Newton's gravitation principle supported the heliocentric viewpoint by clear and unambiguous explanations and offered a useful tool for making very precise predictions. The heliocentric concept proposed by Copernicus is often referred to as "Copernicus' revolution."
In the 18th and 19th century scientists discovered the Sun to be only one star among from an infinitesimal number of stars and in the 20th century it revealed that Milky Way is only one galaxy among the myriads of similar galaxies. Nowadays Einstein's theory of relativity is universally accepted. Relying on the equivalence principle, it has become unnecessary to determine the centre of the Cosmos. In other words, one can elect either the Sun or the Earth to be the centre (or reference point) of the solar system.
All we know, everything is in motion in the Universe. The Earth is spinning around its axis and moves on an orbit around the Sun. These motions cause the alternation of days and nights and that of seasons.
Observing from the Earth it appears as if the Sun were moving in the sky, following a regular path on the celestial heaven. We call Ecliptic the apparent way the Sun makes in a year in the sky. More precisely, it is the intersection of the celestial sphere with the ecliptic plane. The ecliptic plane contains the Earth's mean orbital plane in the solar system. Except for two, most planets in the solar system are moving almost on the same plane, therefore these planets appear from the Earth always close to the Ecliptic. The two exceptions are Mercury and Pluto whose orbital planes deviate from the ecliptic plane by 7 and 17, resp.
The Equator of the Earth does not align with the ecliptic plane, but makes an angle of approx. 23,27. This causes the alternation of seasons, because a particular area gets different amount of sunshine during the annual movement of the Earth.
At first it seems reasonable, seasons differ from one another because the Earth's distance from the Sun is always different. When the Earth is closest to the Sun, it is five million kilometres closer (perihelion) than on its farthest point (aphelion) from the Sun. Each year we are in the farthest distance from the Sun on July 3, and in the nearest distance on January 3. That is, the Sun is the farthest from us in summer and the closest to us in winter. Consequently, we cannot explain the occurrence of seasons by our planet's distance from the Sun. The reason for this is the axial tilt, in other words, the inclination of the angle made between the Earth's rotational axis and its orbital plane. The alternation of seasons is caused by the fact that the axial tilt and the direction to which the axis points remain invariable all over the year. North Pole constantly points to Polaris in Ursa Minor. If we turn to Polaris at any points of the Northern Hemisphere, we invariably turn to the North.
Seasons come about in the following way: On March 22, each year the Sun is above exactly the Equator. This is one of the two days when both daylight and nightlight last for twelve hours all over the world and on the Northern Hemisphere it is vernal equinox. Although the North Pole still points to Polaris, it is shifting towards the Sun during the three months that follow the equinox. In the first quarter of the year the Sun is rising