Planetarium: An optical device for projecting on a screen or domed ceiling the stars and planets and their apparent motions in the sky.

Observatory: Domed structure enclosing telescopes.

Axis: A straight line, real or imaginary, passing through a body, on which it rotates.

Rotation: Turning of a body about an imaginary axis running through it.

Revolution: The motion of one body around another.

Horizon (astronomical): A great circle of the celestial sphere 90 degrees from the zenith. [The imaginary line where the sky appears to meet the land. The horizon can be divided into 360 degrees of azimuth.]

Zenith: The point on the celestial sphere opposite to the direction of gravity; or the direction opposite to that indicated by a plumb bob. [The imaginary point directly overhead.]

Cardinal Points: The four principal points of the compass: North, East, South, and West.

Meridian (celestial): The great circle on the celestial sphere that passes through an observer's zenith and the north (or south) celestial pole.

Meridian (terrestrial):The great circle on the surface of the Earth that passes through a particular place and the north and south poles of the Earth. [A line of longitude.]

Sunrise: The event or time of the daily first appearance of the Sun (upper limb) above the eastern horizon.

Sunset: The event or time of the daily disappearance of the Sun (upper limb) below the western horizon.

a.m.: ante meridiem (Latin), that is, before the Sun crosses the meridian (before noon).

p.m.: post meridiem (Latin), that is, after the Sun crosses the meridian (after noon).

Phases of the Moon: The progression of changes in the Moon's appearance during the month that results from the Moon's turning different portions of its illuminated hemisphere to our view.

New Moon: Phase of the Moon when it's longitude is the same as that of the Sun.

Waxing Crescent: One of the phases of the Moon when its elongation is less than 90 degrees from the Sun and it appears to be less than half full (the right side is illuminated).

First Quarter: One of the two phases of the Moon when its longitude differs by 90 degrees from that of the Sun; the Moon appears half full at these phases (the right half is illuminated).

Waxing Gibbous: One of the phases of the Moon in which more than half, but not all, of the Moon's daylight hemisphere is visible from the Earth (the right side is illuminated).

Full Moon: The phase of the Moon when it is at opposition (180 degrees from the Sun) and its full daylight hemisphere is visible from the Earth.

Waning Gibbous: One of the phases of the Moon in which more than half, but not all, of the Moon's daylight hemisphere is visible from the Earth (the left side is illuminated).

Last Quarter: One of the two phases of the Moon when its longitude differs by 90 degrees from that of the Sun; the Moon appears half full at these phases (the left half is illuminated).

Waning Crescent: One of the phases of the Moon when its elongation is less than 90 degrees from the Sun and it appears to be less than half full (the left side is illuminated).

Waxing: To grow.

Waning: To shrink.

Terminator: The line of sunrise or sunset on a moon or a planet.

Month: Calendar time unit derived from the approximate time for the Moon to go through one cycle of phases.

Blue Moon: The infrequent occurrence of two Full Moons during the same calendar month.

Solar Eclipse: The cutting off of all or part of the light from the Sun by the Moon passing in front of it.

Lunar Eclipse: The cutting off of all or part of the sunlight reflected from the Moon by the Earth passing between the Sun and the Moon.

Total Eclipse: An eclipse of the Sun in which the Sun's photosphere is entirely hidden by the Moon, or an eclipse of the Moon in which it passes completely into the umbra of the Earth's shadow.

Annular Eclipse: An eclipse of the Sun in which the Moon is too distant to appear to cover the Sun completely, so that a ring of sunlight shows around the Moon.

Partial Eclipse: An eclipse of the Sun or Moon in which the eclipsed body does not appear completely obscured.

Umbra: The central, completely dark part of a shadow.

Penumbra: The portion of a shadow from which only part of the light source is occulted by an opaque body.

Ocean Tides: The deformation of the ocean surface by the differential gravitational forces exerted by the Moon and the Sun.

Barycenter: The center of mass of two mutually revolving bodies.

High Water (High Tide): The maximum height reached by a rising tide. The high water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions.

Low Water (Low Tide): The minimum height reached by a falling tide. The low water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions.

Tidal Range: The difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water. The great diurnal range or diurnal range is the difference in height between mean higher high water and mean lower low water.

Spring Tide: The highest ocean tidal range of the month, produced when the Moon is near either the Full or New phase.

Neap Tide: The lowest-ranging ocean tide during the month, which occurs when the Moon is near First or Last Quarter.

Seasons: Winter, Spring, Summer, or Fall (all you gotta do is call)

Equinox: One of the intersections of the ecliptic and the celestial equator. [One of two dates when the noon Sun would be at the zenith along the Equator of the Earth. Literally means, "equal nights," as in the number of hours of daylight and darkness are approximately equal.]

Vernal Equinox: The point on the celestial sphere where the Sun crosses the celestial equator passing from south to north. [The Spring Equinox. "Vernal" literally means greening.]

Autumnal Equinox: The intersection of the ecliptic and celestial equator where the Sun crosses the equator moving from north to south. [The Fall Equinox.]

Solstice: Either of two points on the celestial sphere where the Sun reaches its maximum distances north and south of the celestial equator. [Literally means, "Sun stop," as in the line of latitude on the Earth's surface along which the noon Sun would be at the zenith stops moving northward or southward.]

Inclination or Tilt (of an orbit): The angle between the orbital plane of a revolving body and some fundamental plane, usually the plane of the celestial equator or of the ecliptic. For example, the Earth's axis is inclined 23.5° with respect to the plane of its orbit around the Sun.

Star: A self-luminous sphere of gas (mostly hydrogen and helium).

Sun: The star about which the Earth and other planets revolve.

Planets: Any of the nine solid bodies revolving about the Sun.

Moon: A smaller body which revolves around a planet.

Constellation: A configuration of stars named for a particular object, person, or animal; or the area of the sky assigned to a particular configuration.

Ursa Major (Big Dipper):

Pointer Stars: The two stars in the end of the cup of the Big Dipper (Merak and Dubhe) which point toward Polaris (the North Star).

Ursa Minor (Little Dipper):

Polaris (North Star): A star approximately at the north celestial pole. Polaris is now the pole star; there is no south pole star. Polaris is the moderately bright star at the end of the handle of Ursa Minor (Little Dipper).

Cassiopeia:

Circumpolar: The portion of the celestial sphere near the north celestial pole that is always above the horizon.

Zodiac: A belt around the sky 18 degrees wide centered on the ecliptic.

Astrology: The pseudoscience that deals with the supposed influences of the configurations and locations in the sky on the Sun, Moon, and planets on human destiny; a primitive religion having its origin in ancient Babylonia.

Ecliptic: The apparent annual path of the Sun on the celestial sphere.

Milky Way: The band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of our galaxy.