**Astronomy**—Astronomy is the study of everything that lies beyond the Earth's atmosphere.
**AU**—Astronomical Unit. One AU is equal to the length of the Earth's semi-major axis.

**celestial sphere**—An imaginary sphere around the Earth that all the stars and planets are placed on.

**meridian**—A circle on the celestial sphere that passes through the two poles and the zenith of your position.

**nadir**—The point on the celestial sphere directly opposite the zenith and underneath your feet.

**zenith**—The point on the celestial sphere directly above your head.

**geocentric universe**—A model of the universe with the Earth at the center.

**heliocentric solar system—**A model of the universe with the sun at the center.

**ion—**An atom that has lost or gained electrons.

**Kepler's laws—**Three laws of planetary motion based on the observation that planets orbit the sun in ellipses.

**Law one—**The orbits of planets are ellipses, with the sun at one focus.

**Law two—**An imaginary line from a planet to the sun will sweep over equal areas of the ellipse in equal intervals of time.

**Law three—** A planet's period squared is proportional to a^{3}. *A* is the length of a planet's semi-major axis.

**Newton's law of gravitation—**The force of gravity between any two objects in the universe is equal to the mass of the first object (m1), multiplied by the mass of the second object (m2), multiplied by a gravitational constant (G), all divided by the square of the distance between the two objects.

**Newton's laws of motion—**Three laws of motion that give a more fundamental understanding of the universe than Kepler's laws.

**Law one—** All objects at rest stay at rest. All objects in motion stay in motion, in a straight line and at a constant speed, unless acted upon by a force.

**Law two—** Force equals mass times acceleration, or F = m x a.

**Law three—** For every force one body exerts on a second body, the second exerts an equal and opposite force on the first.

**period—**The amount of time (in Earth years) it takes a planet to orbit the sun once.

**right-hand rule—**A rule to help you determine which way is north on a planet. Point your fingers in the direction the planet rotates. Your fingers should curl around the planet, like you are grabbing a ball. Then stick out your thumb. That way is north.

**retrograde motion—**Backwards (westwards) motion of a planet with respect to the stars.

**revolutional period—**The length of time is takes a planet or moon to cycle around the object that it orbits. Earth's revolutional period is 365 days.

**rotational period—**The length of time it takes a planet or moon to rotate 360 degrees on its axis. Earth's rotational period is 24 hours.

**semi-major axis—**Half the length of the longest diameter of an ellipse.

**spectrum—**The range of visible light: red, orange, yellow, green, blue, indigo, and violet.