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the spherical concavity of the sky; and will mark two points in the heavens that are also called the north and the south poles, around which, or rather their line of junction, all the fixed stars appear to revolve. The north pole of the heavens (the only one visible in our latitude) is a point, situated in the constellation called Ursa Minor, or the little bear; and a bright star, in the tip of the tail of this imaginary animal, is called the Pole-star, or Polarstar, because near to the real pole. Circles supposed to be drawn round the Arctic, and the Antarctic pole, at the distance of about 234 degrees, are termed, respectively, the Arctic and the Antarctic Circle. On the terrestrial globe, those circles surround what are called the two Frozen Zones, or girdles of the earth. AREA.-See Glossary II. ASCENSION, RIGHT.-See Right As
ASTRONOMICAL HORIZON.-See Glossary II., Horizon. ATMOSPHERE.-See Glossary I.
See Glossary II. ATTRACTION.-See Glossary I. AXIS.-See Arctic, and Astronomy, page 6.
OF AN ELLIPSIS, PARABOLA, CONE, &c.-See Glossary I., Cone and Conic Sections.
MAJOR AND AXIS MINOR OF AN ELLIPSIS, are the same as the Transverse and Conjugate Diameters, which see in Glossary I. AZIMUTH.-See Glossary II., Astronomy, pp. 36 and 256, and Navigation, page 27. COMPASS.-See Glossary II.,
and Navigation, page 30.
MAGNETIC.-See Glossary II., article Horizon.
BEARING, in Navigation, is the situation of one place from another with respect to the points of the compass. Thus if Alies in the direction of south-west from B, then to an observer at B, A is said to bear south-west, or to have a south-west Bearing; while to an observer at A, the point B will bear north-east, or have a northeast Bearing.-See Navigation, page 15. BERGS. The Swedish Berg is a hill or mountain; and hence the name of Bergs, or more generally Icebergs, is given to the mountains of ice which are met with in the Polar Seas. Flat sheets of wide-spread ice are termed Fields; and small portions (because found floating) are Floes.
CALENDAR OR KALENDAR. This
term is understood to have been derived from the Greek kaleo, I call or proclaim, because the first appearance of the new moon was watched for and proclaimed; and hence the first days of the several
months were marked Calenda (the Calends) in the Calendar, or Almanac of the Romans.
Time, as measured by the revolutions of the sun and moon, is divided into days, months, and years. The Months (Mooneths) were at first meant to denote the period of a lunar revolution, or the time from one new moon to another; and the years were counted by the revolutions of the sun-from the shortest day (daylight) to the shortest day again, or from the point of time of the vernal equinox to its return. The solar year, however, does not contain an even number of lunations; and calendars were made to shew the connexion between them. The earliest known Roman year consisted of 304 days, divided into ten months, or moons; and hence the names September, October, November, and December, from the Latin words for 7, 8, 9, and 10. Two more moons were prefixed by Numa, making up 354 days, or about 12 lunations. This period, however, being still less than a revolution of the sun, Julius Cæsar reformed the calendar, by lengthening the several months (to the same extent as they now are), so as the year should consist of 365 days; and as the sun is nearly six hours longer in performing his apparent annual circuit, every fourth year was made 366 days, by adding a day to the month of February. This addition of a day to every fourth year is known at present by the name of Leapyear; but the Romans did not add it to the end of February, as we do, but intercalated it after the twenty-third, so that the twenty-fourth day was counted twice: this day, usually termed Sextilis, was therefore Bis-sextilis; and consequently the Bissextile is another name for Leap-year.
