was invaluable.
Johannes Kepler, Tycho's assistant, inherited this data. After years of painstaking calculation, Kepler realized that planetary orbits were not circles, but ellipses. He developed his three Laws of Planetary Motion, which accurately described the orbits and finally made the heliocentric model mathematically sound.
The Confirmation: Galileo
Galileo Galilei was the first to use a telescope to systematically study the heavens, and his discoveries provided powerful evidence against the geocentric model.
Moons of Jupiter: He discovered four moons orbiting Jupiter. This showed that not everything in the universe orbited the Earth.
Phases of Venus: He observed that Venus goes through a full set of phases, just like our Moon. This is impossible in the Ptolemaic model but is a natural consequence of the heliocentric model.
Sunspots and Lunar Craters: His observations of imperfections on the Sun and Moon shattered the ancient idea that the heavens were perfect and unchanging.