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Geocentric Model Earth-centered

Main Claims:

Earth-centered/moves around in perfect circles.

Explained why Venus, Mercury, and the moon would go in front of the sun.

None of the planets would go in front of the moon.

Explained/showed why other planets would pass behind the sun.

Explains why Earth appears to be unmoving.

Stars are all at the same distance from Earth in a sphere.

-Main Flaws

Didn't explain petrograde motion of other planets in the sky.

Also didn't explain shooting stars and comets.

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Geocentric Model

Main Claims:

Ptolemy proposed that the Earth was the center of the universe, with the Sun, Moon, planets, and stars orbiting it in circular paths.

To explain planetary motion, he introduced epicycles (small circular orbits along larger orbits) to account for apparent retrograde motion.

He believed that stars were fixed on a celestial sphere surrounding the Earth, and the heavens were composed of concentric spheres.

Ptolemy also wrote about astrology and light refraction, claiming that celestial bodies influenced human affairs.

-Main Flaws

Earth-centered

System was needlessly complex due to epicycles

His underestimation of Earth's size contributed to navigational errors.

His maps misrepresented the size and placement of landmasses.

Insisted that planets moved in perfect circles, which ignored planetary motion (elliptical orbits).

Ptolemy sometimes manipulated data to fit his model rather than revising his theories based on new observations.

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Heliocentric Model Sun-Centered

Main Claims:

The Sun, not the Earth, was at the center of the universe ( Published a book called De revolutionibus orbium coelestium, in 1543).

Planets, including Earth, revolve around the Sun in circular orbits.

Earth rotates on its axis daily, which explains the apparent movement of the stars.

Earth also orbits the Sun annually, explaining the changing positions of celestial objects.

He explained that the apparent backward motion (retrograde motion) of planets was due to Earth's movement, not epicycles as in Ptolemy’s model.

Unlike the planets, the Moon orbits Earth, which he correctly retained from earlier models.

He suggested that the stars are much farther away than previously thought, making their motion negligible relative to Earth’s movement.

-Main Flaws

He maintained the idea that planets moved in perfect circles rather than elliptical orbits. This led to inaccuracies.

Since circular orbits didn't perfectly match observations, he still included small epicycles to adjust planetary motion.

He lacked direct observational proof (such as stellar parallax, wasn’t observed until the 19th century).

His model was more mathematically elegant than Ptolemy’s but not necessarily more accurate at the time.

He could not explain why planets stayed in orbit around the Sun

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Heliocentric

Main Claims:

Using a telescope, Galileo provided strong evidence for Copernicus’s heliocentric model.

He observed celestial phenomena that contradicted the Ptolemaic geocentric model.

He discovered four large moons orbiting Jupiter (Io, Europa, Ganymede, and Callisto), proving that not everything revolves around Earth.

He observed that Venus goes through phases like the Moon, which could only be explained if it orbited the Sun rather than Earth.

Contrary to the belief that celestial bodies were perfect spheres, he observed mountains and craters on the Moon.

His telescope revealed that the Milky Way was composed of countless individual stars, proving the vastness of space.

He formulated early concepts of inertia, stating that an object in motion stays in motion unless acted upon

He disproved Aristotle’s idea that heavier objects fall faster than lighter ones, showing that all objects fall at the same rate in a vacuum.

-Main Flaws

He incorrectly believed that Earth's motion caused ocean tides, rather than the gravitational pull of the Moon.

While he contributed to the study of motion, he did not develop a complete theory of gravity

He insisted that planets moved in perfect circles rather than ellipses, rejecting Kepler’s more accurate laws of planetary motion.

His strong advocacy for heliocentrism, despite lacking absolute proof at the time, led to his trial and house arrest by the Catholic Church.

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He was an ancient Chinese astronomer and astrologer who lived during the Warring States period (around the 4th century BCE). While he is not as widely known as some other figures in Chinese philosophy, his contributions to astronomy were significant in their time. He also didn't get a complete model of the solar system.

Main Claims:

One of Gan De's most notable claims was that he discovered the moons of Jupiter. He is credited with observing the planet Jupiter and noting that it had accompanying "stars" moving around it. These were the moons of Jupiter, which were later confirmed by European astronomers like Galileo centuries later. Gan De's observations were remarkably ahead of his time.

He also made observations about the motion of the planets, particularly Jupiter, and the apparent movements in the sky. Gan De's understanding of planetary orbits helped shape early Chinese astronomical thought and had a significant impact on later astronomers in the region.

