Astronomical Unit:
A unit of length equal to the average distance between Earth and the Sun. Scientists use it because distances could get to be too big otherwise. 1 astronomical unit = 149,597,870 kilometers. Often abbreviated as A.U.

Christy, James W.:
Discovered the moon of Pluto, Charon in 1978 at the U.S. Naval Observatory nearby Flagstaff.

density:
Mass per unit of volume. Often measured in grams per cubic centimeters.

double planet:
definition to come later

(kilo)meter:
A unit of measurement equal to 3.336342068853e-9 the distance travelled by light in one second. One thousand meters equal one kilometer.

Lowell, Percival (1855-1916):
American astronomer, born in Boston. Best known for his belief that there was or had been life on Mars, as evident in the canals he believed were present. Also known for founding the Lowell Observatory in Flagstaff, Arizona and predicting the existance and location of Pluto.

satellite:
A natural body (i.e. moon) or mechanical device launched from Earth which circles around a planet.

synchronous orbit:
When a satellite takes the same amount of time to rotate and orbit around its planet. When this occurs, the same face of the satellite is facing the same face of the planet.

telescope:
An instrument used to enlarge small and distant objects. Refracting telescopes use lens. Reflecting telescopes use concave mirrors to magnify.

Tombaugh, Clyde W. (1906- ):
American astronomer, born near Streator, Illinois. Discovered the planet Pluto at the Lowell Observatory using information collected by Percival Lowell in 1930.