Astronomy Chpt 2 Vocab

angular size

The angle between lines drawn from the observer to opposite sides of an object

ellipse

A geometric figure related to a circle but flattened into an oval shape.

epicycle

Imaginary, small, and circular orbit superimposed on another circular orbit, proposed by early astronomers to explain the retrograde motion of the planets.

focus

One of two points within an ellipse used to generate the elliptical shape.

geocentric models

Models of the solar system centered on the Earth.

heliocentric models

Models of the solar system centered on the Sun.

moon illusion

The illusion in which the Moon appears larger when near the horizon than when seen high in the sky.

parallax

The shift in an object's position caused by the observer's motion; a method for finding distances based on that shift.

period

The time required for a repetitive process to repeat.

retrograde motion

The drift of a planet westward against the background stars.

satellite

A body orbiting a larger body.

semimajor axis

Half the long dimension of an ellipse.

kepler’s 1st law

Planets move in elliptical orbits with the Sun off-center at a focus of the ellipse.

kepler’s 2nd law

A line joining the planet and the Sun sweeps out equal areas in equal times.

kepler’s 3rd law

Relates a planet's orbital period to the semimajor axis of its elliptical orbit; P^2 = a^3, if P is measured in years and a in astronomical units.