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500 BCE
Spherical Earth
Pythagoras suggested that earth is spherical -
300 BCE
Aristarchus and the distance to the sun and moon
Aristarchus calculated the relative distance between the Earth and the Sun by measuring the angle between the Moon and the Sun during a half moon and using trigonometry -
240 BCE
Eratosthenes and the circumference of the Earth
Eratosthenes was the first person to measure the tilt of the earth and its circumference, by using the summer solstice. -
150
Ptolemy and epicycles
Ptolemy developed Aristotle’s geocentric theory of the universe, and suggested that planets like Mars move in circles as they orbit the Earth, where the circles are called epicycles. -
400
Aristotle's geocentric model of the universe
Aristotle believed the universe is finite in space but exists eternally in time. -
1543
Copernicus and a heliocentric universe
Nicolaus Copernicus reintroduced the idea of a heliocentric universe. Copernicus believed that the planets only travel in perfect circles, and so his heliocentric model needed a similar amount of epicycles to explain their observed motions.. -
1572
Tycho and a changing universe
Tycho disproved Aristotle’s concept of an unchanging universe when he saw a new star in the constellation of Cassiopeia in 1572, which turned out to be a supernova. Tycho also proved that Aristotle’s transparent spheres do not exist by showing that comets would have to travel through them.[12] -
Galileo and the telescope
Galileo created one of the first microscopes and was one of the first people to use the telescope as an astronomical instrument. He became the first person to observe lots of phenomena, including the moons of Jupiter, the phases of Venus, sunspots, and the depth of the craters on the Moon, which he illustrated by showing the change in shadows across a day. -
Kepler’s laws of planetary motion
Tycho’s student, German astronomer Johannes Kepler, first extended Aristotle’s theory of spheres by arguing that they are separated by five polyhedrons. By 1619, Kepler was able to determine the relationship between a planet’s average distance from the Sun and the time it takes to complete one orbit, its period. -
Bradley and a moving Earth
James Bradley attempted to prove that the Earth moves using a phenomenon known as stellar parallax. Stellar parallax occurs because the stars appear to move as we view them from slightly different positions throughout the year. -
The size of the universe
The size of the known universe doubled in 1781 when the British astronomer William Herschel discovered the planet Uranus. The size of the universe increased by a factor of over 8000 in 1838, shortly after German astronomer Friedrich Bessel first measured stellar parallax