The Solar System

Geocentric

A model of the solar system that places Earth at the center, with the sun and planets revolving around it.

Parallax

The apparent shift in position of an object when viewed from different angles or positions.

Retrograde motion

The apparent backward movement of a planet in the sky relative to the stars.

Epicylces

Small circles in which planets were thought to move while circling Earth in the geocentric model.

Heliocentric

A model of the solar system that places the sun at the center, with Earth and other planets orbiting around it.

Claudius Ptolemy

Ancient Greek astronomer who proposed the geocentric model of the solar system.

Nicolaus Copernicus

Astronomer who developed the heliocentric model, suggesting that the sun is at the center of the solar system.

Johannes Kepler

Astronomer known for his laws of planetary motion that describe the orbits of planets around the sun.

Terrestrial planets

Rocky planets that are close to the sun, including Mercury, Venus, Earth, and Mars.

Jovian planets

Gas giants that are farther from the sun, including Jupiter, Saturn, Uranus, and Neptune.

Ellipse

An oval shape that describes the orbit of planets around the sun.

Semi-major axis

The longest radius of an ellipse, measuring the distance from the center to the outer edge.

Aphelion

The point in a planet's orbit where it is farthest from the sun.

Perihelion

The point in a planet's orbit where it is closest to the sun.

Eccentricity

A measure of how much an orbit deviates from being circular; determined by the distance between the foci and the length of the major axis.

Focal points

The two points within an ellipse where the total distance to any point on the ellipse is constant.

Apparent retrograde motion

The illusion that planets move backward in their orbits due to the relative motion of Earth and the planets.

Kepler's first law

Planets move in ellipses with the sun at one focus.

Kepler's second law

A line joining a planet and the sun sweeps out equal areas during equal intervals of time.

Kepler's third law

The square of the period of any planet is proportional to the cube of the semi-major axis of its orbit.

Orbital mechanics

The study of the motions of objects in space, particularly the gravitational interactions that dictate their orbits.

Planetary order

The sequence of the planets in the solar system from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Planetary orbits

The paths that planets follow as they revolve around the sun.

Model of the solar system

A representation that illustrates the arrangement and movement of planets around the sun.

Predicting planetary positions

Using mathematical models and Kepler’s laws to estimate where a planet will be in the future.