CH 2: Copernican Revolution

perihelion

The point in the orbit of a planet where it is closest to the Sun.

aphelion

The point in the orbit of a planet where it is farthest from the Sun.

eccentricity

A measure of how an orbit deviates from being circular.

heliocentric model

The astronomical model that places the Sun at the center of the solar system.

geocentric model

An astronomical model that places Earth at the center of the solar system.

retrograde motion

The apparent backward motion of a planet in the sky.

full phase of Venus

When Venus is on the opposite side of the Sun from the Earth.

new phase of Venus

When Venus lies between the Earth and the Sun.

Kepler's First Law

Planets move in elliptical orbits with the Sun at one focus.

Kepler's Second Law

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

Kepler's Third Law

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

planetary motion

The movement of planets in their orbits around the Sun.

velocity

The speed of something in a given direction.

mass

The amount of matter in an object, typically measured in kilograms.

weight

The force exerted by gravity on an object, measured in newtons.

observation

The action or process of observing something in order to gain information.

theory

A framework of ideas and assumptions that represents our best possible explanation for something.

scientific method

A systematic approach to inquiry that involves forming hypotheses, conducting experiments, and drawing conclusions.

orbital period

The time taken for a body to make a complete orbit around another body.

Doppler shifts

Changes in the frequency of light or sound waves as the source and observer move relative to each other.

stellar parallax

The apparent shift of an object against a distant background when viewed from different positions.

circumference of orbits

The complete distance around the orbit of a planet.