The movements of the Sun

The path of our Sun around the celestial sphere is inclined to its orbital plane by an angle of (currently) ~23.44°. The Sun spends half the year in the southern half of the celestial sphere and the other half in the northern. Its path thus crosses the Celestial Equator twice every year: once at the March equinox, around March 21, as it comes into the northern hemisphere and 6 months later when, at the September equinox around September 22, it returns to the southern hemisphere.

Celestial coordinate system

Just as a location on the Earth’s surface has a “latitude”, defined as its angular distance from the equator towards the poles, so a star has a "declination" (Dec) given as an angle which is either positive (in the northern hemisphere) or negative (in the southern hemisphere).

On the Earth we define the position of a location round the Earth by its longitude. As referred to above, the path of the Sun gives two defined points along the Celestial Equator that therefore might sensibly be used as the zero of Right Ascension (RA) – the points where the ecliptic crosses the Celestial Equator at the March and September equinoxes. The point where the Sun moves into the northern hemisphere was chosen and was given the name ‘The first point of Aries’ as this was the constellation in which it lay. Star positions are ever since measured eastwards around the celestial sphere from the first point in Aries to give the star’s RA.

Sun orbit

In the heliocentric model the Sun has an orbit - around the milkyway – in a period of 250 million years.

In the geo-heliocentric model the Sun has an orbit – around the HELION POINT a.k.a. Sun barycenter – in a MEAN period 365.242229199372 days a year.

AND

The HELION POINT has an orbit around CENTER in a period of 101,920 years.

Because Earth is orbiting CENTER in the opposite direction as the HELION POINT orbits CENTER, the experienced period the Sun orbits around Earth has a MEAN value of 365.25775886599 days a year.

The Sun orbits the HELION POINT, 305,760 times in a Great-Great-Year

The Sun orbits Earth, 305,747 times in a Great-Great-Year

The HELION POINT orbits Earth, 305,744 times in a Great-Great-Year

Let’s look at the moon and the planets.