Models of the Universe: Heliocentric vs Geocentric

Heliocentric Model

A model where the Sun is the center of the universe (proposed by Copernicus).

Geocentric Model

A model where Earth is the center of the universe (Ptolemaic Model).

Epicycle

A small circular orbit that planets follow within their larger orbit in the geocentric model.

Retrograde Motion

The apparent backward movement of planets as seen from Earth.

Prograde Motion

The normal forward movement of planets across the sky.

Ptolemaic Model

The geocentric model where planets move in epicycles around Earth.

Copernican Model

The heliocentric model where planets orbit the Sun.

Galilean Satellites

The four largest moons of Jupiter discovered by Galileo (Io, Europa, Ganymede, Callisto).

Ellipse

An oval-shaped path followed by planets in their orbits.

Foci of an ellipse

The two fixed points inside an ellipse; the Sun is at one focus in planetary orbits.

Eccentricity

A measure of how stretched an ellipse is (0 = circle, 1 = straight line).

Semi-Major Axis

Half the longest diameter of an ellipse, which determines the size of an orbit.

Orbital Period

The time it takes a planet to complete one orbit around the Sun.

Force

A push or pull on an object.

Mass

The amount of matter in an object.

Acceleration

The change in velocity of an object over time.

Gravitational Force

The attractive force between two objects with mass.

Inertia

The tendency of an object to stay in motion or stay at rest unless acted upon by an external force.

Kepler's First Law (Law of Ellipses)

Planets orbit the Sun in an ellipse, with the Sun at one focus.

Kepler's Second Law (Law of Equal Areas)

Planets move faster when closer to the Sun and slower when farther away.

Kepler's Third Law (Harmonic Law)

The square of a planet's orbital period is proportional to the cube of its semi-major axis.

Newton's First Law (Inertia)

An object in motion stays in motion unless acted upon by a force.

Newton's Second Law (F = ma)

Force equals mass times acceleration.

Newton's Third Law (Equal & Opposite Reaction)

For every action, there is an equal and opposite reaction.

Law of Universal Gravitation

Every object with mass exerts an attractive force on other objects.

Effect of Mass on Gravity

When mass increases, gravity increases.

Effect of Distance on Gravity

When distance increases, gravity decreases.