The subsequent reformation of the calendar by Pope Gregory XIII., now called the Gregorian Calendar, and the reasons for that reformation, are amply, though concisely explained, in the History of Astronomy, at pp. 37, 47, and 48, to which we refer. CALENDAR MONTHS contain the number of days marked to each in the calendar, as distinguished from Lunar Months, which are legally of four weeks or twentyeight days of course, neither of these periods corresponds to a lunation. CANICULAR PERIOD. The Dog-star is otherwise called Sirius and Canicula (Latin, Canis, a dog); and what is connected with that star is termed Canicular. The Dog-days (or Canicular days) in the ancient calendars were forty days; reckoning twenty before and twenty after, the heliacal rising of Sirius. That period, being then the hottest of the year, was, by the Greeks, accounted the season of "fevers, plagues, and death." The time of the heliacal rising of the dog-star, however, (what the astrologers of old never
dreamt of,) varies in consequence of the precession of the equinoxes; so that, instead of happening in the warmest season, it has gradually advanced towards the autumn. The modern almanac makers have, therefore, regardless of the star, marked the dog-days as commencing on the third of July and ending on the eleventh of August.
The Egyptian year consisted of 365 days; and supposing it to have begun at the heliacal rising of the dog-star (which they called Soth), its next heliacal rising would be about six hours later every year; so that a period of 1461 years would elapse before that star would again rise heliacally at the beginning of their year. This they knew, and it has been termed the Sothic, or Canicular period.-See Astronomy, PP.
15 and 43.
CHARTS, in Geography and Navigation, are representations of portions of the earth's surface on paper, according to scales which regulate the relative proportions of the parts. Geographical charts are more usually called Maps, being general; but Nautical, or Marine Charts are particularly appropriated to delineations of a coast and part of the adjacent sea. Marine Charts are constructed by two different methods. In a Plane Chart, the meridians as well as the latitudes are represented by equidistant and straight parallel lines; and, consequently, the longitudes and latitudes appear equal, differing from the fact. In a Mercator's Chart, the meridians as well as the circles of latitude are also represented by straight parallel lines; but the distance of the latter increase in a determinate ratio from the equator to the poles.-See Navigation, pages 7 and 17.
CHORD OF AN ARC.-See Glossary I., Angle.
CIRCLE, DIVISION AND PARTS OF. -See Glossary I., Angle, and Astronomy, page 8, note.
-GREAT, OF A SPHERE. A great circle of a sphere is any circle which is formed by a plane supposed to pass through its centre. CIRCLES, CONCENTRIC AND ECCENTRIC.-See Concentric Circles.
OF DECLINATION.- See Declination and Navigation, page 27. SECONDARY.-See Astro
all the fixed stars appear to revolve round the poles. At every place of the earth, except on the equinoxial, one or other of the poles is always elevated above the horizon to a degree equal to the latitude of that place. Certain stars, according to their vicinity to the pole, notwithstanding they also apparently move round it, will never fall so low as the horizon; those never set, and are termed Circumpolar Stars.
CLEPSYDRA, an instrument used by the ancients for measuring time. It received various improvements, but the general principle was that of the gradual dropping of water from one vessel into another, in the manner of a sand-glass; to which latter, the name of Clepsydra was sometimes incongruously given.
CLIMATE. In its geographical and technical application, Climate, or Clime, denotes an imaginary narrow belt of the globe, parallel to the equator; and is so called from the Greek clima, inclination, because the difference of climates depends on the inclination, or obliquity of the sphere. The belts constituting the several climates are small, depending on the average length of the longest day: that of each increasing by half an hour, from the equator to the polar circles, when the climates are counted by months, till they reach the poles. In a more popular and accurate sense, a Climate is designated (as hot or cold, dry or moist) from the comparative degree of heat and moisture which generally exists in its atmosphere. -See Physical Geography, page 34. COLURES. See Ecliptic and Astronomy, page 62.
COMPASS.-See Glossary II. COMPLEMENT OF AN ANGLE OR ARC is what it wants to complete the quadrant, or right angle.
CONCENTRIC CIRCLES are such as have the same centre, the one surrounding the other as with a ring. Circles that are wholly, or partially, included in
another, but have different centres, are termed Eccentric. CONCENTRIC THEORY.-See Epicycle. CONE, RIGHT AND OBLIQUE.-See Glossary I., Cone.
