(9)

tor. Excentric anomaly, is the angle at the centre of the elliptical orbit, formed by the line of apses and a line drawn to the point in which an ordinate passing through the planet's true place in its orbit, meets the circumference of a circle, described on the line of apses as a diameter.

Mean anomaly, is a sector of the elliptical orbit over which the radius vector has passed, from the aphelion to the place of the planet in its orbit; and is proportional to the time of description.

Antarctic circle. See Arctic circle.

Antipodes, those who inhabit parts of the earth diametrically opposite to each other.

Anticipation of the equinoxes or seasons, the excess of the civil Julian year of 365d. 6h. above the solar tropical year of 365 days 5 hours 48 minutes 48 seconds. This constitutes the difference between the Julian and Gregorian calendars, or old and new styles.

Aphelion, is that point of a planet's orbit which is at the greatest distance from the sun.

The places of the aphelia of the several planets are all different, and have each a small progressive motion, occasioned by the mutual attractions of the planets on each other.

Apogee, is that point of the moon's orbit which is at the greatest distance from the earth. This term is also frequently applied to the sun, to signify that point in which he is at the greatest distance from the earth.

Apses or apsides, are the extremities of the greater axis of the planets' elliptical orbits: the axis itself being called the line of the apses.

Arctic circle, is a small circle parallel to the equator, and at the same distance from the north pole that the tropics are from the equator. A circle similarly situate round the south pole, is called the antarctic circle. These are also frequently termed the north-polar, and south-polar circles, respectively.

Ascension of a celestial body, is an arch of the equator, reckoned from west to east, and intercepted between the equinoctial point Aries, and that point which rises with the body. This is distinguished into right, and oblique ascension, according to the angle in which the equator cuts the horizon.

Aspect, is a term applied to signify the situation or apparent distance, in longitude, of any two celestial bodies in the zodiac, from one another, and is particularly denominated, and designated by appropriate characters, according to this distance-as conjunction 6, sextile ✶, quartile П, trine A, opposition, and some others, which see.

B

Asteroids, star-like bodies, a term of recent invention, and applied to three small bodies lately discovered in the solar system, between the orbits of Mars and Jupiter. Their orbits are considerably more excentric than that of any of the other planets; though their elements are still but imperfectly ascertained-See note subjoined to the Table of the solar system, page 73.

Astronomy, is that science which explains and demonstrates the phenomena of the heavens.

Atmosphere, usually termed the air, is that transparent elastie fluid which surrounds the earth. It is indispensably necessary to animal and vegetable life, combustion, and many other functions in nature. The atmosphere being a perfectly elastic, compressible, and ponderous fluid, its density must decrease upwards, in a geometrical ratio, of the heights taken in arithmetical ratio. The whole weight of any column of the atmosphere, on the surface of the earth, is found, by experiment, to equal, in a mean state, that of a column of mercury of an equal base and about 30 inches high; that is, about 15 pounds avoir. on every superficial inch. The planets, if not the sun and fixed stars, are all probably furnished with similar atınospheres. Attraction, is that power, either continually exerted by the Deity, according to a fixed law, or by him communicated to matter; by which all bodies, or particles of bodies, whether in contact, or at a distance, adhere, or tend towards each other. Attraction, according to the manner or circumstances of its operation, is commonly distin guished into that of gravity, that of cohesion, that of electricity, &c.

Axis of a planet, is that imaginary line passing through its centre, round which it performs its diurnal rotation. Azimuth of a celestial body, is an arch of the horizon intercepted between the meridian of the place and the azimuthcircle passing through the body. The true azimuth of a body may readily be calculated by the resolution of a spheric triangle; and then the difference between this, and that observed by a compass furnished with a magnetic needle, will be the variation of the compass.

Azimuth-circles, are those great circles of the sphere which pass through the zenith and nadir, and consequently cross the horizon at right angles.

Barometer, is an instrument for measuring the weight of a superincumbent column of the atmosphere, at any given time and place. It is commonly made of a long glass tube, of a moderate bore, open at one end; which being filled with well-purified mercury is inverted, with the

11 )

open end downwards, into a bason, of the same fluid. The mercury in the tube will then subside, leaving a vacuum in the upper part of the tube; and the height of the column of mercury in the tube, thus sustained by the pres sure of the atmosphere on the surface of the mercury in the bason, will be a just measure of its weight.

It is found by experiment that the height of the column of mercury is not always the same in the same place, but varies generally between 28 and 31 inches, on the surface of the earth. The barometer has been applied with success to the measuring of accessible altitudes. For this purpose let the height of the mercury in a barometer, both at the bottom and top of the eminence or depth to be measured, be observed as nearly as may be at the same time. Also observe the temperature of the air by ther mometers both attached to the barometers, and at a distance from them, in the shade. Then let the column of mercury in the colder barometer be increased by its 9600th part for every degree of difference in the two attached thermometers (Fahr. scale). Subtract the common logarithm of the less column of mercury from that of the greater, and the difference multiplied by 10000 will be the alt. nearly, in fathoms. For a correction apply, by addition or subtraction, one 435th part of the above alt. for every degree of the mean temperature of the two detached thermometers above or below 31 degrees, and the result will be the true alt.

