Copernican vs Ptolemaic Models

Ptolemy established two rules that linked the motion of planets to the Sun. These rules helped explain why planets appeared to move forward, then backward, and then forward again in the sky. This is known as retrograde motion.

Ptolemy underestimated the size of the solar system because he assumed that the stars were all at the same distance from the Earth. This led him to believe that the planets were much closer to Earth than they actually are.

Even though Ptolemy's measurements were inaccurate, his measurements were important because they provided a foundational model for understanding the motions of the planets. This model paved the way for later astronomers to refine our understanding of the cosmos.

1. Retrograde Motion: Planets appear to temporarily change their direction of motion in the sky.
2. Changing Brightness: Planets appear to change their brightness due to their varying distance from Earth.
3. Changing Apparent Speed: Planets appear to move at different speeds across the sky.

Plato believed that the scientific method involved seeking perfect forms and explaining the natural world through abstract ideas, rather than through observation and experimentation.

Plato believed that the celestial bodies were perfect, unchanging, and moved in perfect circles. This belief led to the development of the idea that the Earth was the center of the universe and served as the foundation for the geocentric model of the universe.

Eudoxus believed that planets reside on rotating spheres, called celestial spheres. Each planet needed four spheres to explain its motion. This was because the spheres rotated independently from each other producing complex motions in the planets.

Aristotle proposed a thought experiment that argued that if the Earth were moving, objects thrown upward would land behind the point from which they were thrown. He concluded that since this doesn't happen, the Earth must be stationary.

The angle between the horizon and the Sun, measured in degrees.

The Moon's phases are caused by the changing angles at which we see the sunlit portion of the Moon as it orbits the Earth.

The time it takes for the Moon to complete one full cycle of phases, approximately 29.5 days.

The period of the lunar cycle where the illuminated portion of the Moon appears to be increasing.

The period of the lunar cycle where the illuminated portion of the Moon appears to be decreasing.

A solar eclipse occurs when the Moon passes between the Sun and Earth, blocking the Sun's light from reaching a portion of Earth.

A lunar eclipse occurs when the Earth passes between the Sun and Moon, casting a shadow on the Moon.

The region of complete shadow cast by an object during an eclipse.

The change in the apparent position of an object when viewed from two different locations.

Tycho observed that the supernova of 1572 remained stationary against the background stars, which contradicted the Aristotelian belief that the celestial realm was unchanging.

Tycho's observations were incredibly accurate and detailed because he used large and sophisticated instruments, including a huge quadrant and a mural quadrant.

The Tychonic system places the Earth at the center of the universe and has the Sun revolving around the Earth. The other planets orbit the Sun. The stars are fixed and distant.

The Tychonic system does not explain the phases of Venus or the apparent changes in the size of Mars.

Kepler's model in Cosmographic Mystery was a geocentric model, with the orbits of the planets being nested regular solids inscribed within spheres.

Kepler believed that the planets were driven by a force emanating from the Sun, with the force weakening further away from the Sun.

Kepler realized that if the planets were driven by a force emanating from the Sun, their speeds could not be constant because the force would weaken as they moved further from the Sun.

The path of a planet is not a circle, but an ellipse with the Sun at one focus point.

The point where the planet is farthest from the Sun.

Direct light feels warmer because it carries a higher concentration of energy from the Sun.

Latitude determines how high the Sun can appear in the sky. The Sun can be directly overhead for observers between the Tropics of Cancer and Capricorn.

The point where the planet is closest to the Sun.

The length of the longest diameter of the ellipse.

This occurs when a planet is farthest from the Sun, so it has less gravitational pull and its speed slows down.

This occurs when a planet is closest to the Sun, so it has strongest gravitational pull and its speed increases.

A period of time when eclipses can occur.

The point at which a planet first becomes visible after being obscured by the Sun’s rays.

The Dresden Codex contains numerous astronomical tables that predict eclipses for a specific time period.

Ancient Babylonian astronomers were motivated by the need to predict the best times for agriculture due to the unpredictable and arid climate.

The Babylonians used a base-60 system for their mathematical calculations.

The Babylonians selected their gods based on the planet's color or visual appearance.

Venus' orbit around the Sun makes it appear as a morning star for roughly 8 months, then disappears and reappears as an evening star for another 8 months.

The apparent movement of stars across the night sky is caused by the Earth's rotation on its axis.

If you are standing on the Earth's North Pole, the stars appear to move in circles around the North Celestial Pole, with this pole being the center of their rotation.

As you travel from the North Pole towards the equator, the center of rotation (North Celestial Pole) appears to move lower in the sky.

When facing north from the equator, stars appear to move in arcs from east to west, parallel to the horizon.

From the equator, facing south, the stars appear to move in a circle around the South Celestial Pole, which is located directly above the South Pole.

The North Star (Polaris) marks the North Celestial Pole, while no single bright star marks the South Celestial Pole.

The angle between the horizon and the North Star is equal to your latitude on Earth in the Northern Hemisphere.

The Earth's orbit and tilt cause us to face different parts of the sky at different times of the year, resulting in different visible constellations.