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Constellation
A constellation is a recognized pattern of stars as seen from Earth.
Epicycle
In a geocentric model, a small circular motion of a planet whose center itself moves on a larger circle around Earth, used to reproduce observed planetary motions.
Galaxy
A galaxy is a vast system of stars and matter held together by gravity.
Geocentric model
A model of the solar system in which Earth is at the center and the Sun, Moon, planets, and stars move around Earth.
Heliocentric model
A model of the solar system in which the Sun is central and the planets (including Earth) orbit the Sun.
Retrograde motion
The apparent temporary reversal of a planet’s motion across the sky relative to the background stars.
Solar system
The Sun and all objects that orbit it or are otherwise gravitationally bound to it, including planets, moons, dwarf planets, asteroids, comets, and dust.