Bissextile, a year consisting of 366 days, by adding a day to the month of February every 4th year. This day was by Julius Cæsar appointed to be the 24th of March (called by the Romans the 6th of the calends) which being reckoned twice, the year was on this account termed bissextile. This year is, on another account, called leap. year.

Calendar, is a table, almanac, or distribution of time, suited to the several uses of society.

Various calendars have been adopted by different nations in different ages of the world. The Roman calendar, as corrected and established by Julius Cæsar, and thence called the Julian calendar, made the year to consist of 3651 days; viz. three years each containing 365, and the 4th 366. But as the solar year actually falls short of the Julian by about 11 minutes, Pope Gregory XIII, in 1582, reformed this calendar, by striking out the surplus days that the seasons had then got a-head of the calendar; (viz. 10 days) and ordering that, in future, 3 days should be stricken out of every 400 years of the Julian account, by calling every centurial year not devisible by 4 (as 1700,

1800, 1900, 2100, &c.) a common year, instead of a leapyear. The year is divided into 12 calendar months, viz. 7 of 31 days, 4 of 30, and 1 of 28 or 29.

Central forces, are those by the influence of which the planets and comets perform revolutions round their centres of motion, and are retained in their orbits. Those forces are of two kinds, viz. the centrifugal, and the centripetal. Centrifugal or projectile force, may be considered as a single impulse, given by the Creator, and which, agreeably to the laws of motion, would carry the body with a uniform velocity, in a rectilineal direction.

Centripetal force, or force of gravity, may be considered as a continually-operating influence, urging the body down towards the centre of motion: and according to the proportion between these two forces the body will describe a circular, or an elliptical orbit.

Chronology, is that science which treats of time, comprehending its remarkable æras or epochs, divisions, subdivisions, and measures.

Circle, is a plane figure bounded by a uniformly-curved line called the circumference, every part of which is equally distant from a certain point within the same, called the centre. Diameter is a right line passing through the centre, and terminated on each side by the circumference. Radius, or semidiameter, is the distance from the centre to the circumference.

Circles of the sphere are of two kinds-great, and small. Great circles, are those which divide the sphere into two equal parts; the chief of which are, the equator, the ecliptic, meridians, horizon, azimuth-circles, and circles of celestial longitude. Small circles, are those which divide the sphere into two unequal parts; the chief of which are, parallels of altitude and of depression, parallels of terres. trial, and parallels of celestial latitude.

Circles of celestial longitude, are those great circles of the sphere which cross the ecliptic at right angles. Circum-polar stars, are those which appear to perform daily circuits round the pole, without rising or setting; and such are all those whose polar distance does not exceed the latitude of the place.

Colures, are those two meridians which pass through the equinoctial and solstitial points of the ecliptic, and are hence distinguished into the equinoctial and solstitial colures.

Comets, are certain bodies in the solar system, moving in very excentric orbits, in various planes and directions, and visible but for a short time when near their perihelia; and then generally appearing with a lucid tail or train

of light, on the side of the comet opposite to the sun. Frequently, however, comets are seen without this lucid train; the body or nucleus being surrounded with a bearded or hairy-like atmosphere. The whole list of comets that have been hitherto observed amounts to upwards of 500; of which about 170 have been observed with accuracy, and the elements of their orbits computed. Conjunction, is that aspect in which two celestial bodies, in the zodiac, have the same longitude.

Constellation, this term is applied to any assemblage or number of neighbouring stars in the heavens, which astronomers have classed together under one general name. They are generally designated by the names and figures of some living creatures, and thus delineated on the celestial globe or atlas. The number of constellations, according to the ancients, was 48, viz. 12 near the ecliptic, called the 12 signs of the zodiac, 21 on the north side of the zodiac, and 15 on the south side. Modern astronomers, by forming new constellations out of such stars as were not included in the above, have increased the number to about 70-The several stars in each constellation are distinguished either by letters of the alphabet, or by numbers: and some few by proper names; as, Aldebaran, Castor, Pollux, &c. Crepusculum or twilight-circle, is a circle of depression, 18 degrees below the horizon; for, it is found by observation that when the sun crosses this circle, before rising, or after setting, twilight begins or ends. This is occasioned by the rays of light from the sun being refracted and reflected by the earth's atmosphere.

Culmination of a star, is the point of its greatest elevation

above the horizon, or where it crosses the meridian. Cusps, the horns of the moon, or any other planet, when less than half its illuminated part is visible.

Cycle, is any certain period of time in which the same circumstances, to which the cycle has a reference, regularly return. The most noted chronological cycles are—

1. The cycle of the sun, a period of 28 years, after which the same day of the month will happen on the same day of the week, as in the same year of a former cycle.

2. The Metonic or lunar cycle, a period of 19 years, after which the change, full, and other phases of the moon, will happen on the same days of the month, as in the same year of a former cycle.

3. The cycle of Indiction, a period of 15 years, instituted by Constantine A. D. 312, probably as a stated period of