Ancient astronomy and the Copernican revolution
People have studied the sky for thousands of years. The shift from an Earth-centred to a Sun-centred universe changed how humans saw their place in the cosmos.
Ancient observers
Ancient cultures tracked the sky for calendars, agriculture, navigation, and religion. Stonehenge in England aligns with solstice sunrise and sunset. Babylonians recorded planetary positions and predicted eclipses. Maya astronomers tracked Venus precisely. Polynesian navigators used stars to cross the Pacific.
Greek astronomy
Greek thinkers developed geometric models. Aristotle argued Earth was spherical, noting its curved shadow on the Moon during lunar eclipses. Around 240 BCE, Eratosthenes estimated Earth's circumference by comparing the Sun's angle in two Egyptian cities, getting a value close to the true 40,000 km. Aristarchus even proposed a Sun-centred model, but it was not widely accepted.
The geocentric model
Ptolemy, around 150 CE, refined an Earth-centred (geocentric) model in which the Sun, Moon, and planets moved on circles called epicycles, which themselves moved on larger circles. It predicted planetary positions reasonably well and was used for about 1,400 years, especially through translations preserved and improved by Islamic astronomers.
Copernicus
In 1543, Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres, placing the Sun at the centre (heliocentric model), with Earth as one of several planets orbiting it. His model explained retrograde motion naturally and gave the planets' order and relative distances, though it still used circular orbits and was not more accurate at first.
Tycho and Kepler
Tycho Brahe made the most precise naked-eye observations of planetary positions in the late 1500s. His assistant Johannes Kepler used Tycho's data on Mars to discover that planets move in ellipses, not circles, publishing his first laws in 1609. Kepler's laws made the heliocentric model far more accurate.
Galileo's evidence
From 1609-1610, Galileo Galilei turned a telescope to the sky. He saw mountains on the Moon, four moons orbiting Jupiter (showing not everything orbits Earth), countless stars in the Milky Way, and the full set of phases of Venus, which made sense only if Venus orbits the Sun. His support for Copernicus led to his trial by the Inquisition in 1633.
The scientific method in astronomy
The Copernican revolution shows science at work: competing models were judged by how well they explained and predicted observations. Simpler, more accurate explanations won out, even against strong tradition. Today, astronomy still advances by testing models against new data.
Worked example
Eratosthenes knew that at noon on the summer solstice the Sun was directly overhead at Syene but about 7.2 degrees from vertical at Alexandria, some 800 km north. Since 7.2 degrees is one-fiftieth of a full circle, Earth's circumference must be about 50 x 800 = 40,000 km, remarkably close to the modern value.
Key takeaways
Ancient cultures used the sky for calendars, navigation, and ritual. Greeks showed Earth is round; Eratosthenes measured its size. Ptolemy's geocentric model with epicycles lasted about 1,400 years. Copernicus proposed a heliocentric model in 1543. Tycho's precise data let Kepler discover elliptical orbits. Galileo's telescope revealed Jupiter's moons and Venus's phases, supporting Copernicus. Models win by explaining and predicting observations.
Flashcard
Aligned with solstice sunrise and sunset
Check your understanding
Which of Galileo's observations most directly showed that Venus orbits the Sun?