the meridian. The interval between one apparent noon and another, or the Solar day, is subject to continual variation; first, because the rate of motion of the earth in its orbit is unequal, and causes a corresponding inequality in the apparent ecliptic motion of the sun; and, secondly, because of the obliquity of the ecliptic path with regard to the earth's axis and equator.

86. TRUE OR MEAN NOON is twelve o'clock, or the exact completion of twenty-four hours, as shown by a correct time-keeper, the going of which is exactly adjusted to the average solar day. Hence, the time as shown by such a clock, is called mean time.

87. EQUATION OF TIME is the difference, on any given day, between apparent time, as shown by the sun-dial, and mean time, as given by a correct clock.

That portion of this discrepancy which arises from the obliquity of the ecliptic, is all that can be found by the globe; and, if this portion were the whole, the sun-dial and clock would agree on the equinoctial and solstitial days. But, in consequence of the united operation of the two causes, (see Def. 85,) the days on which the sun and clock agree, are the 15th April, the 16th June, the 1st September, and the 24th December; the clock being before the sun by fourteen minutes and a half about the 12th February: and after the sun as much as sixteen minutes and a quarter, about the 1st of November.

88. Lunar Day, is employed by some authors to designate the period elapsing between two consecutive times of any semi-meridian's coinciding with the moon; and is of the average length of 24 hours, 50 minutes, 48 seconds.

It consists of the sidereal day, or rotation of our meridian, and that additional portion of time which we occupy in passing, by the earth's rotation, from the plane in which we last left the moon, to coincide with the plane into which she has shifted, by performing a part of her monthly eastward revolution around us.

89. ASPECT of a planet or star, is its apparent situation with regard to another planet or star.

Aspects are marked as under :

Conjunction, or in the same sign and degree of Longitude. 8 Opposition, or in the exactly opposite sign and degree. A Trine, or one third of the ecliptic, or four signs apart. Quartile, or one quarter of the ecliptic, or three signs apart. Sextile, or one sixth of the ecliptic, or two signs apart.

N.B.-The Conjunction or Opposition of the moon, i. e., her being new or full, is called her Syzygy; and her Quartile Aspect, as when she is "half-moon," her Quadrature.

DEFINITIONS, ETC.

*43

90. DIRECT, ȧ planet's motion in the Zodiac is said to be direct when, as we view it night by night, it is going eastward like the moon; or according to the order of the signs.

91. RETROGRADE. A planet's motion in the Zodiac is said to be retrograde when it appears to go contrary to the order of the signs.

92. STATIONARY.

A planet is said to be stationary, when, in consequence of the earth's position and motion, as combined with its own, it appears for some time in the same point of the heavens.

93. NODES. Are the two points at which the orbit, or path, of one planet, intersects the orbit of another planet. With reference to the plane of our orbit or ecliptic, that point at which the planet crosses to come northward, is called its ascending node, and is marked thus ; and the point at which it is going southward, or its descending node, thus 8.

An imaginary line joining these points, and thus being in the plane of both orbits and passing through the centre of the sun, is called the Line of the Nodes.

94. APOGEE, (año, from, and yn, the earth.) That point in the elliptical orbit of the moon, which is most distant from the earth.

95. PERIGEE, (Tepɩ, near, and yŋ, the earth.) That point in the orbit of the moon at which she makes her nearest approach to the earth.

96. PERIHELION, (πepɩ, and ŋλios, the sun.) The nearest approach of a planet to the sun.

97. APHELION, (añw, and ŋλɩs, the sun.) That point in the elliptical orbit of a planet which is most distant from the sun.

Perijove and Apojove, Perisaturnium and Apesaturnium, are employed with reference to the satellites of Jupiter and Saturn.

98. APSIS, (plural Apses or Apsides,) are the points of greatest and least distance of a planet from its primary, or the body around which it revolves. Thus the Perihelion or the Perigee is called the lower Apsis, and the Aphelion or the Apogee, the higher Apsis; and an imaginary line joining these extreme points, is called the line of the Apses, or Apsides."

99. GEOCENTRIC place of a planet, is the position in which it would appear if viewed from the centre of the earth.

100. HELIOCENTRIC place of a planet, is the position in which it would appear if viewed from the sun's centre.

101. ELONGATION is a term employed to denote the apparent distance, in degrees of the sky, between an inferior planet and the sun. Our separation from Mercury

is such as to cause his distance from the sun of 36 millions of miles, to appear to be 29 degrees of our sky, when his position renders that distance most visible to us. The greatest elongation of Venus is 47 degrees.

