Antæci, a name given to those inhabitants of the earth, who live under the same meridian, and at equal distances from the equator, but on opposite sides of it.

Antecedentia, a motion of any of the heavenly bodies which is contrary to the order of the signs; as from Aries towards Pisces, &c.

Antipodes, those inhabitants of the earth who live diametrically opposite to each other, or walk feet to feet.

Aphelion, that point in the orbit of a planet in which it is at its greatest distance from the sun.

Apogeon, that point in the orbit of a planet in which it is at its greatest distance from the earth.

Apsides, two points in the orbit of a planet in which it is at its greatest and least distance from the sun: the line joining those points is called the line of the Apsides.

Armillary Sphere, an instrument composed of the principal circles which are usually drawn upon an artificial globe.

Ascii, the inhabitants of the torrid zone, so called because the sun being twice a year in their zenith, their bodies at those times cast no shadow.

Ascensional Difference, an arc of the equinoctial contained between that point of it which rises with the sun, moon, or star, and that which comes to the meridian with them; or it is the time the sun rises or sets before or after six o'clock.

Atmosphere, that collection of vapours, or body of air, which surrounds or encompasses the earth.

Attraction, a property of matter, by which bodies approach towards each other, without any sensible agent either drawing or impelling them.

Aurora, the morning twilight, which begins to appear when the sun is about eighteen degrees below the horizon.

Axis of the earth, or of a planet, an imaginary line passing through the centre from one pole to the other; or that round which they are supposed to perform their diurnal rotations.

Azimuths, great circles which pass through the zenith and nadir, and are perpendicular to the horizon. The azimuth of any celestial object is an arc of the horizon contained between the east or west points of the heavens, and a vertical circle passing through the centre of that object.

C

Centrifugal Force, that force by which any revolving body endeavours to fly off from the centre of motion, in a tangent to the circle which it describes.

Centripetal Force, that force by which any revolving body is made to tend towards the centre. See page 438.

Conjunction, is when two stars, seen from the sun or the earth,

appear in the same point of the heavens, or answer to the same degree of the ecliptic.

Constellation, a number of stars lying in the neighbourhood of each other, which astronomers, for the sake of remembering them with more ease, suppose to be circumscribed by the outlines of some animal, or other figure.

Cosmical rising or setting of a planet or star, is when it rises with the sun in the morning, or sets with him in the evening. Consequentia, a motion of the planets according to the order of the signs; as from Aries towards Taurus, &c.

Culminating, a term applied to the sun or a star when it comes to the meridian of any place.

Cycle of the moon, a revolution of nineteen years, in which time the conjunctions and lunar aspects are nearly the same as they were nineteen years before.

D

Day, (natural,) that portion of time in which the earth completes an entire revolution upon its axis.

Day, (artificial,) the time between the sun's rising and setting; to which is opposed night, or the time between his setting and rising.

Day, (astronomical,) the time between two successive transits of the sun's centre over the same meridian; which always begins and ends at noon.

Declination of the sun, moon, or stars, is their distance north or south from the equator, reckoned in degrees, minutes, &c. upon a circle which is perpendicular to it

Direct, a planet is said to be direct, when it moves according to the order of the signs; as from Aries towards Taurus, &c. Disk of the sun, or moon, is its round face, which, on account of the great distance of the object, appears flat, or like a plane surface.

Digit, in Astronomy, the 12th part of the sun's diameter, which is often used in the calculation of eclipses.

Diurnal, of or belonging to the day; thus, the diurnal motions of the planets are the spaces they move through in a day.

Dominical Letter, one of the first seven letters of the alphabet, which is usually marked in red, and employed in the Almanac for distinguishing the Sundays throughout the year.

E

Eccentricity, the distance between the centre of an ellipsis and either of its foci.

Eclipse of the sun, an obstruction of his light, occasioned by the interposition of the dark body of the moon between him and our sight.

Eclipse of the moon, a deprivation of her light, occasioned by the interposition of the earth between the sun and moon.

Ecliptic, a great circle of the sphere, in which the sun always appears to move; so called, because eclipses generally happen when the moon is in or near this circle. The obliquity of the ecliptic is the angle it makes with the equator, which is about 23° 27′ 53′′, in 1822.

Elevation of the Pole, is an arc of the meridian contained between the pole and the horizon; which is always equal to the latitude of the place, or the distance of the zenith from the equator.

Elongation, the angular distance of a planet from the sun, as it appears to a spectator upon the earth.

Elements, in Astronomy, the requisites necessary to determine the theory of a planet, in order to calculate its position, motion, &c.

Ellipsis, a figure formed by cutting a cone obliquely to its axis: it is in a curve of this kind that the planets move round the sun, and the satellites round their primaries.

