ASTRONOMICAL GLOSSARY

MIDNIGHT: Astronomically, the time when the Sun is opposite its highest point in the sky. Both 12 hours before and after noon (so, technically, both a.m. and p.m.), midnight in civil time is usually treated as the beginning of the day. It is displayed as 12:00 a.m. on 12-hour digital clocks. On a 24-hour cycle, 00:00, not 24:00, usually indicates midnight.

MOON ON EQUATOR: The Moon is on the celestial equator.

MOON RIDES HIGH/RUNS LOW: The Moon is highest above or farthest below the celestial equator.

MOONRISE/MOONSET: When the Moon rises above or sets below the horizon.

MOON'S PHASES: The changing appearance of the Moon, caused by the different angles at which it is illuminated by the Sun. First Quarter: Right half of the Moon is illuminated. Full: The Sun and the Moon are in opposition; the entire disk of the Moon is illuminated. Last Quarter: Left half of the Moon is illuminated. New: The Sun and the Moon are in conjunction; the Moon is darkened because it lines up between Earth and the Sun.

MOON'S PLACE, Astronomical: The position of the Moon within the constellations on the celestial sphere at midnight. Astrological: The position of the Moon within the tropical zodiac, whose twelve 30° segments (signs) along the ecliptic were named more than 2,000 years ago after constellations within each area. Because of precession and other factors, the zodiac signs no longer match actual constellation positions.

MORNING STAR: A planet that is above the eastern horizon at sunrise and less than 180° west of the Sun in right ascension.

NODE: Either of the two points where a celestial body's orbit intersects the ecliptic. Ascending: When the body is moving from south to north of the ecliptic. Descending: When the body is moving from north to south of the ecliptic.

OCCULTATION (OCCN.): When the Moon or a planet eclipses a star or planet.

OPPOSITION: The Moon or a planet appears on the opposite side of the sky from the Sun (elongation 180°).

PERIGEE (PERIG.): The point in the Moon's orbit that is closest to Earth.

PERIHELION (PERIH.): The point in a planet's orbit that is closest to the Sun.

PRECESSION: The slowly changing position of the stars and equinoxes in the sky caused by a slight wobble as Earth rotates around its axis.

RIGHT ASCENSION (R.A.): The celestial longitude of an object in the sky, measured eastward along the celestial equator in hours of time from the vernal equinox; comparable to longitude on Earth.

SOLSTICE, Summer: When the Sun reaches its greatest declination (23½°) north of the celestial equator, around June 21. Winter: When the Sun reaches its greatest declination (23½°) south of the celestial equator, around December 21.

STATIONARY (STAT.): The brief period of apparent halted movement of a planet against the background of the stars shortly before it appears to move backward/westward (retro grade motion) or forward/eastward (direct motion).

SUN FAST/SLOW: When a sundial is ahead of (fast) or behind (slow) clock time.

SUNRISE/SUNSET: The visible rising/setting of the upper edge of the Sun's disk across the unobstructed horizon of an observer whose eyes are 15 feet above ground level.

TWILIGHT: See page 106. ■

Note: These definitions apply to the Northern Hemisphere; some do not hold true for locations in the Southern Hemisphere.