Absolute Magnitude - The magnitude an object would have if it were 10 parsecs away.

Annular Eclipse - A solar eclipse in which a ring of solar photosphere still remains visible.

Aphelion - The farthest point from the sun in a planet's orbit.

Apparent Magnitude - The magnitude an object has for a visual observer.

Astronomical Unit (A.U.) - The average distance from the earth to the sun.

Autumnal equinox - The point at which the ecliptic intersects the celestial equator when the sun moves into southern declinations.  At this time the sun is directly overhead at the equator and day and night are of equal lengths.  Around September 23rd.  (see vernal equinox)

Celestial equator - The earth's equator projected into space.

Conjunction - The alignment of two planets (or anything else) which occurs when they reach the same celestial longitude.

Declination - Latitude, projected onto the sky.  It is typically measured in degrees, minutes and seconds north (+) or south (-) of the celestial equator.  For example, the north star is at +90 dec.  With right ascension it is used to locate stars.

Double (or binary) star - Two or more stars in close proximity either visually (they only appear near each other) or actually (they orbit each other).

Lunar eclipse - The movement of the moon into earth's shadow.

Ecliptic - The path the sun (and somewhat the moon and planets) follows across the sky throughout the year.

Gibbous - A waxing or waning moon larger than a half moon.

Inferior conjunction - A conjunction of the earth with mercury or venus.

Light year - The distance light travels in one year.  A light year is 5,865,696,000,000 miles

Magnitude - A logarithmic scale of brightness used for objects in the sky.  Each increase in magnitude is a decrease in brightness by a factor of 2.5.

Meridian - A circle projected into the sky which passes through each of earth's poles and your zenith (the point directly overhead).  (A circle at a "right angle" to the celestial equator from the observer's perspective).

Occultation - The hiding of one celestial body by another.

Parsec - The distance from which 1 astronomical unit covers 1 second of arc.

Perihelion - The closest point to the sun in a planets orbit.

Precession - The wobble of the earth on its axis which takes about 26,000 years to complete.

Right ascension - Longitude, projected onto the sky.  Measured in hours, minutes and seconds.  Used with declination to locate stars.

Solstice - the position of the sun when it reaches its northern most declination or southern most declination.  The longest and shortest day of the year, respectively, around June 22nd and December 22nd.

Transit - the passage of Mercury or Venus in front of the sun, or the passage of a planet's moon in front of the planet, as seen from earth.

Vernal equinox - The point at which the ecliptic intersects the celestial equator when the sun moves into northern declinations.  At this time the sun is directly overhead at the equator and day and night are of equal lengths.  Around March 21st.  (see autumnal equinox)

Zenith - the point directly overhead of an observer's position, 90 degrees above the horizon.

Zodiac - a set of 12 constellations through which the sun and planets pass in the course of a year.