Astronomy as known today has a very interesting history. It is a science which was
first based on observation and in which religion had a great influence. Some thinkers
help to build the nowadays theory, sometimes at the risk of their lives.
Born in 1473, Copernicus was convinced very young, probably after reading "Aristarchus of Samos",
that the earth is not at the center of the world. He devoted his spare time to accumulate
observations of celestial bodies and their orbits calculations in order to develop a new
system of the world.
In 1543, he published a work arguing that the sun was the center of the world
and it is around it that the other planets turned in the following order:
Mercury, Venus, Earth, Mars, Jupiter and Saturn.
Furthermore, Copernicus was convinced that the orbits of celestial bodies should
be circular and traveled at a constant speed as Aristotle and Ptolemee. To explain the irregular
motion of the planets, he had to introduce epicycles and build a very complex system.
After questioning the Earth as the center of the world by Copernicus, the concept of immutability of the heaven,
another idea from Aristotle, collapsed at the end of the sixteenth century. People saw a new star in the immutable sky!!!!
This happened in 1572 ; a new star was visible in daylight for a month and continued to shine for a year and a half.
Astronomers at this time, which referred to the immutability of the heaven of Aristotle, thought that the phenomenon could have
occurred only within the sphere of the Moon, so close to the Earth.
But thanks to precise measurements of the position of the new star, the Danish astronomer Tycho Brahe showed that it was
absolutely immobile and fixed relative to the other stars. However, if the new moon was actually closer to Earth, it would
have had to move across the sky like planets. Tycho Brahe therefore leads to the only possible conclusion: the new star would
be very far from the planets. The heaven were not immutable, but subject to change as the Earth, and
doubt began to settle on the dogma of Aristotle. These doubts were confirmed five years later, in 1577. Tycho Brahe observed
the passage of a bright comet and analyzed its movement. His observations showed that the comet was moving relative
to the background formed by the stars, but much more slowly than the Moon.
The observations of this comet therefore confirmed the results of 1572, highlighting a second celestial object subject to change.
The observations of the comet went further. Analyzing them, Tycho Brahe showed that the trajectory of the body was not circular,
but elliptical. The last piece of Aristotelian thought, the circular motion of the planets, also began to tremble.
Johannes Kepler, born in 1571, began his career as an assistant to Tycho Brahe. Upon the death of Tycho, all the valuable observations
of planets accumulated over twenty years became Kepler's legacy.
The German astronomer was particularly interested in the movement of Mars, which no existing system was able to reproduce accurately.
After a very laborious calculations, Kepler determined the origin of irregularities in Mars's movement. the planet's orbit around the Sun,
was not circular, but elliptic. Kepler published this result in 1609, in Astronomia Nova (New Astronomy) and finally buried
the old dogma of circular planets orbits.
Kepler also showed that Mars does not travel around its orbit at a constant speed, but at one depending on the distance of the planet from the Sun.
Copyright : Moulong Armel Rodrigue, 2012