# Contributions of Aristotle, Aristarchus, Ptolemy, Copernicus, Brahe, Kepler, Galileo, Newton, and Einstein to Astronomy

Classified in Physics

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## Aristotle:

• Proved that the Earth is spherical
• Believed that it was at the center of the universe

## Aristarchus:

• First to say that the Sun, and not the Earth, was the center of our universe
• Used simple geometry to estimate the size of the sun and moon and their relative distance

## Ptolemy:

• Made a model called the Ptolemaic system
• Concluded all astronomical objects move at constant speeds in circular orbits
• Built his model to fit this idea
• The Ptolemaic model is one of the longest upheld scientific theories in history: it was the cornerstone of astronomy for 1,500 years

## Copernicus:

• Concluded that the Sun was the center of the universe instead of the Earth
• Heliocentric

## Brahe:

• Made super precise instruments for observing space before the telescope was created
• Believed the universe was a blend of the Ptolemaic and Copernican models
• Created his own model in which the planets orbit the Sun and the Sun orbits the Earth

## Kepler:

• Kepler's 1st Law: All planets move about the Sun in elliptical orbits, having the Sun as one of the foci
• Kepler's 2nd Law: A radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time
• Kepler's 3rd Law: The squares of the sidereal periods (of revolution) of the planets are directly proportional to the cubes of their mean distances from the Sun

## Galileo:

• Invented the telescope
• Discovered that 4 moons revolved around Jupiter

## Newton:

• Invented Calculus
• Proposed force of gravity was what was holding the solar system together

## Einstein:

• Theory of Relativity: Determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers
• The photoelectric effect is the emission of electrons or other free carriers when light is shone onto a material. Electrons emitted in this manner can be called photo electrons

## Newton:

• Newton's first law of motion - sometimes referred to as the law of inertia. An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force
• Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object