In a leap-year, the letter thus found will be the Dominical letter till the 28th of Feb. and the preceding one will be the Dom. let. from that time till the end of the year. Earth, the third planet in order from the sun; at the distance of about 95 millions of miles; furnished with one

moon.

Eclipse. When any secondary planet passes through the shadow of its primary, it is said to be eclipsed; as the moon by the shadow of the earth, or any of Jupiter's satellites by his shadow. But when the shadow of a secondary planet falls on its primary, then, with respect to that part of the primary on which the shadow falls, the sun is said to be eclipsed.

Ecliptic limit, is a certain distance from the node of the secondary's orbit, beyond which no eclipse can happen. This limit with respect to a solar eclipse is about 17°. and with respect to a lunar eclipse, about 120. Ecliptic, a great circle of the sphere in the plane of which the earth performs its annual revolution round the sun. Ellipse or ellipsis, a plane curvilineal figure, which may be described round two centres thus.--Take a thread of any determinate length, tie its two ends together, and throw the loop round two pins stuck into a plane board-then moving round a pencil, or the like, within the loop, so as to keep it always tight, the curve described will be an ellipsis. The two central points are called the foci of the ellipsis; a right line passing through the two foci, and terminated by the curve on each side, is called the transverse axis or diameter, and one bisecting this at right angles is called the conjugate.

Elongation of a planet, (generally applied to Mercury and Venus) their angular distance from the sun as seen from the earth.

Embolismic, or intercalary, a term applied to a lunar month occasionally thrown in to bring up the lunar to the solar years. It is also applied to the 29th of February, thrown in every 4th year to make the civil years correspond with the solar.

Emersion, the end of an eclipse or of an occultation.
Epact, the excess of solar time, above lunar. In the Gre-

gorian calendar it is the moon's age at the beginning of the year, which may be found by the following rule, till the year 1900.

Subtract 1 from the Golden number, multiply the remainder by 11, and the product, rejecting the 30's, will be the epact.

Epoch. See Era. Equation of time, the difference between apparent, and mean solar time. This arises from two causes, viz. the elliptical figure of the earth's orbit in which the diurnal arches will of consequence be unequal; and the inclination of the the ecliptic to the equator, whence equal arches of the former, in which the earth moves, will not correspond to equal arches of the latter, on which time is measured. Equator, that great circle which cuts the axis of rotation at right angles. Equinoctial points, the beginning of the signs Aries and Libra, those two points of the ecliptic in which it crosses the equator: the former being called the vernal, and the latter the autumnal, equinoctial point.

Equinoxes, the times when the sun appears to enter the equinoctial points; viz. the 21st of March, and 22d of September.

Excentricity, or eccentricity, of a planet's orbit, is equal to half the distance between the two foci of the elliptical orbit.

Focus, foci. See Ellipsis.

Frigid zones, those round the poles, bounded by their respective polar circles.

Geocentric place of a planet, is its place, (generally expressed in latitude and longitude, or right ascension and declination) as it appears from the earth. Globes (artificial) small spheres of paste-board, or the like, on one of which (called the terrestrial globe) are drawn the principal circles of the sphere, together with the several continents, islands, &c. of the earth, in their relative situations and magnitudes. On the other, (called the celestial globe) besides the circles of the sphere, are inserted all the visible fixed stars, distributed into their respective constellations. The use of the Globes, explains the manner of solving geographical and astronomical problems, by means of artificial globes.

Golden number, is the year of the lunar cycle, increasing annually by unity from 1 to 19.

Gravity, that species of attraction which takes place between bodies at a distance from each other, and by which, if not otherwise prevented, they would mutually approach each other, with a continually-accelerated velocity. Gravity is directly proportional to the quantity of matter, and inversely, to the square of the distance.

Heliocentric place of a planet, is its place in the heavens, as if viewed from the sun.

*( C )*

Herschel, or Georgium Sidus-the 7th primary planet in order from the sun, at the distance of about 1800 millions of miles. It is furnished with 6 satellites.

Horizon, that great circle of the sphere which, extended to the heavens, is the boundary of our vision. It is usually distinguished into sensible or visible, and rational or

true.

Hour, the 24th part of a natural day.

Horary angle of a celestial body, an angle at the pole of the equator, included between the meridian of the place and that passing through the body.

Immersion, the beginning of an eclipse, or of an occultation. Inclination of the axis of a planet, the angle which it makes with the axis of the plane of its orbit.

Inclination of the orbit of a planet, the angle in which it crosses the ecliptic.

Indiction, (Roman). See Cycle.

Jupiter, the fifth primary planet from the sun, at the distance of about 490 millions of miles. It is the largest in the system, and is furnished with four satellites.

Latitude of a place on the earth, its distance from the equator, measured on the meridian of the place.

Latitude of a celestial body, its distance from the ecliptic, measured on a circle of celestial longitude passing through the body.

