In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects like satellites, planets, stars, etc. The origin of the coordinate can be anywhere, including Earth. If the coordinate system is Earth-centered, we call it a geocentric coordinate system.
Flat-Earthers are often triggered by the term ‘geocentric’. They would search inside astronomy books and scientific journals to find the word ‘geocentric’. If they can find it, they would use the fact as ‘evidence’ of geocentrism, or that the Earth is the center of the universe, and the Sun is revolving around the Earth.
They are wrong. In many cases, the term ‘geocentric’ refers to the origin of a coordinate system. And it has nothing to do with whether the Sun revolves around the Earth or the other way around.
There are many celestial coordinate systems to choose from. Each has an origin which denotes the center of the coordinate system. Some of which are:
- Topocentric (observer-centered)
- Geocentric (Earth-centered)
- Heliocentric (Sun-centered)
- Selenocentric (Moon-centered)
- or it can be anywhere else, like jovicentric, which is centered on Jupiter.
The most used geocentric coordinate system is the equatorial coordinate system, where the origin is the center of the Earth. The position of a celestial object is denoted using two coordinates:
- Declination: the angle between the object and the celestial equator.
- Right ascension: the angle of the object from the vernal equinox.
We use a geocentric coordinate system because we live on Earth and it is convenient for many purposes. It has nothing to do with geocentrism, the superseded description of the universe. Geocentrism has nothing to with flat-Earth either. Flat Earth is an even more archaic description of the Earth. But apparently, that doesn’t stop flat-Earthers to piggy-back geocentrism to advance their agenda.