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VOCABULARY OF DEFINITIONS, &c.

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Altair, a star otherwise called a Aquilæ. Altazimuth Instrument. See p. 677. Altitude (altitudo, height); the angular elevation of a heavenly body above the horizon measured towards the zenith on any great circle.

Alwaid, a star otherwise called B Draconis.

Amplitude (amplitudo, largeness); the

angular distance of a heavenly body from the east or west point of the horizon, measured on the horizon. Angle (angulus, a corner); the inclination of two straight lines to each other in the same plane gives rise to a plane rectilineal angle, or simply an angle. When a straight line, standing on another straight line, makes the adjacent angles equal to each other, each of those angles is called a right angle; but if the adjacent angles are not equal to each other, then the greater is called an obtuse and the lesser an acute angle.

Angle of eccentricity. See p. 31. Angle of position (chiefly used with reference to double stars). This is the angle formed by a line joining two stars with the meridian line which passes through the larger one. It is reckoned from 0 to 360 from the north point of the field of the telescope, by east, south, and west, round to north again.

Angle of situation; a term introduced by Sir J. Herschel to indicate the angle formed at any star by arcs passing through the zenith and the Pole respectively. This angle, sometimes called the parallactic angle, was formerly known as the angle of position,' but it has been thought best to limit this term as above.

Angular velocity of any heavenly body is the rate of increase or decrease of the angle contained between the radius vector of the body and a fixed straight line.

Annual equation. See p. 72.

Annual Variation in the right ascension or declination of a star is the change produced in either element by the combined effects of precession of the equinoxes and proper motion. Annular (annulus, a ring); a term applied to a certain kind of solar eclipse, because the appearance presented is a ring of light. See p. 169.

Anomalistic Period. The time of revolu

tion of a planet in reference to its line of apsides (q. v.) In the case of the Earth, the period is called the anomalistic year. See p. 424. Anomaly (a not, and duaλós even), eccentric. "An auxiliary angle employed to abridge the calculations connected with the motion of a planet or comet in an elliptic orbit. If a circle be drawn, having its centre coincident with that of the ellipse, and a diameter equal to the transverse [major] axis of the latter, and if from this axis a perpendicular be drawn through the true place of the body in the ellipse to meet the circumference of the circle, then the eccentric anomaly will be the angle formed by a line drawn from the point where the perpendicular meets the circle, to the centre, with the longer diameter of the ellipse."-Hind. Anomaly, mean. The angular distance of a planet or comet from the perihelion or aphelion, supposing it to have moved. with its mean velocity.

Anomaly, true. The true angular distance of a comet or planet from perihelion or aphelion.

Ansa ansa, a handle); a term applied to those opposite extremities of the ring of Saturn which, viewed foreshortened from the Earth, appear as projec tions or handles to the ball. Antares (avri opposed to, "Apns Mars, i. e. rivalling Mars), a red star, otherwise called a Scorpii.

Aphelion (and from, Aios the Sun); that

point in the orbit of a planet or comet farthest from the Sun. See p. 34. Apogee (and from, y the Earth); the correlative of the preceding, applied in three general senses, (1) to that point of the Moon's orbit farthest from the Earth; (2) to that point of the Earth's orbit farthest from the Sun; and (3) to that point of any planetary or cometary orbit farthest from the Earth. Apparent (ad to, and parere to appear);

employed astronomically as the opposite to true or real. Thus, the apparent sunset differs from the real sunset in consequence of the effect of refraction. The apparent equinox differs from the real equinox in conse

quence of the effect of nutation. The apparent position of a star differs from the real position in consequence of the effect of refraction, aberration, nutation, &c.

Apsis (ais, a curve); applied to the opposite extremities of a planetary or cometary orbit, which are also its perihelion and aphelion points. So the line of Apsides is the line joining these two points, which is also the major-axis of the ellipse.

Arcturus, a star otherwise called a Boötis. Argument (argumentum, a thing taken

for granted) is a term used to denote any mathematical quantity by which another may be found.

Aries, first point of; the origin or station from which are reckoned right ascensions on the equator and longitudes on the ecliptic.

Arneb, a star otherwise called a Leporis. Ascension, oblique. The oblique ascension is the arc of the equator between the first point of Aries and the point of the equator which rises with a heavenly body, reckoned forwards according to the order of the signs. Ascension, right. The right ascension of a heavenly body is its distance from the first point of Aries reckoned on the equator. It is so called because in a right sphere the meridian passing through the object will coincide with the horizon when the object is rising or setting. Ascensional Difference is the difference between the right and oblique ascensions.