Leap-year, one of 366 days, occurring every 4th year, and so called, because in that year the Dominical letter falls back two letters, or leaps over one. See Bissextile. Libration of the moon, a small apparent libratory motion, arising chiefly from her equable rotation round her axis, combined with her unequal motion in her orbit.

Longitude of a place on the earth, an arch of the equator intercepted between the prime meridian, and that passing through the place, and is denominated east or west, according to its situation with respect to the prime meridian. Longitude of a celestial body, an arch of the ecliptic, reckoned according to the order of the signs, from the equinoctial point Aries to the circle of celestial longitude passing through the body.

Lunar cycle. See Cycle.

Mars, the fourth primary planet from the sun, at the distance of about 144 millions of miles.

Meridians, great circles crossing the equator at right angles. Meridian of the place, that passing through the north and south points of the horizon.

Midheaven, that point of the ecliptic, or of the equator, which is in the meridian.

Minute, the 60th part of an hour, or of a degree.

Month, the 12th part of a year. It is variously distinguished according to circumstances, viz.

Lunar illuminative month, the time between the first appearance of one new moon, and of the next. The an

cient Jews, with the Turks and Arabs, reckon by this month.

Lunar periodical month, the time in which the moon appears to make a revolution through the zodiac 27 d. 7 h. 43 m. 8 s.

Lunar synodical month, or lunation, the time between one new moon, or conjunction of the sun and moon, and the next: at a mean = 29d. 12h. 44m. 3s. 11t.

Solar month, the 12th part of a solar tropical year = 30d. 10h. 29m. 5s.

Calendar months, those made use of in the common reckoning of time, as in Almanacs or Calendars.

The judicial month, consists of 4 weeks or 28 days. Moon, the satellite or secondary of the Earth, at the distance of about 240 thousand miles.

Nadir, the lower pole of the horizon.

Nodes of a planet's orbit, those two points in which it crosses the ecliptic. That in which the planet passes from the south side of the ecliptic to the north, is called its ascending node or dragon's head S, and the opposite point, its descending node, or dragon's tail. The nodes of all the planets' orbits have a slow retrograde motion, occasioned by their moving in different planes, and their mutual attraction on each other.

Vonagesimal, that point in the ecliptic which is 90° from the horizon.

Nutation of a star, a small apparent motion, occasioned by

the variable attraction of the sun and moon on the spheroidal figure of the earth; by which the axis is made to revolve with a conical motion, the extremities or poles describing in 18y. 7m. the lunar period, or revolution of the moon's nodes, a small ellipse whose transverse diamcter 19".1 and conjugate = 14".2. The correction of the right ascension and declination of a star arising from this cause may be readily found by the following theorems: in which A the right ascension of the star (per table), D = its declination, and N = the longitude of the moon's ascending node.

Th. 1.-8.3 cos. (N— A) tan. D—1".25 cos. (N + A) tan. D 16′′.25 sin. N. the nutation in Rt. as. in

seconds of time.

Th. 2.+ 9.55 cos. N. sin. A 7".05 cos. A sin. N = the nutation in declin. in seconds of a degree. The upper signs are to be used when the star has north dec. and the under signs when it has south dec. See Aberration.

Oblique ascension of a celestial body, that point of the equator which rises at the same time with the body in an oblique sphere.

Obliquity of the ecliptic, the angle in which the ecliptic crosses the equator.

Occultation of a star, the moon's passing between the star and the observer, and thereby, for a time, hiding it from his sight.

Olympiads. Games celebrated by the Greeks every 4 years. See Era.

Opposition, that aspect in which the difference of longitude of the two bodies is 180°.

Orbit of a planet, the path in which it revolves round its centre of motion. The orbits of all the planets, whether primary or secondary, are elliptical, though of but small excentricity; and all (with the exception of Herschel's satellites) nearly in the plane of the ecliptic, or earth's

orbit.

Parallax of a celestial body, is equal to the angle at the body, subtended by a semidiameter of the earth terminating in the place of the observer. Hence the horizontal parallax of a body will be the greatest, and in the zenith it will entirely vanish, The fixed stars, from their immense distance, have no sensible parallax.

Parallax of the earth's annual orbit, at a planet, is the angle at that planet subtended by the distance between the earth and sun.

Penumbra, a faint or imperfect shade, observed in eclipses,

and occasioned by a partial interception of the sun's light. Perigee, that point of the moon's orbit which is nearest to the earth. The term is sometimes applied to signify that point in which the sun is nearest to the earth. Perihelion, that point of a planet's orbit which is nearest to the sun.

Periodical time of a planet, that in which it performs a complete revolution round its centre of motion.

Perioeci, such as live in opposite points of the same parallel of latitude.

Periscii, those whose shadows turn quite round during the day, the sun not setting-and such, at certain times of the year, are the inhabitants of the frigid zones.