Emersion, the time when any planet which is eclipsed begins to recover its light again.

Epact, the moon's age at the end of the year, or the difference between the solar year and the lunar one.

Equations, certain quantities by which we estimate the inequalities in the motion of a planet: the moon, being subject to many irregularities, has a great number of equations.

Equation of time, the difference between equal time and apparent, or that shown by a perfectly true clock and a sun dial. Equator, a great circle which separates the northern from the southern hemisphere, and being referred to the heavens is called the Equinoctial.

Equinoxes, the two points where the ecliptic cuts the equator; so called, because, when the sun is in either of these situations, the days and nights are equal to each other.

Evection, an inequality in the motion of the moon, by which, at her quarters, her mean place differs from her true one by about 24 degrees more than at her conjunction and opposition.

F

Foci of an ellipsis, two points in the longest or transverse axis, on each side of the centre; from each of which, if any two right lines be drawn to meet in the periphery, their sum will be always equal to the transverse axis.

G

Geocentric place of a planet, is that position which it has when seen from the earth.

Golden Numbers, a series of numbers proceeding from one to nineteen, which are used in the almanac for determining the imes of new and full moons.

Gregorian Year, so called from Pope Gregory XIII, who reformed the calendar in the year 1582; which reformation was not used in England till 1752.

H

Heliacal rising of a star, is when it emerges from the sun's ray's, and appears above the horizon before him in the morning. Heliacal setting of a star, is when it is so hidden in the sun's beams, as not to be seen above the horizon after him in the evening.

Heliocentric place of a planet, is that in which it would appear to a spectator placed in the sun.

Hemisphere, the half of a globe or sphere, when it is cut through its centre in the plane of one of its great circles.

Heteroscii, a name given to the inhabitants of the temperate zones, because their shadows at noon always fall one way.

Hour Circles, the same with meridians, or great circles which pass through the poles of the world, and are perpendicular to the equator.

Hypothesis, a supposition, a system formed upon some principle which has not been proved.

I

Immersion, the moment when an eclipse begins, or when a planet enters into the dark shadow.

Inclination, the angle which the orbit of one planet makes with that of another.

Indefinite, or Indeterminate, that to which the human mind cannot fix any certain bounds or limits.

Inferior Planets, those which move at a less distance from the sun than the earth; as Mercury and Venus.

Ingress, is the sun's entrance into any sign or other part of the ecliptic.

Intercalary Day, the odd day, which is made up of the six hours every fourth or leap year.

L

Latitude of a star or planet, is its distance from the ecliptic, reckoned in degrees, minutes, &c. upon the arc of a great circle which is perpendicular to it.

Libration, an apparent irregularity of the moon's motion, which makes her appear to librate about her axis, in such a manner that the parts of her eastern and western limbs becomes visicle and invisible alternately.

Longitude of a place, is its distance east or west from the

first meridian, reckoned in degrees, minutes, &c. upon the equa

tor.

Longitude of a star or planet, is its distance from the first point of Aries, reckoned in degrees, minutes, &c. upon the ecliptic.

Lunation, a lunar synodical month, or the space of time between one new moon and another, which is generally about 29 days, 12 hours, 44 minutes, and 3 seconds, being greater than the periodical month by two days and five hours.

Luni-Solar Year, a period arising from multiplying the cycle of the moon 19 by that of the sun 28.

M

Macula, dark spots, appearing on the face of the sun, moon, and some of the planets, being contradistinguished from Faculæ, which are bright or shining spots, that by means of the telescope are sometimes to be seen on the face of the sun, &c.

Magnitudes. The stars are divided into six sizes, or classes, of which the brightest are called stars of the first magnitudes; the next in brightness to these, stars of the second magnitude; and so on.

Mean motion of a planet, is that which would take place if it moved in a perfect circle, and equally every day.

Meridian, a great circle of the sphere, which passes through the zenith and poles, and is perpendicular to the horizon; it is so called because when the sun is upon the circle it is always mid-day, or noon.

Micrometer, an instrument by which the apparent magnitudes of objects, viewed through telescopes or microscopes, are measured with great exactness.

Microscope, an optical instrument, by means of which very minute objects are represented much larger, and viewed distinctly at small distances.

Month, (lunar or periodical) a period of about twenty-seven days, seven hours, and forty-three minutes, which is the time the moon is in passing from one point of her orbit to the same point again.

N

Nadir, that point in the heavens which is directly opposite to the zenith, or immediately under our feet.

Nebula, clusters of small stars which have been discovered by the telescope in different parts of the heavens.

Nocturnal Arc, is that space of the heavens which the sun apparently describes from the time of his setting to the time of his rising.