If the sun orbits the earth daily in the geocentric model, what causes a year?
A viewer notes that in heliocentrism the earth circles the sun in about 365 days, but in geocentrism the sun circles the earth in one day, and asks what causes a year.
Answer
In the geocentric model the stars, sun, and moon all revolve around the earth in one day; it is the whole universe, not just the sun. But those three move at slightly different speeds. The stars and the universe move on sidereal time: from one crossing of a meridian to the next takes 23 hours, 56 minutes, 4.09 seconds, a sidereal day. The sun takes a little longer, 24 hours, so the sun is traveling slightly slower than the stars, a retrograde motion of about one degree per day against them. There are 360 degrees in a circle, and that one-degree-per-day drift carries the sun backward through the twelve constellations of the zodiac, starting at Aries and returning to Aries in 365.25 days; that is where our year comes from. The moon has a larger retrograde motion of thirteen degrees per day, completing its circuit in about twenty-seven days, or about twenty-nine allowing for the sun's own motion. So the year comes from the retrograde motion of the sun against the daily rotation of the stars, all around a fixed earth.