There are 2 models
- Geocentric model
- Heliocentric model
Geocentric model:
- The geocentric model is an astronomical model that suggests the earth is at the center of the universe, and that all other planets revolve around the earth.
- The geocentric model was developed by a Greek astronomer named exodus$^2$
- This model is false
Heliocentric model
- The heliocentric model is an astronomical model that suggests the sun is at the center of the universe, and all other planets (including earth) revolve around the sun
- This model is true
- Retrograde motion was used to justify this
Kepler’s three laws of planetary motion (Out of portion)
- The law of orbits: All planets move in elliptical orbits, with the sun at one focus
- The law of areas: A line that connects a planet to the sun, sweeps out equal area in equal intervals (times)
- The law of periods: The square of the period of any planet is proportional to the cube of the semi major axis of its orbit
- In the geocentric view, the sun and the planets have been held in place by slowly rotating crystal spheres arranged around the earth in concentric circles, Kepler’s realisation showed that the orbits were elliptical, and not spherical. But now, what held the planets in their orbits? Gravity.