Astrology (aorhp a star, and Ayos a word); identical with astronomy in grammatical meaning, but conventionally applied to the science (or delusion) of fortune-telling by aid of the stars. Astrometry (dorp a star, and μérpov a measure); conventionally applied to the measurement of the apparent magnitudes of stars. Photometry (q. v.) is used in a similar sense. Astronomy (dorp a star, and vouos a law); the science of the laws of stars. Augmentation of the Moon's semi-diameter is the increase due to the distance of the Moon from the observer being less than its distance from the centre of the Earth, to which calculations are referred.

Axis (awv an axle); an imaginary line joining the north and south poles of a planet, and upon which it is assumed to revolve. Azimuth (samatha to go towards, Arabic); the angular distance of an object from the north or south points of the hori

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Calendar, or kalendar (kalendæ, the first day of every month), is the name applied to the tabular statement of a system of reckoning time.

Canopus, a star otherwise called a Argûs. Capella, a star otherwise called a Auriga. Castor, a remarkable binary star, otherwise called a Geminorum.

Celbalrai, a star otherwise called 8 Ophiuchi.

Chaph, a star otherwise called B Cassiopeiæ.

Circle, Mural. See p. 678. Circle, Transit. See p. 650. Circumpolar (circum around, and Polus); the two regions of the heavens lying near the north and south poles respectively. The word is also used to refer to those portions of the sky which at the place of observation are always above the horizon.

Co-latitude, the complement of the latitude or what it wants of 90°. Collimation, line of (cum with, and limes a limit); the optical axis of a telescope. Colures (Koλouw, I cut in the middle) are two great circles passing through the poles of the heavens at right angles to each other. The equinoctial colure passes through the poles and equinox and corresponds to the hour circles of oh and 12h of R. A., and the solstitial colure passes through the poles and solstices and corresponds to the hour circles of 6h and 18h of R. A. Comet (Kouhrns, hairy). See p. 260. Cometography (comet, and ypάow I de

scribe); that branch of astronomy which treats specially of comets.

Compression of the poles (cum together, and premere to squeeze); the amount of the flattening of the polar regions of a planet whereby the polar diameter becomes less than the equatorial. Conjunction (cum together, and jungere to join); two or more heavenly bodies are said to be in conjunction when they

have the same longitude or right ascension. See p. 450. Constant (cum together, and stare to stand); a numerical quantity always of the same value in a mathematical computation or in an astronomical reduction is called a constant. Thus the ratio of the circumference of a circle to its diameter is constantly 31416: 1, so, knowing the diameter, we can always determine the circumference. Constellation (cum together, and stella a star); a group of stars whose outline is conceived to represent in the imagination some mundane object. See P. 464.

Cor Caroli, a star otherwise called a Canum Venaticorum.

Cor Hydra, a star otherwise called a Hydræ.

α

Cor Leonis, a star otherwise called Regulus or a Leonis.

Cor Serpentis, a star otherwise called Unukalhay or a Serpentis. Cosmical (xóouos the world). A heavenly body is said to rise or set cosmically when it rises or sets at sunrise, Culmination (culmen the top); the meridian passage of a heavenly body, which is then at the top of its course. Cursa, a star otherwise called B Eridani. Curtate distance (curtare to shorten) is

the distance of a planet or comet from the Sun or Earth projected upon the plane of the ecliptic.

Cusp (cuspis a sharp point); the extremities of a crescent moon or inferior planet.

D. Declination, the angular distance of a heavenly body from the equator-symbol, 8. According as the declination is north or south, it is + or -8. Degree (degredior to go down; from de down, and gradus a step). The circle is conventionally divided into 360 equal parts, each of which is called a degree. Deneb, a star otherwise called a Cygni. Denebola, a star otherwise called 8 Leonis. Diameter (did through, and μéтρov a measure); the breadth of anything. Dichotomy (dixa in two, and Téμvw I cut); a bisection. Applied to an inferior planet or the moon, it means the moment of quarter phase when the phase is a perfect semi-circle.

Dubhe, a star otherwise called a Ursæ Majoris.

E.

