MEDIEVAL

Instruments and observing methods were restricted to positional measurements of celestial bodies, and this did not change through the Middle Ages. The view of the universe that days was the geocentric system established by Greek astronomer Ptolemy around 120 AD: A sphere with fixed stars on it rotates daily around the spherically shaped Earth, with Sun, Moon, and planets being guided around Earth by a complicated machinery of epicycles; many had even forgotten about the Earth's spherical shape.

The events that brought astronomy to the state of modern science were (a) the introduction of the heliocentric system, and (b) the invention of the telescope around 1600.

The Heliocentric System
While already considered by ancient Greek Aristarchus around 300 BC, the heliocentric system was finally established in 1543 by Nicolaus Copernicus (1473-1543) from Poland when his book, De Revolutionibus ("On Revolutions") appeared. This model considered the Sun and no more the Earth to be the center of planetary motions, and the apparent annual motion of the Sun as an illusional effect caused by this motion, while the diurnal rotation of the stellar sky is explained by a rotation of the Earth around its axis. The observed apparent motion of the planets can be understood as their motion around the Sun, viewed from a moving Earth. However, as Copernicus kept the circular orbits, he also considered an epicycle system to describe planetary motion accurately.

After Copernicus, Danish astronomer Tycho Brahe (1546-1601) proposed a hybrid model of Moon and Sun orbiting the Earth and the other planets moving around the Sun, still needing epicycles for accurate description of their orbits. Strangely, he kept the idea that the sky and all planets encircle a static Earth daily, and got in conflict with Nikolaus Baer who thought Earth was rotating. Tycho also established the nature of comets as objects of translunar space and not atmospheric phenomena, as had been postulated by Aristotle, by measuring a lower limit of the distance of several times the Lunar distance for one comet, and observed a supernova in 1572, thus proving that the stellar skies are not so unchangeable as people had believed previously.

German astronomer Johannes Kepler (1571-1630) used Brahe's Mars observations to establish that planets move on elliptical orbits around the Sun, and derived his three laws of planetary motion:

1. The orbit of each planet is an ellipse with the Sun in one focus.
2. The radius vector from Sun to planet sweeps equal areas at each time, meaning that the planet moves faster when closer to the Sun.
3. The squares of the revolution periods are proportional to the cubes of the mean distances from the Sun for all planets.

The establishment of the Kepler laws of planetary motion was the last great achievement of the pre-telescopic era of astronomy, although Kepler himself had also developed a type of telescopes.

It was finally left to Galileo to give evidence for the heliocentric model with his telescopic discoveries of the moons of Jupiter and the phases of Venus. However, he got in serious trouble with the Roman Inquisition for his advocation of the Copernican system, and the Church authorities kept the old geocentric system of Ptolemy as their doctrine for a long time.

The first rigorous proof of the Earth's motion around the Sun came finally over a century later in 1729, when James Bradley discovered the aberration of light from the stars, a small apparent displacement caused by the combination of Earth's motion with the finite velocity of light (which had to be discovered previously, see below). The other predicted effect, stellar parallaxes, had to wait for their discovery until 1838, when Friedrich Wilhelm Bessel discovered the parallax of star 61 Cygni.

Telescopes were invented around 1600, and about 1609 the first people began to use them for observing the sky. As already mentioned, Italian astronomer and physicist Galileo Galilei (1561-1642) was one of the first astronomers using a telescope.