Like many ancient astronomers, Gan De contributed to predicting solar and lunar eclipses. This was an important part of Chinese astronomy, as eclipses were often viewed as omens or significant events, and accurate predictions were crucial for royal court officials to prepare for these phenomena.

Gan De worked on the cataloging of stars and the understanding of the constellations, helping to organize and systematize astronomical knowledge. His work contributed to the foundation of Chinese star charts and astrology.

-Main Flaws

While Gan De is credited with observing Jupiter's moons, he did not have the technological tools or the understanding to grasp what these "wandering stars" actually were. His claim was not based on an accurate understanding of moons as natural satellites, but rather as "stars" that seemed to follow Jupiter. He could not have known the true nature of these moons because he lacked the telescope and the framework of modern astronomy.

Much of Gan De's work was tied to astrology, which often incorporated mystical or symbolic interpretations of celestial events. This could be seen as a flaw in his scientific approach, as it mixed empirical observation with beliefs that did not necessarily align with the later scientific method.

Geocentric Model: Like many astronomers of his time, Gan De adhered to the geocentric model of the universe, which placed Earth at the center of the cosmos. This understanding was a common belief in ancient civilizations, but it was later replaced by the heliocentric model (where the Sun is at the center of the solar system) by figures like Copernicus. Gan De's observations were confined within this outdated model, limiting the accuracy of his theories.

Although Gan De made important observations, he did not possess the mathematical tools or methods to explain or quantify the phenomena he observed with great precision. Without advanced mathematical frameworks, his astronomical theories were often imprecise by modern standards.

While Gan De made important strides in early Chinese astronomy, his scope of observation was still limited by the technological constraints of his time. His understanding of celestial bodies and their behavior was based on naked-eye observations, which naturally limited his accuracy and the scope of his claims.

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Heliocentric model

Main Claims:

Earth orbits the Sun at an average speed of 107,000 km/h and also rotates on its axis.

Heliocentric Model, The Sun is at the center of the solar system, and Earth orbits around it along with the other planets.

The Sun and Earth both orbit a common center of mass, which is very close to the center of the Sun, but since the Sun contains 99.87% of the total mass of the solar system, the center of mass is close to it but not at the exact center.

The solar system consists of the Sun, eight planets (including Earth), their moons, and smaller objects like asteroids and comets, all orbiting the Sun due to gravitational forces.

Main Flaws:

The Sun's center of mass moves slightly due to the gravitational influences of the planets, which complicates the simplicity of a fixed Sun-centered model.

Planets do not orbit in perfect circles; their orbits are elliptical, which means that the distance between the planets and the Sun varies over time.

The Sun is not at the center of the universe, and the solar system is just one part of a much larger galaxy. The idea of a "center of the universe" is no longer considered accurate in modern cosmology.

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Elliptical Orbits(Heliocentric)

Main Claims:

Kepler’s First Law, 1609

Kepler discovered that planets orbit the Sun in ellipses, not perfect circles.

This corrected the inaccuracies in Copernicus’s heliocentric model, making planetary motion more predictable.

Kepler’s Second Law, 1609

He found that planets move faster when they are closer to the Sun and slower when farther away.

This explained why planetary speeds change depending on their position in orbit.

Kepler’s Third Law, 1619

He established a mathematical relationship between a planet’s distance from the Sun and its orbital period: T2∝R3T^2 \propto R^3T2∝R3

This showed that planetary motion followed a precise mathematical pattern.

Other

He theorized that some force (later identified as gravity) keeps planets in their orbits.

While he didn’t fully understand it, this idea set the stage for Newton’s law of universal gravitation.

Unlike Galileo, Kepler correctly theorized that the Moon, not Earth’s motion, caused ocean tides.

By observing sunspots, he suggested that the Sun itself rotates, which was later confirmed.

Main Flaws

Kepler believed in a divine mathematical harmony and tried to fit planetary motion into geometric shapes, such as nested Platonic solids, which was incorrect.

His belief in the "music of the spheres" was based on numerology rather than science.

While he suspected the Sun exerted a force on planets, he didn’t define gravity mathematically.

Newton later explained planetary motion with the law of universal gravitation.

Despite his scientific contributions, Kepler practiced and defended astrology, believing celestial bodies influenced human affairs.

He speculated that planetary motion was driven by the Sun’s magnetism rather than a gravitational force.

This idea was incorrect and later replaced by Newton’s gravitational theory.

He tried to justify the distances between planets using mathematical ratios that ultimately had no physical basis.