body from its rising to its setting; and is measured in time.

34. NOCTURNAL ARC of any heavenly body, is, in like manner, measured by the time of its absence from the celestial hemisphere of any latitude, or from its setting to its rising again.

35. CARDINAL POINTS, (cardo, Lat. a hinge,) are those which are principal or reckoning points on the earth, or in the heavens; viz., the North, South, East, and West: the first degree of the signs Aries, Cancer, Libra, and Capricornus the Zenith, Nadir, &c.

36. THE COMPASS consists, First, of a circular card or paper, with the 360° of its circumference divided into thirty-two parts; each of these parts, therefore, containing 1140, and being called "Points of the Compass." Secondly, of a magnetised needle, which, being poized at the centre of the card, and constantly pointing towards the northpole of our world, determines the position in which the card should be placed, in order that it may show the relative direction, or "bearing," of any part of the horizon.

The Mariner's Compass differs from the compass attached to Globes, or usually employed to adjust them, in having the card poized on its centre and moved by the magnet attached to its under surface.

Magno N

NW

NE

37. VARIATION OF THE COMPASS. By the apparent positions and motions of the heavenly bodies, as caused by the rotation of the earth, it may be seen that the magnetised needle does E not point along our meridian, or duly to the north; but, at present, 2410 westward of it (at Greenwich). The compass card therefore, must be moved about until its Fleur de Lis or north point be placed 2440 eastward of the pointing of the magnet, as a correction of this variation. (See Magnetism, Index.)

W

S

38. THE ZENITH is that point in the celestial sphere which is immediately overhead, and which is equally distant from all points of the horizon.

39. THE NADIR is that point in the heavens which is diametrically opposite to our feet. It is the Zenith of our Antipodes.

DEFINITIONS, ETC.

33

Thus the Zenith and Nadir are the two poles of our horizon, and each is 90o distant from every part of it.

40. ZENITH DISTANCE of any celestial object, is the distance of its centre from the zenith, or the complement of its altitude above the horizon. (See illustr. H, p. 19).

41. POLAR DISTANCE of any celestial object, is an arc of the meridian contained between the centre of that object and the pole of the Equinoctial.

42. VERTICAL CIRCLES are imaginary circles of any bearing whatever, passing through the Zenith and Nadir of an observer, and therefore cutting his horizon at right angles. Thus the brazen meridian represents a vertical circle passing north and south.

43. Prime VERTICAL is that vertical circle, which passing through the east and west points of the horizon, is at right angles with the meridian.

44. ALTITUDE of any heavenly body, is its distance above the horizon, reckoned in degrees on a vertical circle: hence it is the complement of the zenith distance.

45. AZIMUTH of any heavenly body, is the distance or bearing of its present vertical circle from the meridian, reckoned in degrees on the horizon.

Hence vertical circles are also called azimuth circles. Azimuth, therefore, means nothing more, than the distance at which any heavenly body would be found from the north or south of the horizon, if it were instantly to glide down, the shortest way, from its present elevation. The variation of the magnet from the meridian is ascertained at sea by comparing the true azimuth, found by calculation from the observed latitude and altitude, or by the Globe, with that shown by the Azimuth Compass.

46. AMPLITUDE is the number of degrees intercepted between the centre of an object when rising or setting, and the east or west point of the horizon.

The variation of the magnet may be found by contrasting the true amplitude, found by calculation, or by the globe, with that shown by the compass.

47. ANGLE OF POSITION, (or apparent bearing) of a given distant place, is the bearing of the zenith of that place from the meridian of the observer; and is reckoned on his horizon from that point (north or south,) over which the pole of the Equinoctial is elevated.

Hence it corresponds to the azimuth of that point of the heavens, which, at the time, is in the zenith of the distant place.

48. QUADRANT OF ALTITUDE is the graduation of 90° of a large circle of the sphere, on a thin flexible strip of brass, provided with a pivot and screw. When attached by this means to the uppermost part of the brass meridian, it serves to trace out and to measure the arcs of vertical circles, the altitudes of stars, their azimuths, &c. There is an additional graduation of 18° in a contrary direction, for the purpose of measuring the depression of the sun, &c., which, thus serving to ascertain the duration of twilight, is called the "Crepuscular" graduation. (See Twilight, Index.)

POSITIONS OF THE SPHERE are three, RIGht, Parallel, and OBLIQUE.

49. That position of the Sphere is called Right, which they have who live at the Equator; and who, having always a point of the equinoctial in the Zenith, have the Polar points in their horizon. To such inhabitants all the heavenly bodies rise and set, and have their daily courses at right angles with the horizon.

50. That position of the sphere is called Parallel, which would be presented to an inhabitant of either Pole; who, having the Polar point in his zenith, would have his horizon coinciding with the Equinoctial, and the heavenly bodies of one hemisphere only, constantly circling around him in lines parallel to it.

