In old times the stars and astronomy were mainly considered
for two reasons:
and philosophical contexts, and time determination.
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
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)
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
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
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.