User:Matteogiliooo/sandbox

The Commentariolus is subdivided into nine paragraphs which explain planets movements.

While the first paragraph has the function of introducing the whole treaty, the second one contains seven postulates:


 * 1) Celestial bodies do not all revolve around a single point.
 * 2) The centre of the Earth is the centre of the lunar sphere—the orbit of the moon around the Earth.
 * 3) All the spheres rotate around the Sun, which is near the centre of the Universe.
 * 4) The distance between the Earth and the Sun is an insignificant fraction of the distance from the Earth and the Sun to the stars, so parallax is not observed in the stars.
 * 5) The stars are immovable; their apparent daily motion is caused by the daily rotation of the Earth.
 * 6) The Earth is moved in a sphere around the Sun, causing the apparent annual migration of the Sun; the Earth has more than one motion.
 * 7) The Earth’s orbital motion around the Sun causes the seeming reverse in direction of the motions of the planets.

Afterwards he describes the order of the planets assuming their revolution periods. The furthest from the Sun is Saturn, then Jupiter, Mars, Earth, Venus and Mercury. The planets' velocity depends on the distance from the sun; the greater the distance between the planet and the sun, the more the planet takes to complete a full revolution.

Then he describes the three Earth motions. The first one is the revolution motion: the Earth revolves around the Sun and sweeps out equal areas in equal intervals of time. This motion causes an illusory Sun motion which would lead to thinking that the Earth is in the centre of the Universe. The second motion is the rotation, which occurs from West to East. The third motion is the declination, in which the rotation axis is inclined by about 23°.

Copernicus states that the duration of the year depends on the fixed stars. So the year has always last 365 days, 6 hours and about 20 minutes.

Copernicus observes the Moon and its five motions. The Moon revolves around the Sun in one year time and around the Earth in one month. Then it completes an epicycle in a little more than a month following the opposite direction of the deferent. The last two motions are two revolutions (a month) which follow the opposite direction of the epicylce's centre. These motions seem to explain why the Moon sometimes slows down and others speeds up, and why sometimes it seems to complete a retrograde motion.

The seventh paragraph is about Saturn, Jupiter and Mars, which have similar motions. To complete a revolution Saturn takes 30 years, Jupiter 12 years and Mars 29 months.

In the last two paragraphs Copernicus talks about Venus and Mercury. The first has a system of circles and takes 9 months to complete a revolution. Mercury's orbit is harder to study because it is visible just for some days a year. Mercury, just like Venus has two epicycles, one greater than another. It takes almost three months to complete a revolution.