CONIC SECTIONS.-See Glossary I. CONJUGATE DIAMETERS.-See Glossary I., Conic Sections. CONJUNCTION OF THE SUN AND PLANETS. The planets, relatively to the earth, are separated into two divisions, Inferior and Superior: the former having their orbits within, and the latter without, that of the earth. When a planet, as seen from the earth, is in the same direction as the sun, it is said to be in conjunction with the sun. This, however, in the case of an inferior planet, may be either when it passes between the sun and the earth, or when it is on the further side of the
sun; the former is the Inferior, and the latter the Superior Conjunction. A superior planet, never passing between the sun and the earth, is only once in conjunction with the sun during its revolution. In the point of its orbit, when the earth is between it and the sun, the planet is said to be in Opposition to the sun. The Conjunctions and Oppositions of the moon have the general name of Syzygies.—See Quadratures. The angle under which we see the distance of a planet from the sun (reduced to the ecliptic) is called its econgation.-See Astronomy, page 75. CONSTELLATIONS. The whole of the fixed stars are apportioned into separate groups termed Constellations, each of which is included within the outline of a certain figure (chiefly of an animal) which is imagined to be drawn on the concave of the sky. Forty-eight of these constellations are of unknown antiquity. The twelve that occupy the zodiac are termed the Twelve Signs.-See Zodiac. CO-SECANTS,
These are the Secants, Sines, and Tangents, of arcs that are the complements of those in CO-TANGENTS. question. COSMICAL, a term applied to the rising or setting of a star, and opposed to Acronycal, which see.
COTYLEDON. The embryo, which constitutes the vital principle of a vegetable seed, is termed the Corculum, being a Latin diminutive, signifying a little heart, which in many cases its shape resembles. This Corculum consists of two parts: the Rostellum, or radicle, which, descending, becomes the root; and the Plumula, or feather, which, ascending, becomes the stem and leaves. A pair (for some eminent botanists assert that they are never single) of roundish or compressed bodies constituting the chief bulk of most seeds, and immediately attached to the Corculum, have been named Cotyledons or Seed-lobes, from a Greek word signifying a cavity. These generally rise out of the ground with the Plumula, and assume the appearance of leaves (though unlike those of the future plant); and when the real foliage comes forth, they droop and die. In the natural classification of Jussieu, all plants are divided, in the first instance, into three divisions: those of which the seeds have only one Cotyledon, termed Monocotyledonous; those which have two Cotyledons, termed Dicotyledonous; and those in which Cotyledons are altogether wanting, or Acotyledonous.
CRUSTACEOUS. For the explanation of
this term, and its distinction from Testaceous, see Physical Geography, page 49,
CRYPTOGAMOUS. In the sexual system of Linnæus the fructification of plants is ascribed to certain essential parts of the
flower; and such plants as, having no flowers, wanted those parts, were classed under one head by the name of Cryptogamia, or hidden marriages. Cryptogamous plants are therefore those that bear no flowers, and flowering plants have sometimes been termed Phanerogamous. CULMINATION is the transit of a star over the meridian, or the point of its highest altitude.
CURVATURE, RADIUS OF.-See Radius of Curvature. CURVE.-See Glossary I,
CYCLE is a period of time during which certain natural phenomena complete their round; so as to begin anew, and conti nually occupy another cycle equal to the past. The Greek cyclos is literally a circle. METONIC.-See Epact. CYCLOID.—See Glossary I., and Trochoid in the present.
DAY. In common language, day is opposed to night, as light to darkness; but, in this usage, its lengths are very unequal, varying with the latitude of the place, and the time of the year; for, within the polar circles, there are weeks and months in which the sun never sets, and equal periods in which he never rises. In all other climates, however, the day and night together, that is, from sunrise to sunrise, always make up a period, nearly equal, which is called the Solar Day. This day is divided into twenty-four equal hours; and these hours, counted from noon and enumerated from one to twenty-four, make the solar day of the astronomers. The Civil Day begins at midnight, and is counted in two portions of twelve hours each from midnight to noon, and from noon to the succeeding midnight.
