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work, arranged in alphabetical order; with such remarks and examples interspersed, as were judged necessary for illustration: together with Tables of the periodical times, distances, magnitudes, and other elements, of all the planets, both primary and secondary, in the solar system; according to the latest observations.

This, it is presumed, cannot fail to be considered as a valuable appendage to the work-especially by the young student of astronomy: as the glossary will tend greatly to facilitate his progress, and the tables will present him with a comprehensive view of the whole science-the result of the observations and researches both of past and present times.

Philadelphia, Feb. 14th, 1806.

Explanation of the principal Terms relating to Astronomy, Chronology, and the astronomical parts of Geography; with occasional Illustrations and Remarks.

Aberration of a star, is a small apparent motion, occasioned by a sensible proportion between the velocity of light and that of the earth in its annual orbit. From this cause, every star will, in the course of a year, appear to describe a small ellipsis in the heavens, whose greater axis = 40′′ and its lesser axis, perpendicular to the ecliptic, = 40" x cos. of star's lat. (to radius 1.) In astronomical calcu lations, where great accuracy is required, and the place of a star concerned, a correction on account of aberration, as well as on other accounts, ought to be applied to the star's place as found in the tables. This correction may readily be found by the following theorems; in which A = the star's right ascension, D = its declination, and S = the Sun's longitude.

Theorem 1. (-1.272 cos. (A-S)) cos. D + (0.055. cos. (A + S)) cos. D aberr. in R. A. in seconds of time.

Theorem 2.-20 cos. A. sin. S. sin. D + 18.346 sin. A. cos. S. sin D-7.964 cos. S. cos. D = aberr. in dec. in seconds of a degree: observing that the sine, cosine, &c. of all arches between 90° and 270° are to be considered as negative, and those of all other arches as affirmative.

When the star has south declination, let the sign of the last term in the 2d theorem be changed.

Acceleration (diurnal) of a fixed star, is the difference between the sidereal and the mean solar day, which = 3′ 55".9 or S′ 56′′ of mean time nearly; and so much sooner will any fixed star rise, culminate, or set, every day, than on the preceding day. A planet is said to be accelerated in its motion, when its velocity, in any part of its orbit, exceeds its mean velocity; and this will always be the case when its distance from the Sun is less than its mean distance.

Era, or epoch, any noted point of time, in chronology, from which events are reckoned, or computations made. Different nations or people make use of different epochs: as the Jews, that of the creation of the world; the christian nations, that of the nativity of Christ, A. M. 4007; the Mahometans, that of the Hegira, or flight of Mahomet from Mecca, A. D. 622; the ancient Greeks, that of the Olympiads, commencing B. C. 775: the Romans, that of the building of Rome, B. C. 752; the ancient Persians and Assyrians, that of Nabonasser, &c. Altitude of a celestial body, is its elevation above the horizon, measured on the arch of an azimuth-circle intercepted between the body and the horizon. The apparent altitude, or that measured by an instrument, re uires to be corrected in order to obtain the true altitude-1. by subtracting the refraction; 2. by adding the parallax; 3. by subtracting the dip corresponding to the height of the observer's eye above the surface of the earth; and 4. when the lower or upper limb of the sun or moon is observed, by adding or subtracting the apparent semidiameter. Altitude, meridian, is that of a body when on the meridian. Amplitude of a celestial body, is an arch of the horizon intercepted between the east or west points thereof, and that point where the body rises or sets. The true amplitude of a body may be found by the following proportion :

Rad: cos. lat. :: sin. dec.: sin. amp. which will be of the same name (north or south) with the declination.

The difference between the true, and the magnetic am. plitude of a body, or that observed by a compass furnished with a magnetic needle, will be the variation of the compass. Angle is the inclination of two converging lines meeting in a point, called the angular point. A plane angle is that drawn on a plane surface. The measure of a plane angle is the arch of a circle comprehended between the lines including the angle, the angular point being the centre. A spheric angle is that formed by the intersection of two great circles on the surface of a sphere. The measure of a spheric angle is the arch of a great circle comprehended between the two arches including the angle, the angu lar point being its pole. A right angle is one whose measure is an arch of 90°. An acute angle is one less than 90°. An obtuse angle, one greater than 90° Inomaly is the angular distance of a planet from its aphelion. It is distinguished into true, excentric, and mean. True anomaly of a planet, is the angle at the sun or focus of the elliptical orbit, formed by the line of apses and radius vec

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 II, 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 elastic 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 distinguished 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 sphe ric 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 a 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

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