Polaris is a particular star. It is almost exactly over the North Pole. This position makes this star perfect to do some calculation in relation to the Earth.
Sun, moon and planets
Planets longitude changes continuously in respect of the fixed stars (the dome). They have their own motion on the celestial sphere. Currently a double reason is given for this particular motion: first planets move around the sun on an almost circular trajectory (according to Kepler laws) and, second , the Earth moves around the sun and thus the visual direction Earth-planet that projects the planet on the celestial sphere is continually changing.
What should be the different consequences when considering a fixed and central Earth?
This article is under revision.
Consider the picture aside. It represents the celestial sphere. The ecliptic is the maximum circle inclined of about 23°27’ on the celestial equator . It is the projection of the trajectory of the sun (they say the Earth) on the celestial sphere (we say the dome or the firmament).
and are the points where the ecliptic intersects the equator otherwise known as the equinox points.
The angle of 23°27’ is the obliquity of the ecliptic and it is not always constant but it registers in the course of the year some small variation.
In the antiquity astronomers had already understood that the trajectory of the sun in the heavens is inclined.