retrograde motion
the apparent westward drift of the planets with respect to the stars
sidereal day
the time interval it takes for earth to make one complete rotation with respect to a star other than the sun and is 23 hours, 56 minutes and 4 seconds
heliocentric
-aristarchus
-sun centered universe
sidereal month
-the true period of the moons revolution around earth
-27 1/3 days
geocentric
belief that earth was a sphere that stayed motionless at its center
rotation
turning or spinning of a body on its axis
astronomical unit
unit that is the average distance from earth to the sun, about 150 kilometers
solar eclipse
when the moon moves in a line directly between earth and the sun, it casts a dark shadow on earth
declination
the angular distance north or south of the celestial equator denoting the position of a stellar body
perturbation
any variance in the orbit of a body from its predicted path
perihelion
the pint in the orbit of a planet where it is closest to the sun
ecliptic
the apparent annual path of the sun against the backdrop the celestial sphere
celestial sphere
imaginary, transparent, hollow sphere on which the ancients believed the stars traveled daily around earth
constellation
an apparent group of stars originally named in honor of mythological character or great hero
plane of the ecliptic
the imaginary plane that connects earths orbit with the celestial sphere
ptolemaic system
earth centered model of the universe proposed in a.d. 141 that uses epicycles and deferents to describe a planets motion
revolution
the motion of a body, such as a planet or moon, along a path around some point in space
phases of the moon
refers to the progression of changes in the moons appearance during the month
right ascension
the angular distance of a stellar object measured eastward along the celestial equator from the position of the vernal equinox
synodic month
the cycle of the moon through its phases requires 29 1/2 days
lunar eclipse
-during the full moon phase
-when the moon moves within the shadow of earth
equatorial system
a method used to locate stellar objects that is very similar to the latitude-longitude system used on earths surface
aphelion
the point in orbit of a planet where it is farthest from the sun
precession
the very slight movement of earths axis over a period of 26,000 years
mean solar day
-the time interval from one noon to the next
-about 24 hours