Eccentricity (ex from, and centrum a centre);

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Eutin of epiloma; the diference be tween the mean and apparent places of the equicI

Equation of the centre; the ¿ference be tween the true and meas aizsales f a planet or comet. Equal to 20. (See Get & Eccatrinity, p. 7942 Equation of time; the £ference between

meat and apparent time. See p. 422. Easter equal, equally diving the sphere aierial; an iza mary procegation of the equator of the earth, ann a circle frequently referred

to in astroDGY.

Equatorial, intrument Iterally, an instrument which moves in the equator. See p. 634

Equinze Cage equal, and soz night); tive two points where the ecliptic intersects the equator, so called becase on the Son's arrival at either of them the day is equal to the night throughout the world. See p. 237.

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PRADO 2. the centre of the Su Eanmar

um the NEI, ADÅ AFTNER The BOSE Ase of an inKOKINES WL a thing more than a Tided-ecject-glass micrometrical Hemisphere ve hat and recibe a

sphere, a halfaghere - half the surface of the beaters The celestial equator Cvides the cciestial sphere into the northern and southern halfspheres, or bemİNÇCÆTER

Homum, a star otherwise called (Pegasi. Honiton koji, I band or it The

amalie birixe as that circle of the beavens witch as car view, and whose plane woches the Earth at the place of the cheerver, and is at right angles to the nenith. The rational or tree burice is parallel to the former, and passes through the centre of the Earth.

Hour angle, the distance of a heavenly body from the meridian, expressed in hours, minutes, &c.

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the disappearance of an object before undergoing occultation. Inclination (inclinare, to bend down, to slope); one of the elements of the orbit of a planet or comet-namely, the angle which the plane of the orbit makes with the plane of the ecliptic. Inequalities (in not, and æquus equal); conventionally applied to irregularities in the motions of planets. Izar, a star otherwise called Boötis.

J.

Jovicentric (Jupiter, gen Jovis, and KévTрov a centre); as viewed from, or having relation to, the centre of Jupiter.

K.

Kaus Australis, a star otherwise called Sagittarii.

Kocab, a star otherwise called B Ursæ Minoris.

Korneforos, a star otherwise called B Herculis.

L.

Latitude (latitudo, breadth); the angular distance of a heavenly body from the ecliptic, north or south as the case may be.

Libration (librans, swinging); an (apparent) oscillatory motion of the Moon. See p. 71.

Longitude (longitudo, length); the angular

distance of a heavenly body from the first point of Aries measured in the ecliptica.

Longitude, mean; the longitude of a planet or comet, supposing it to have moved with its mean (average) velocity. Longitude, true; the real angular distance of a planet or comet from the first point of Aries.

Lumière cendrée (French, ash-coloured light). See p. 79.

Lunar (luna, the Moon); having reference to the Moon.

Lunation (lunatio, a change in the Moon); the period of the Moon's revolution round the Earth, in which it goes through all its phases, otherwise called its synodic period.

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Menkalinan, a star otherwise called B Auriga.

Menkar, a star otherwise called a Ceti. Meridian (Meridies, mid-day); the great

circle of the heavens passing through the zenith and the poles. Micrometer (μικρός small, μέτρον a measure); an instrument for measuring small distances. See p. 621.

Minor axis of an orbit of a planet or comet. See p. 34.

Mintaka, a star otherwise called & Orionis. Mira (mirus, wonderful), a star otherwise called, Ceti. See P. 485.

Mirach, a star otherwise called B Andromedæ.

Mirjak, a star otherwise called a Persei. Mirzam, a star otherwise called B Canis Majoris.

Mizar, a star otherwise called Ursæ Majoris.

Moon-culminating Stars are certain stars

which, being situated near the Moon, at any particular time are suitable points from which the angular distance of our satellite may be measured for the determination of the longitude of the place of observation.

Motion, direct. A body is said to have a direct motion when it advances in the order of the signs, or when its longitude continually increases. Motion, retrograde. A body is said to have a retrograde motion when it advances contrary to the order of the signs, or when its longitude continually diminishes.

N.

Nadir (natara, to correspond); the point immediately beneath an observer, and therefore exactly opposite the zenith. Nath, a star otherwise called ẞ Tauri. Nebula (nebula, a fog). See p. 496. Nekkar, a star otherwise called B Boötis. Nodes (nodus, a knot) are those points in the orbit of a planet or a comet where it intersects the ecliptic. The ascending node (8) is the point where the orbit passes from the south to the north side of the ecliptic; the descending node (8) is the opposite point, where

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