Epact, the excess of the solar above the lunar year of 12 mean lunations, or 354 days.

Equator is an imaginary circle drawn round the centre of the earth from east to west.

Equation is a correction applied to the mean time or place to reduce it to the true.

Equinoxes, or equinoctial points, are two points in the ecliptic, where it is cut by the equator; one called the vernal, the other the autumnal equinox.

Eccentricity is the distance between one of the foci and the centre of a planet's orbit.

Focus is the point in the elliptical orbit of a planet, round which the planet revolves.

Foci, the plural of focus, are two points in the transverse axis of a planet's orbit, distant from each other twice the extent of the eccentricity.

Galaxy, the milky way.

Geocentric motion is the apparent movement of a planet, as seen from the earth.

Gibbous, protuberant, convex; applied to the moon between the quadrature and full. It is also applied to some of the planets.

Globes, spheres representing the earth and visible heavens. Golden number, the cycle of the moon, a period of 19

years.

Heliocentric motion, the motion of a planet as seen from the sun.

Hemisphere is the half of a globe or sphere, divided by a plane passing through its centre.

Horizon is a circle round the earth, ninety degrees from the zenith of a place, the plane of which divides the earth into upper and lower hemispheres. This is called the rational horizon. The sensible horizon is the circle which bounds our sight.

Inclination of a planet's orbit, the angular distance between the orbit and the ecliptic.

Latitude of a heavenly body is its distance north or south from the ecliptic. Degrees of latitude are reckoned on secondaries to the ecliptic, passing through the body.

Latitude on the earth is the distance north or south from the equator, reckoned in degrees and minutes.

Libralian of the moon is a periodical irregularity in her motion, by which exactly the same face is not always presented to the earth.

Limits in a planet's orbit are the two points farthest distant from the nodes.

Longitude on the earth is the distance east or west from some fixed meridian, assumed as first.

Longitude of a heavenly body is the distance on the ecliptic from the first of Aries to the intersection of a secondary passing through the body. This longitude is reckoned eastward, 360°.

Meridian is a great circle of the sphere, drawn from north to south through the poles.

Nebula are telescopic stars, having a cloudy appearance. Nodes are two points, at which a planet's orbit crosses the plane of the ecliptic. That intersection, where a planet passes to the north, is called the ascending node; the opposite, the descending node.

Nonagesimal is the ninetieth degree of the ecliptic above

the horizon.

Notanda, things to be noted, or observed.

Oblate spheroid, a spherical body flatted at the poles. Obliquity of a circle to the ecliptic, the angular distance between that circle and the ecliptic.

Oblique sphere is a position of the sphere, in which the equator and its parallels cut the horizon in an oblique direction.

Opposition, opposite part of the heavens. Two bodies are said to be in opposition, when they are 180° distant, though they may not be in the same degree of celestial: latitude.

Orbit is the figure, which a planet describes in its revolution round the sun, or round its primary.

Parallax is the difference between the true and apparent place of a heavenly body. When the body is in the horizon, it is called horizontal parallax. A planet would appear in its true place, if seen from the centre of the earth. It appears in the apparent, when seen from the earth's surface.

Parallel sphere is a position of the sphere, in which the equator and circles of latitude are parallel to the horizon. Penumbra is the moon's partial shadow.

Perigee is the point in the moon's orbit nearest the earth. The term is sometimes applied to the sun, when nearest to the earth.

Periaci are inhabitants living in the same parallel, but opposite meridians.

Perihelion is the point in a planet's orbit nearest the sun.

Phases are the different appearances of the moon, Mercury and Venus, as the illuminated side is differently presented.

Phenomenon, appearance.

Phenomena, the plural of phenomenon, appearances, generally, unusual appearances.

Planet, a revolving heavenly body.

Plane of a planet's orbit is the imaginary surface in which it lies; passing through the centre of the planet, it extends indefinitely into the heavens.

Polar circles are two circles drawn round the earth from east to west, parallel to the equator, about 23°, 28* from the poles; the northern called the Arctic, the southern the Antarctic, circles.

Poles of a planet's orbit are the extremities of its axis. Precession of the equinoxes, the retrograde motion of the equinoxes from a fixed point in the heavens.

Primary planets are those which revolve immediately round the sun.

* See Obliquity,

Prime vertical is that vertical circle which crosses the meridian at right angles, cutting the horizon in the cardinal points, east and west.

Projectile force, is that which impels a body in a right line.

Quadrature is the point in the celestial sphere, ninety degrees from the sun.

Quadrant, the fourth part of a circle.

Radius, the extent from the centre of a circle to the circumference.

Refraction, the incurvation of a ray of light from its rectilinear course.

Retrograde motion of a planet, apparent motion from east to west.

Right ascension of a heavenly body is its distance from the first of Aries, reckoned on the equator. If the body be not in the celestial equator, right ascension is reckoned from the point, where the secondary, passing through the body, cuts the equator.

Secant, a line drawn from the centre of a circle through one end of an arch till it meets the tangent.

Secondary planets, or satellites, are those which revolve round some of the primary planets.

Secondary to a great circle, is a great circle crossing it at right angles.

Segment of a circle is any part, greater or less than a semicircle, cut off by a chord.

Sidereal revolution is the time in which a planet moves from a star to the same star again.

Sine is a right line drawn from one end of an arch perpendicular to the radius.

Solstices are two points of the ecliptic, ninety degrees from the equinoxes.

Star, a luminous heavenly body appearing always in the same, or very nearly the same situation; hence called fixed

star.

Supplement of an arch, what it wants of 180°.

Synodical revolution is the time intervening between a conjunction of a planet with the sun, and the next conjunction of the same bodies.

Syzygy is the conjunction or opposition of a planet with the sun.

Tangent is a line touching the circumference of a circle, perpendicular to the radius.

Tide, the alternate ebb and flow of the sea.

Transverse is the longest axis of an ellipse.

Tropical revolution is the time intervening between a planet's passing a node, and coming again to the same node.

Tropics are two circles drawn round the earth parallel to the equator, at the distance of about 23° 28'; the northern called the tropic of Cancer, the southern the tropic of Capricorn.

Twilight, the partial light observed before sunrise in the morning, and after sunset in the evening.

Vector radius is an imaginary line from a planet in any part of its orbit to the sun.

Versed sine, that part of a diameter or radius which is between the sine of an arch and the circumference; or it is what the co-sine wants of being equal to radius.

Vertical circles are circles drawn through the zenith and nadir of a place, cutting the horizon at right angles.

Zenith is the point in the heavens directly over the head of the observer. The opposite point is called the Nadir. These are the poles of the horizon.

Zodiacal light, a pyramid of light, appearing before the twilight of the morning, and after the twilight of the evening. Zodiac is a broad circle included between two lines drawn parallel to the ecliptic, at eight degrees distance on each side. This zone includes the orbits of all the planets formerly known.

Zone, a large division of the earth's surface; literally, a girdle.