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Libration of the Moon, 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, two points farthest distant from the nodes.
Longitude of a heavenly body, its distance on the ecliptic from the first of Aries to the intersection of a secondary passing through the body. It is reckoned eastward 360°.
Longitude on the Earth, the distance east or west from a fixed meridian.
Meridian, a great circle of the sphere, encompassing the Earth from north to south. Half of this is sometimes called a meridian.
Nadir, the point in the heavens directly under the observer, and opposite to the zenith.
Nebulæ, telescopic stars cloudy in appearance.
Node, a point at which the orbit of a planet crosses the plane of the ecliptic. The intersection where the planet passes to the north is denominated the ascending node ; where it passes to the south, the descending node ; above being often used for north, and below for south, in astronomical terms.
Oblate spheroid, a spherical body flatted at the poles.
Obliquity, inclination, the angular distance of a circle from the eclíptic.
Oblique sphere, a position of the sphere, in which the equator and parallels cross the horizon in an oblique direction.
Opposition, opposite part of the heavens. Two bodies are said to be in opposition, when their distance of longitude is 180°, though they may not be in the same degree of celestial latitude.
Orbit, the figure described by a planet in its revolution round the Sun, or its primary.
Parallax, the angular difference between the true and apparent place of a heavenly body.
Parallel sphere, a position of the sphere, in which the parallels of latitude and the equator appear parallel to the horizon.
Penumbra, the partial shadow of the Moon.
Perigee, the point in the Moon's orbit nearest to the Earth ; sometimes applied to the place of the Sun, when nearest to the Earth.
Periæci, inhabitants in the same parallels, but under opposite meridians.
Perihelion, the point in the orbit of a planet nearest to the Sun.
Phases, the different appearances of the Moon, Mercury and Venus, as the illuminated side is differently presented to a spectator.
Phenomenon, appearance, often a novel appearance.
Planet, a heavenly body revolving round the Sun, or some primary planet.
Plane of a planet's orbit, that imaginary surface in which it lies; or a supposed even surface between every part of its circumference.
Polar circles, two circles drawn round the Earth from east to west, parallel to the equator, about 23° 28' from the poles.
Poles of a planet or the Sun, the extremities of its axis.
Precession of the equinoxes, their retrograde motion in the heavens.
Primary planets, those which perform their revolutions immediately round the Sun.
Projectile force, that which impels a body in a right line.
Quadrature, a quarter, a point in the celestial sphere 90° from the Sun.
Quadrant, the fourth part of a circle.
Radius, a right line from the centre of a circle to the circumference.
Refraction, the turning of a ray of light from a straight Retrograde motion, apparent motion from east to west.
Right angle, 90°. When a line falls on another line, making the angles on each side equal, each is a right angle.
Right ascension, the distance of a heavenly body from the first of Aries on the equator, or referred to that circle by a secondary. It is reckoned from the first of Aries to the point where the secondary, passing through the body, cuts the equator.
Secondary planets, satellites or moons, small planets revolving round some of the primary planets.
Secondary to a great circle, a great circle crossing it at right angles.
Sidereal revolution, the time of a planet's revolving from a star to the same star again.
Sine, a line drawn from one end of an arch perpendicular to the radius.
Solstices, two points in the ecliptic, 90° from the equinoxes. Star, a luminous heavenly body shining by its own light.
Synodical revolution, the time intervening between the conjunction of a planet with the Sun, and the succeeding conjunction of the same bodies.
Syzygy, the conjunction or opposition of a planet with the Sun; as the change or full of the Moon.
Tangent, a right line touching the circumference of a circle perpendicular to the radius.
T'ide, the alternate ebbing and flowing of the sea.
Transverse, the longest axis of an ellipse.
Tropical revolution, the time intervening between a planet's passing a node and coming to the same node again.
Tropics, two circles parallel to the equator, at the distance of about 23° 28'.
Twilight, crepusculum, the partial light before sunrise in the morning and after sunset in the evening.
Vector radius, a line from a planet, in any part of its orbit, to the Sun.
Vertical circles, circles cutting the horizon at right angles, and passing through the zenith and nadir of a place.
Zenith, the point in the heavens directly over the observer. The zenith and nadir are the poles of the horizon.
Zodiacal light, a pyramid or triangular beam of light, rounded a little at the vertex, appearing before the twilight of the morning and after the twilight of the evening.
Zodiac, a broad circle in the heavens between two lines on each side of the ecliptic, and parallel to it at eight degrees distance.
Zone, literally a belt or girdle ; a large division of the Earth's surface.
The Sun with his attendant planets and comets constitute the solar system.
Conceive a large gilt ball suspended in open space, with several smaller balls moving around it from west to east, at different distances and with unequal velocity; imagine those nearest the large ball to have the swiftest motion, and that the movement of the others is more and more slow, as you pass to those most remote; imagine further, that several of the revolving balls have others moving round them, and carried with them, or round the central ball, and that all these motions are perpetual, and you will have some imperfect idea of the solar system. The idea will be more complete, if occasionally a ball with a fiery train, or tail, be conceived moving with great velocity in a direction nearly to the central ball; but that, passing round this, it recedes with retarded motion, the train increasing as it draws towards the centre, and diminishing as it recedes.
It is important that every instructer in astronomy should be furnished with an orrery. In want of this, balls may
be formed of soft materials. But the whole may be supplied by a fruitful imagination. This alone may bear the student from the balls or the orrery to the great heavenly bodies, their movement, and distances. To give a clear view of the whole, as suspended and re