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Astronomy (as-tron´om-i). The science which treats of the heavenly bodies, explaining the motions, times and causes of the motions, distances, magnitudes, gravities, light, etc., of the sun, moon, and stars, the nature and causes of the eclipses of the sun and moon, the conjunction and apposition of the planets, and any other of their mutual aspects, with the times when they did or will happen.

Aberration (ab-er-ā´shun). A small apparent motion of the fixed stars, occasioned by the progressive motion of light and the earth’s annual motion in its orbit. By this they sometimes appear twenty seconds distant from their true situation.

Amplitude (am´pli-tud). An arc of the horizon intercepted between the true east and west points and the center of the sun, or a star at its rising or setting.

Anomaly (an-om´al-i). The angular distance of a planet from its perihelion, as seen from the sun; either true, mean, or eccentric.

Aphelion (af-ēl´yun). That point of a planet’s orbit which is most distant from the sun.

Apogee (ap´o-jē). That point in the orbit of the moon which is at the greatest distance from the earth.

Apparition (ap-par-ish´un). The first appearance of a star or other luminary after having been obscured.

Ap´pulse. The approach of a planet towards a conjunction with the sun or any of the fixed stars.

Apsis (ap´sis). The two points of a planet’s orbit in which it is at its greatest and least distance from the sun.

Aquarius (a-kwā´ri-us). The eleventh sign of the zodiac, which the sun enters about the 21st of January.

Asteroids (as´ter-oids). The small planets that circulate between the orbits of Mars and Jupiter.

Ax´is (ax´is). The imaginary line passing through the center and poles of the earth, on which it performs its diurnal revolutions from west to east.

Azimuth (az´im-uth). An arc of the horizon intercepted between the meridian of the place and the vertical circle passing through the center of a celestial object.

Can´cer. The fourth sign of the zodiac, being that of the summer solstice, which the sun enters about the 21st of June.

Capricorn (kap´ri-korn). The tenth sign of the zodiac, which the sun enters about the 21st of December, at the winter solstice.

Colure (kol´ur). Two great circles, supposed to intersect each other at right angles in the poles of the world, one of them passing through the solstitial and the other through the equinoctial points of the ecliptic, viz., Cancer and Capricorn, Aries and Libra, dividing the ecliptic into four equal parts.

Coma (kō´ma). A dense, nebulous covering, which surround the nucleus or body of a comet.

Com´et. A member of the solar system, commonly consisting of three parts: the nucleus, the envelope or coma, and the tail; but one or more of these parts is frequently wanting.

Conjunc´tion. The meeting of two heavenly bodies in the same point or place in the heavens.

Constella´tion. A number of stars which appear as if situated near each other in the heavens, and are considered as forming a particular division.

Cynosure (sin´o-shōōr or sī´). A name of the constellation Ursa Minor, or the Lesser Bear, which contains, in the tail, the pole star by which mariners are guided.

Declination (dek-lin-a´shun). Distance of any object from the celestial equator, either northward or southward.

Disk. The face or visible projection of a celestial body, usually predicated of the sun, moon, or planets; but the stars have also apparent disks.

Eclipse´. An obscuration or interception of the light of the sun, moon, or other luminous body.

Eclip´tic. The great circle of the heavens which the sun appears to describe in his annual revolution.

Equa´tor. The great circle of the sphere, equally distant from the two poles of the world, or having the same poles as the world.

Equinox (ē´kwi-noks). The precise time when the sun enters one of the equinoctial points, making the day and night of equal length.

Faculae (fa´ku-lē). Certain spots sometimes seen on the sun’s disk, which appear brighter than the rest of his surface.

Fixed Stars. Those which retain the same or very nearly the same position with respect to each other.

Gal´axy. The Milky-Way.

Gemini (jem´i-nī). The third sign or constellation in the zodiac, which the sun enters about the 21st of May.

Geocentric (jē-o-sen´trik) Par´allax. The apparent change of a body’s place that would arise from a change of the spectator’s station from the surface to the center of the earth.

Ha´lo. A luminous circle, usually prismatically colored round the sun or moon, and supposed to be caused by the refraction of light through crystals of ice in the atmosphere.

Heliocentric (hē-li-o-sen´trik) Par´allax. The arc of the great circle of the celestial sphere, drawn from the heliocentric to the geocentric place of a body.

Heliometer (hē-li-om´e-ter). An instrument for measuring with exactness the apparent diameter of the sun, moon, planets, etc.

Hori´zon. A circle touching the earth at the place of the spectator, and bounded by the line in which the earth and skies seem to meet.

Le´o (Lat., the Lion). The fifth sign of the zodiac which the sun enters about the 22d of July.

Libra (lī´bra), the Balance. The seventh sign of the zodiac, which the sun enters at the autumnal equinox, in September.

Luna´tion. The period of a revolution of the moon round the earth, or the time from one new moon to the next.

Maculae (mak´u-lē). Dark spots on the surfaces of sun and moon, and on some of the planets.

Moon. A secondary planet or satellite of the earth, whose light, borrowed from the sun, serves to dispel the darkness of night.

Nadir (nā´dir). The point of the heavens or lower hemisphere directly opposite the zenith.

Neb´ulae (neb´u-lē). Misty appearances among the stars, usually, but not always, resolved by telescope into myriads of small stars.

Nodes (nōdes). The two points in which the orbit of a planet intersects the ecliptic.

Nuta´tion. A vibratory motion of the earth’s axis, arising from periodical fluctuations in the obliquity of the ecliptic.

Occulta´tion. The hiding of a heavenly body from our sight by the intervention of some other of the heavenly bodies.

Or´bit. The path described by a heavenly body in its periodical revolution.

Par´allax. The change of place in a heavenly body in consequence of being viewed from different points.

Penum´bra. A partial shadow or obscurity on the margin of the perfect shadow in an eclipse, or between the perfect shadow, where the light is entirely intercepted, and the full light.

Perigee (per´i-jē). That point in the orbit of the sun or moon in which it is at the least distance from the earth.

Perihelion (per-i-hē´li-on). That part of the orbit of a planet or comet in which it is at its least distance from the sun.

Plan´et. The name given to a few bright and conspicuous stars which are constantly changing their apparent situations in the celestial sphere.

Precession (pre-sesh´un) of the Equinoxes. A continual shifting of the equinoctial points from east to west.

Radius Vector. An imaginary line joining the center of the sun and the center of a body revolving about it.

Retrocession (rē-tro-sesh´un) of the Equinoxes. The going backward of the equinoctial points.

Sagittarius (saj-i-tā´ri-us). One of the twelve signs of the zodiac, which the sun enters about November 22.

Sat´ellite. A small planet revolving round another planet.

Scor´pio. The eighth sign of the zodiac, which the sun enters about October 23.

Selenography (sel-en-og´raf-i). The description of the surface of the moon.

Sign. The twelfth part of the ecliptic.

Solstice (sol´stis). The time when the sun, in its annual revolution, arrives at that point in the ecliptic farthest north or south of the equator, or reaches its greatest northern or southern declination.

Star. An apparently small, luminous body in the heavens, that shines in the night, or when its light is not obscured by clouds or lost in the brighter effulgence of the sun.

Sun. The central body of our system, about which all the planets and comets revolve, and by which their motions are regulated and controlled.

Taurus (taw´rus). The second sign of the zodiac, which the sun enters about the 20th of April.

Virgo (ver´go). The sixth sign of the zodiac, which the sun enters in August.

Ze´nith. The point in the heavens directly overhead.