51. That position of the sphere is called Oblique, which they have who live neither at the Pole nor at the Equator; but who, having those stars culminating* vertically, which have declination answering to their latitude, have the Equinoctial stars culminating northward or southward at a corresponding zenith distance, and a portion of the heavenly bodies of both hemispheres rising and setting in courses oblique with regard to the horizon.

52. RIGHT ASCENSION of any heavenly body, is that degree of the Equinoctial which comes to the meridian with it. It is called Right Ascension, because, at all places situated on the Equator, and having, consequently, a Right sphere, this degree not only culminates with the heavenly body, but also rises and sets with it. Right Ascension is reckoned from the first point of Aries eastward all round the globe, either in degrees and parts of a degree, or in hours, minutes, and seconds.

* G on page 9.

DEFINITIONS, ETC.

35

53. A SIDEREAL CLOCK is a clock so constructed, as that its hands may perform their revolutions, not in 24 hours, but in 23 hours, 56 minutes, 4 seconds, and nine hundredth-parts of a second ;* and which, being set at the instant that the first degree of Aries comes to the meridian, will point out to the astronomer the right ascension which may be on the meridian, at any instant of its being consulted.

54. OBLIQUE ASCENSION of any heavenly body, is that degree of the Equinoctial which rises with it in an Oblique Sphere. It is reckoned, like right ascension, from the first point of Aries; and varies with the obliquity of the sphere, that is, with the latitude.

55. OBLIQUE DESCENSION of any heavenly body, is that degree of the Equinoctial which sets with it in an Oblique sphere; and, like the oblique ascension, is greater or less according to the latitude.

56. ASCENSIONAL DIFFERENCE is the number of degrees between that point of the Equinoctial which, in a given latitude, rises with a heavenly body, and its right ascension or the point which rises with it at the Equator and every where culminates with it.

Similarly, the corresponding number of degrees between the right ascension of a heavenly body, and that degree of the Equinoctial which sets when it is setting, is called its Descensional Difference.

57. DISC (SLOKoç, a quoit,) is the circular face of the sun or moon, so called from its apparent flatness. The diameter of a disc is supposed to be divided into twelve equal parts, called "Digits." Any portion of the circumference of a Disc is called its " Limb."

58. AN ECLIPSE (EKλens, a falling away,) is "a privation of the light of one of the luminaries, by the interposition of some opaque body," either between that body and the eye, or between that body and the sun.

The term Occultation also, is applied to the interposition of the moon between us and a star or planet: Transit, to the passage of the inferior planets, Mercury and Venus, across the sun's disc. In both cases the commencement of the occurrence is called the Immersion of the star or planet, and the termination of it, its Emersion. The term transit is also constantly employed for the daily culmination of any heavenly body.

* See definition Sidereal Day.

59. ECLIPSE OF THE SUN is the occultation or hiding of his disc, or of a portion of it, by the intervention of the moon.

Hence, at the time of a Solar Eclipse, the moon is vertical, or nearly so, where the sun is vertical.

60. ECLIPSE OF THE MOON is the obscuration of her disc by the shadow of the earth.

Hence, at the time of a Lunar eclipse, the sun is vertical, or nearly so, to the Antipodes of those who have the moon vertical.

61. THE ECLIPTIC, in the heavens, or on the celestial globe, is a great circle drawn, or supposed to be drawn, over all those stars which are situated in the same plane with our earth and the sun, and over which that luminary therefore appears to pass as we revolve around him. The ecliptic derives its name from the circumstance of the moon's being situated in it at the time of eclipse. It is sometimes called the sun's path; but sometimes also, and more correctly, "the* heliocentric circle of the earth," as it marks out the real annual path of our planet, as it would be seen from the sun's centre. (See Ecliptic Illustrations, Index.)

The "Ecliptic" on the terrestrial globe, is nothing more than a record, given in a continuous line, of the degrees of latitude which, day by day, answer to the declination of the sun, as our earth revolves around him whilst keeping its axis invariably directed to the Polar point.

62. THE OBLIQUITY OF THE ECLIPTIC is the angle at which the ecliptic is inclined to the Equinoctial, and consists of 23° 28'. It is the angle at which the earth inclines its axis and, consequently, its Equator, whilst annually moving around the sun in the ecliptic plane. (See Ecliptic Illustrations, Index.)

63. EQUINOCTIAL POINTS. Those points of the celestial sphere, where the ecliptic is intersected by the Equinoctial, are called Equinoctial points. (See Ecliptic Illustrations, Index.)

64. SOLSTITIAL POINTS, OR "SOLSTICES," (Lat. sol, the sun, and stare, to stand,) are those points of the celestial sphere where the sun's apparent motion to the south or to the north ceases; and at which he, consequently, appears in that respect to stand still.

*λos (helios) the Sun.