How did the sun, moon and stars move and at what speed in the geocentric model? What flaw was observed?
In the geocentric model, it was believed that the sun, moon, and stars revolved around the Earth. They were considered to be fixed on celestial spheres or crystalline spheres. The movement of these celestial bodies in the geocentric model was thought to be a combination of two motions: the daily rotation around the Earth and the yearly revolution along the ecliptic.
The sun was believed to move eastward during the day and then go below the Earth at night, only to rise again in the morning from the east. This was attributed to the sun's daily rotation around the Earth.
Similarly, the moon was observed to rise in the east and set in the west, following a similar pattern as the sun but with a slightly different trajectory. The moon's motion was also explained as its rotation around the Earth.
The stars were observed to appear to have fixed positions in the sky, moving around the North Star (Polaris in the Northern Hemisphere). These observations were explained by attributing the stars to be fixed on a celestial sphere rotating around the Earth once a day.
The flaw observed in the geocentric model was the retrograde motion of some planets, especially Mars and Jupiter. Retrograde motion refers to the temporary westward movement of planets against their normal eastward motion in the sky. This observation was challenging to explain in the geocentric model because it contradicted the assumption that all celestial bodies circled the Earth. It was particularly challenging to account for these irregular retrograde motions using purely circular motions. This inconsistency laid the foundation for the development of the heliocentric model.
In the geocentric model, which was widely accepted up until the 16th century, it was believed that the Earth was at the center of the universe with the sun, moon, and stars moving around it.
According to this model, the movement of celestial objects was described as follows:
1. Sun: The sun was believed to move along a path called the ecliptic, completing one full rotation around the Earth in approximately 24 hours.
2. Moon: The moon was thought to orbit the Earth once in about 27.3 days. It was observed to move across the sky at a slightly different rate than the stars, completing one full revolution in approximately 24 hours and 50 minutes.
3. Stars: The stars were seen as fixed objects positioned on a celestial sphere. They appeared to rise in the east and set in the west, completing a full rotation around the Earth in approximately 23 hours and 56 minutes.
However, over time, a flaw was observed in this geocentric model. The flaw was known as the retrograde motion of the planets. While most celestial objects appeared to move in a consistent pattern across the sky, the planets occasionally appeared to reverse their motion temporarily, moving backward (east to west) before returning to their regular path. This phenomenon could not be adequately explained within the geocentric model and raised questions about its accuracy and underlying assumptions. This flaw ultimately led to the development and acceptance of the heliocentric model proposed by Nicolaus Copernicus, which placed the sun at the center of the solar system.
In the geocentric model, which was widely accepted in ancient times, it was believed that the Earth was at the center of the universe and that the Sun, Moon, and stars revolved around it.
According to this model, the Sun was thought to move along a circular path called the ecliptic, completing one revolution in 365.25 days, which corresponds to a year. The Moon was believed to move in a similar manner, with an orbital period of about 27.3 days, which corresponds to a lunar month. The stars were presumed to be fixed in the celestial sphere, moving across the sky in a daily motion.
Regarding the speeds of these celestial bodies, the geocentric model did not assign specific velocities to them. It was believed that the Sun, Moon, and stars had a uniform and constant motion, without any mention of specific speeds.
However, observations over time revealed a significant flaw in the geocentric model. Astronomers noticed that the planets, including Mars, Jupiter, and Saturn, did not always move at the same speed or in a perfectly circular orbit. Instead, they observed that planets sometimes appeared to slow down, reverse their direction, and speed up again in a phenomenon called retrograde motion.
This flaw in the geocentric model led to the development of the heliocentric model, proposed by Nicolaus Copernicus in the 16th century, which placed the Sun at the center of the solar system and explained the observed motions of the celestial bodies more accurately.