Celestial Sphere

Constellation

A recognized pattern of stars in the sky, historically named after mythological figures or objects, used to map regions of the celestial sphere.

Zodiac

The band of constellations along the ecliptic through which the Sun, Moon, and planets appear to move.

Astrology

An unscientific belief system that suggests positions of celestial bodies influence human affairs.

North Celestial Pole

The point in the sky directly above Earth's North Pole, near Polaris.

South Celestial Pole

The point in the sky directly above Earth's South Pole.

Celestial Equator

The projection of Earth's equator onto the celestial sphere, dividing it into northern and southern halves.

Ecliptic

The apparent annual path of the Sun across the celestial sphere, tilted ~23.5° to the celestial equator.

Local Sky

The sky as seen from a specific location on Earth, described using coordinates like altitude and azimuth.

Horizon

The line where the sky appears to meet the ground from an observer's point of view.

Zenith

The point directly overhead an observer.

Azimuth

A horizontal angle measured clockwise from due north, specifying the direction of an object.

Altitude

High high above the horizon an object is seen, measured in degrees.

Circumpolar

Describes stars that never set below the horizon for a given latitude, circling the celestial pole.

Latitude

The angular distance of a location north or south of Earth's equator.

Longitude

The angular distance of a location east or west of the prime meridian.

Meridian

The imaginary north-south line on the celestial sphere that passes through the zenith and celestial poles. When a star crosses here, it is at its highest altitude and best observed.

Declination

Angular distance of an object north or south of the celestial equator.

Right Ascension

Angular distance of an object east of the vernal equinox, measured in hours, minutes, and seconds.

Summer Solstice

The point when the Sun reaches its farthest northern position on the ecliptic.

Winter Solstice

The point when the Sun reaches its farthest southern position on the ecliptic.

Spring Equinox

The point when the Sun crosses the celestial equator moving northward, with nearly equal day and night.

Fall Equinox

The moment the Sun crosses the celestial equator moving southward, with nearly equal day and night.

Precession

The slow, roughly 26,000-year wobble of Earth's axis that shifts the orientation of the celestial poles and equinoxes.

Apparent Retrograde Motion

The temporary westward motion of a planet against the background stars, caused by Earth overtaking it in orbit.

Apparent Magnitude

A measure of a star's brightness as seen from Earth, with lower numbers meaning brighter appearance.

Absolute Magnitude

A measure of a star's true luminosity, defined as how bright it would appear if placed 10 parsecs away.