Let S be the sun and E the earth; then M and V may represent Mercury and Venus at their greatest elongations of 29° and 47°. By referring to P on p. 19, the pupil will understand, that taking the orbits to be as drawn, circular,* the triangle SVE is right-angled at V; and S V (the distance of Venus from the sun,) must bear the same relation to S E, (the distance of the sun from our earth, which is known to be 95 millions of miles,) that the sine of 47° bears to radius. The dis

tance S V is, accordingly, 68 millions; and S M (that of Mercury) from the same considerations, 36 millions of miles.

The Phases (H, C, and G,) of Venus in our sky, as she varies her positions, h, c, and g, in her orbit, may perhaps be understood from the diagram. She is most brilliant when she appears, through the telescope, of the crescent form, (C) because of her being nearer to us, as (c); and consequently giving light from a portion of a disc of much greater diameter.

102. PHASE, (paris, an appearance,) is a term applied to the changes of appearance of the discs of the moon and other planets.

As not more than half of the surface of any planet can be enlightened at once by the sun-beams, it is evident, that our position with regard to that half-surface must determine the extent of it which is visible to us.

*They are nearly circular.

DEFINITIONS, ETC.

45

Remark. The moon revolves eastward, completely round the earth, in a portion of time during which the sun changes his apparent place comparatively little; and hence, at one time, presents towards us the whole of the light she reflects from him; at another time, no portion of it. The same phases result from the varying positions of the discs of the two inferior planets, when viewed in those positions by the telescope.

Mercury and Venus, performing their orbits around the sun within the compass of our own circuit, like the moon are alternately presenting towards us the whole of their enlightened sides, and various portions of those parts of their surfaces from which the sun-beams are absent.* With respect to the first superior planet, Mars, we must remember, that as he performs his circuit at a distance beyond us, half as great as our own whole distance from the sun, (and thus beholds our orbit at nearly 500 millions of miles from him); our greatest elongation, or utmost remove from the sun in his sky, is only about 48 degrees; and our earth must, necessarily, rise and set to him like Venus to ourselves: in other words, that we can never spend much time in his night-sky: accordingly, we find that a large proportion of his enlightened disc, or day hemisphere, is viewed through our telescopes. He is sometimes round, and sometimes gibbous like our moon when within two days of being full; but never horned, or even halved, like that satellite, or like Venus or Mercury.

The pupil is now prepared to answer a difficulty which may have arisen in his mind; viz., Why the discs of Jupiter and Saturn, and of their satellites, are always complete? At the vast distance of Jupiter, the apparent size of the earth's orbit is so minute, that our greatest elongation from the sun is inconsiderable, and our planet almost invisible in the sun's rays; and thus, as that portion of Jupiter which is turned towards us is likewise turned toward the sun, we constantly see him and his satellites without these changes or phases.

The same remarks apply, a fortiori, to the more distant planets, Saturn and Uranus.

For COSMICAL, ACHRONYCAL, and HELIACAL rising and setting, see Precession of the Equinoxes, Index.

103. ABERRATION. A very slight apparent displace

* See the foregoing wood-cut.

ment of the fixed stars, amounting only to about of a degree, occasioned by a want of correspondence in the direction of the orbitual motion of the earth, and of the motion of the light proceeding from them to the observer. (See Light, Index.)

Days and Hours of different Nations.

"The recurrence of light and darkness, of comparative warmth and cold, of noise and silence, of the activity and repose of animals; the rising, mounting, descending, and setting of the sun; the varying colours of the clouds, generally, notwithstanding their variety, marked by a daily progression of appearances *** Man calls each portion of time in which this series of appearances and occurrences come round, a day and such a group of particulars are considered as appearing or happening in the same day."-Whewell's Inductive Sciences, Vol. I, p. 112.

:

104. THE CIVIL DAY. The natural day is called the civil day with reference to the mode of its division by different nations for civil purposes. The ancient Hebrews computed their Civil Day, as the modern Jews do, from evening to evening: the compound word "evening-morning," is employed by the prophet Daniel to denote that period. This mode of computing the day, as well as that of the beginning of the year and of the week, originated in the command of God by Moses, as one means of distinguishing them from the neighbouring idolatrous nations. It is probable that, like the ancient Babylonians, Syrians, and Persians, whose chief idol was the sun, they, previously to their departure from Egypt, began their day at sun-rising.

The ancient Athenians began their civil day, like the Jews, at sunset. This is the practice also of the Chinese, and of modern Italy, where the twenty-four hours are reckoned in succession.* The modern nations of Europe, with this remarkable exception, follow our own method of counting the twenty-four hours in two equal portions of twelve hours, from midnight to midnight.

“The natural day commenced with darkness. The darkness and the light which succeeded the first chaos, are declared to have constituted the first day. In the same manner are reckoned the five

* The rising generation of Italy are now taught the French time.