# Celestial sphere

## Equatorial coordinate system

The equatorial coordinate system is the most basic coordinate system around the Earth. If you draw a large imaginary circle extending the Earth's equator on the celestial sphere, it becomes the celestial equator. And if the north pole of the Earth is extended and marked on the celestial sphere, this point becomes the celestial north pole. Similarly, the point marked on the celestial sphere by extending the South Pole of the Earth is called the "South celestial pole". The position of celestial bodies in the equatorial coordinate system is expressed in terms of right ascension and declination.

## Horizontal coordinate system

The horizontal coordinate system is a celestial coordinate system that uses the local horizon of the observer as the fundamental plane. The coordinates of an object in the sky are expressed in terms of altitude (or elevation) angle and azimuth.

## Constellations

If you look up at the sky where there is no light on a clear night, there are so many stars that you can't count them. Thousands of years ago, the ancients linked the stars to create constellations with names of animals, objects and mythical figures. Each star in the constellation is only in a similar direction. Distances to the stars vary. There is no real connection between the stars in the constellation.

## The celestial equator and the ecliptic

The celestial equator is the extension of the Earth's equator to the celestial sphere. Therefore, the celestial equator can be represented by a line fixed on the celestial sphere. The ecliptic is a line joining the place where the sun is located on the celestial globe during a year. The ecliptic can be represented by a line fixed on the celestial sphere. The Earth's axis of rotation is tilted 23.5° with respect to the axis of revolution. Therefore, the celestial equator and the ecliptic are inclined 23.5° to each other.

## Pole Star

The Pole Star is located on an extension of the Earth's axis of rotation and therefore helps us to know where north is.