2.9: Key Terms

• asteroid: a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets
• celestial equator: a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator
• celestial poles: points about which the celestial sphere appears to rotate or intersections of the celestial sphere with Earth’s polar axis
• celestial sphere: the apparent sphere of the sky or a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere
• circumpolar zone: those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon
• comet: a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun some of its material vaporizes, forming a large head of tenuous gas and often a tail
• declination: the angular distance north or south of the celestial equator
• ecliptic: the apparent annual path of the Sun on the celestial sphere
• equator: the circle at 0° latitude that divides Earth into the Northern and Southern Hemispheres
• equinox: the two points on the celestial sphere at which the ecliptic intersects the celestial equator
• great circle: a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center
• horizon: a great circle on the celestial sphere 90° from the zenith or the circle around us where the dome of the sky meets Earth
• latitude: angular distance north or south of Earth's equator measured in degrees along a meridian
• longitude: angular distance to a position on a meridian measured east or west and expressed in degrees from the Prime Meridian
• lunar eclipse: an eclipse of the Moon in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon
• meridian: a great circle on the terrestrial or celestial sphere that passes through the poles
• phases of the Moon: the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle from new moon to full moon and back to new moon
• planet: any of the larger objects orbiting around the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars
• right ascension: the coordinate for measuring the east-west positions of celestial bodies or the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body
• sidereal day: Earth’s rotation period as defined by the positions of the stars in the sky or the time between successive passages of the same star through the meridian
• sidereal month: the period of the Moon’s orbit around Earth measured with respect to the stars
• solar day: Earth’s rotation period as defined by the position of the Sun in the sky or the time between successive passages of the Sun through the meridian
• solar eclipse: an eclipse of the Sun by the Moon caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon
• solstice: the two times of the year when the Sun is at its greatest angular distance from the celestial equator
• synchronous rotation: when an object rotates at the same rate that it orbits around another body
• zenith: the point on the celestial sphere opposite the direction of gravity or the point directly above the observer
• zodiac: a belt around the sky about 18° wide centered on the ecliptic