Though in the course of the year the earth must have revolved 3651 times on its axis, making so many real solar days, those days are not of uniform length, some being longer and some shorter than the average. This average is termed Mean Time, and a chronometer regulated to this average agrees with the true solar day only at four points of the year. The accumulated difference of the two modes of measurement is called the Equation of Time.
SIDEREAL, is the time which elapses between that of a star being in the meridian of a place to the moment when it arrives at the meridian again. This period is always the same, not being affected by the motion of the earth in her orbit, as the solar day is. The sidereal day is about four minutes less than the mean solar day. DECLINATION. The declination of a celestial body is its perpendicular distance from the equator, measured on a meridian or great circle passing through the object
INVERSE PROPORTION, or Ratio. See Glossary II. Ratio. DISC. The Discus of the ancients was a circular piece of wood, stone, or metal, which they used in their games. Astronomy it is a name for the apparent face of the sun or moon. The faces of the planets may also be termed Discs when viewed through the telescope. DRIFT IN NAVIGATION denotes the angle which the line of a ship's motion makes with the nearest meridian, when she drives with her side to the wind, and is not governed by the power of the helm ; and also the distance which the ship drives on that line only in a storm. The Drift of a current is its velocity and the direction of its motion.
ECCENTRIC CIRCLES.-See Concentric Circles.
ECCENTRICITY. Referring to this word in Glossary II. we observe an error: for "the distance between," read "half the distance between."
ECLIPTIC. The position of this great circle in the heavens is given under the head of Equator. The Ecliptic, like all other great circles, is supposed to be divided into degrees and minutes, and has its poles, through which other imaginary circles are drawn. These latter may be compared to the circles of longitude, or meridians, on the terrestrial globe; the longitude of a star being counted on the Ecliptic, in degrees and minutes, from the first degree of Aries (which is the vernal equinoctial point) in the same way as the terrestrial longitude is counted upon the equinoctial line from the standard me
The lowest point of the Ecliptic, that in which the sun seems to pause in his orbit and begins again to ascend, is called the Winter Solstice; and the highest point in his career, when he begins again to descend, is the Summer Solstice, which two Solstices (Latin Sol, the sun, and sto, to stand) make, respectively, in our latitude, the shortest and the longest day. Circles drawn parallel to the Equator through these points are termed Tropical Circles, from the Greek trope, a turning; and the included corresponding portion of the earth, forming the Torrid Zone, is said to lie between the Tropics. In the celestial sphere, the Tropical circle at the Summer Solstice is the Tropic of Cancer; and that of the Winter Solstice is the Tropic of Capricorn: because (at one time) terminating respectively at those signs of the Zodiac. We say, at one time, for these points, though they retain the same designation, continually, though slowly, change their situation in the heavens, on account of the Precession of the Equinoxes. A great circle, crossing the Equator at right angles, and passing through the two Solstitial points, is called the Solstitial Colure; while another, passing through the Equinoctial points, is the Equinoctial Colure. See Astronomy, pages 17 and 62. ELLIPSIS.-See Orbit, and Glossary I. ELONGATION.-See Conjunction. EPACT. The year of 365 days contains twelve lunations and nearly eleven days more; so that, were it to begin with the new moon, she would be eleven days old on the first day of the succeeding year; the next year she would be twenty-two days; and on the third new year's day she would have passed a whole lunation and about three days more. The age of the moon (thus varying) on the first day of any year is termed the Epact, from a Greek word signifying adscititious. Those Epacts will form a varying series for nineteen years, when the new moon will again nearly coincide with the close of the year. This period of nineteen years is called the Metonic Cycle, from Meton its inventor; and the number of the years that have passed since the last coincidence (when the Epact was nothing) is called the Golden Number. -See Calendar.
