## Celestial coordinate system

A celestial sphere is an abstract sphere centered on an observer. All objects in the sky can be conceived as projected upon the celestial sphere’s inner surface.

## Equatorial system

The equatorial coordinate system is the most basic coordinate system around the Earth. Most celestial bodies use this equatorial coordinate system to indicate their location and make observations in modern astronomy.

If you draw an imaginary large circle extending the Earth’s equator on the celestial sphere, it becomes the celestial equator. And if you extend the Earth’s north-Pole and mark it on the celestial sphere, this point becomes the celestial North Pole. The closest observable star to the celestial North Pole is called the ‘Polaris.’

Similarly, the point marked on the celestial sphere by extending the South Pole of the Earth is called the ‘celestial South Pole.’ The intersection of the ecliptic and the celestial equator is called the equinox. The semicircle that passes through the celestial North Pole and the celestial South Pole is called the hour circle. Just as the longitude and latitude drawn on a globe are perpendicular to each other, the celestial equator and the time zone are perpendicular also.

The position of celestial bodies in the equatorial coordinate system is expressed in terms of right ascension and declination.

The angle measured counterclockwise from the time zone passing through the vernal equinox to the celestial body’s time zone to be observed is called the ‘right ascension.’ Right ascension is expressed as 0˚ to 360˚ or 0 o’clock to 24 o’clock.

In other words, since the vernal equinox is the standard, the time at the vernal equinox is 0, and the right ascension increases counterclockwise. Next, the celestial equator’s angle measured along the celestial body’s time zone to be observed is called ‘declination.’ Declination can range from -90˚ to 90˚. Since the celestial equator is the standard, the celestial equator’s declination is 0˚, the celestial pole’s declination is 90˚, and the declination of the celestial south pole is -90˚.

To observe a celestial object represented by the equatorial coordinate system from an observer’s perspective, it must undergo a coordinate system transformation. However, the equatorial coordinate system has the advantage of being able to indicate the position of a celestial body regardless of the measurement location and time. For this reason, the equatorial coordinate system is mainly used in modern astronomy.