
Astronomical unit
the avg distance between the earth and the sun (93 million miles)

Light Year
the distance light travels in a year (6 trillion miles)

Asterism
an unofficial star picture

Constellation
88 different regions of the night sky

circumpolar
stars that never set ex: cassiopeia

seasonal
stars that do rise and set ex: Orion

angular size
Diameter, width

angular seperation
the distance between two points

degree
unit of measurement for angles, 60 arc minutes

arc minute
unit of measurement for angles, 60 arc seconds,

arcsecond
unit of measurement for angles, 60 arc seconds go into an arc minute

celestial sphere
the imaginary sphere of celestial objects that seem to center on earth

N + S celestial Poles
projections of the earth axis on the celestial sphere

celestial equator
projection of earth's equater on the celestial sphere

ecliptic
apparant path of the sun across the celestial sphere

zodiac
the set of constellations the ecliptic passes through

Prime Meridian
0 degrees longitude located in greenwhich england

horizon
The line at which the earth's surface and the sky appear to meet

zenith
highest point in the sky

altitude
the distance north or south a point is from the celestial equator

solstice
the longest or shortest day of the year

equinox
2 days of the year when there is exactly 12 hours of daylight and 12 hours of night

latitude
measure of how far north or south you are from the equator

longitude
the measure of how far east or west you are from the prime meridian

retrograde motion
when planets appear to be moving backwards from the path across the ecliptic

elongation
the angle between a planet and the sun

precession
one of the three apparant motions of the earth; known as the wobble of the earths axis



synodic period
when the moon is lined up with the sun from the earths point of view which takes 29.5 days between each one

sidereal period
true orbital period 27.3 days for the moon

solar day
noon to noon 24 hours

sidereal day
true rotational period; 23 hours 56 min

synchronous rotation
the moons orbit, where the rotational period and orbital period are equal

tidal locking
when one side of a body is always facing another, ie the moon to the earth

eclipse season
occurs approx every 6 months when the line of nodes points to the sun

node
the points where the moon's orbit crosses the ecliptic



Annular Solar Eclipse
when the moon is smaller than the sun and doesnt completely block out the sun. a ring is usually visible around the moon

epicycle
a circle in which a planet moves around; suggested by ptolemy to explain retrograde

deferent
A larger circle in which the epicycle revolves around

geocentric
the theory of the earth being at the center of the universe

heliocentric
the sun being at the center of the earth

ellipse
the shape in which the planets travel around the sun, not a complete circle