EPICYCLE, a Greek derivative signifying a Circle upon a Circle. It was a prejudiced opinion among the ancient astronomers, that the motions of the heavenly bodies must necessarily be in circles: and, in order to make that doctrine tally with observation, they invented, in succession, the two theories of Epicycles and Eccentrics. In the former, called also the Concentric Theory, the earth was supposed to be placed in the centre of a circle on the circumference of which the centre of another circle revolved; and on the circumference of this second circle (called an
EPICYCLOID.-See Cycloid. If a circle roll upon the circumference of another circle instead of a straight line, points either on, within, or without its circumference, if on the same plane, will form varieties of Epicycloids. EQUATION OF THE CENTRE.-See Orbit, and Radius Vector.
OF TIME.-See Day. EQUATOR. An imaginary great circle of a sphere, equally distant from the poles of its rotatory motion, is termed, in Astronomy, the Equator, whatever that sphere may be thus we speak of the Equator and the Equatorial portion of Jupiter. On terrestrial globes and maps of the earth it is usually called the Equinoctial Line, or simply the Line.
On celestial globes that figure the concavity of the heavens, the Equator is crossed at an angle of about twenty-three and a half degrees, by another great circle called the Ecliptic, which represents the apparent path of the sun, through the twelve signs, in his annual course.
Ecliptic crosses the Equator in two opposite points called the Equinoxes (Latin equi and noctes), because it is only when the sun is in one or other of those points in the heavens that the length of the day is exactly equal to that of the night. The two Equinoxes are denominated, one the Vernal and the other the Autumnal, because they are crossed by the sun respectively in the spring and autumn. The Equinoctial Points, where the ecliptic thus intersects the equator, are not stationary with respect to the fixed stars, but are regularly, though slowly, moving backwards; and this retrograde motion is called the Precession of the Equinoxes. For a particular explanation of this latter subject, see Astronomy, pages 36-40. EQUINOX, AND EQUINOCTIAL POINTS.-See Equator.
EVECTION is one of the most consider
able of the lunar irregularities, and was discovered by Ptolemy. It is periodical, running through all its changes in about twenty-seven days.-See Astronomy, page 200.
EXTREME AND MEAN RATIO.-See History of Astronomy, page 63. A line is so divided, when the rectangle under the whole line and the lesser segment is equal to the square of the greater segment; and hence the whole line is to the greater segment as that greater segment is to the lesser. The segments of such a division, being incommensurable with the whole line, cannot be exactly given in numbers; but the geometrical construction is easy. -See Euclid's Elements, Book II. prop. xi. FIELDS OF ICE.-See Bergs. FLOES-See Bergs. FLOOD-TIDE.-See Tides. FOCUS.-See Glossary I. FORCE, CENTRIFUGAL.-See sary I.
CENTRIPETAL.-See Glossary II. FORMULA.-See Glossary II.
GENUS. In Natural History, a number of objects, such as animals and plants, are found to possess certain characteristics in common; and such are classed together, by the makers of systems, under one head, or kind, termed a Genus. The other permanent differences between the individuals of the same genus constitute Species; and the accidental differences found among the species are termed Varieties. Genus (kind) is Latin; and, in transferring the word into English, we have also adopted its plural, genera.
GEOCENTRIC. The Greek ge, the earth, is the root of a numerous class of wellknown scientific terms, such as Geography, Geology, Geometry, &c. Geocentric is having the same centre as the earth, or having the earth for its centre. Thus the moon's orbit is Geocentric; but the orbits of the other planets, and of the earth itself, are Heliocentric (Greek helios, the sun), having the sun as their centre of motion. The Geocentric place of a planet is the place in which it would appear to an eye in the centre of the earth. The Geocentric Latitude of a planet is its latitude as seen from the earth; or it is the inclination of a line connecting the planet and the earth to the plane of the ecliptic. The Geocentric Longitude of a planet is the distance, measured on the Ecliptic, in the order of the signs, between the Geocentric place and the first point of Aries. GEODESICAL, OR GEODETICAL, denotes something belonging to, or connected with, the mensuration of the earth's surface.
GIBBOUS. The Latin gibbus is protuberant, in the manner of a Hunchback. In English, the term is applied to designate that appearance of the moon (some days before and after the full) in which more than half her disc is enlightened; the line between light and dark being curved, or bulged outwards.-See Phases.