Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/837

distant from the equator, with regard to which the varia tions of declination on either side are perfectly symmetrical

and uniform.

The observations of the sun s right ascensions and meridional altitudes, which have been made daily during so great a number of years, and under so many different meridians, furnish complete proof that the projection of the sun s orbit is a great circle of the celestial sphere, and that the orbit itself is wholly confined to the same plane.

The great circle which the sun describes in virtue of his proper motion is called the Ecliptic. It has received this name from the circumstance that the moon, during eclipses, is either in the same plane or very near it. These pheno mena can, in fact, only happen when the sun, earth, and moon are nearly in the same straight line, and, conse quently, when the moon is in the same plane with the earth and the sun. The angle formed by the planes of the ecliptic and equator, which is measured by the arc of a circle of declination intercepted between the equator and nitions. tropic, is called the Obliquity of the Ecliptic. The two points in which the equator and ecliptic intersect each other are called the Equinoctial Points ; they are also denomi nated the Nodes of ike Equator ; and the straight line con ceived to join them is the Line of the Equinoxes, or the Line of the Nodes. The node through which the sun passes on coming from the south to the north of the equator is called the Ascending Node, and is usually distin guished by the character ft ; the opposite node is the Descending Node, and is marked by y. A straight line passing through the centre of the earth, perpendicular to the plane of the ecliptic, is called the Axis of the Ecliptic, and the points in which its prolongation meets the sphere are called its Poles these denominations being analogous to those of the axis and poles of the equator. The two small circles of the sphere which pass through the poles of the ecliptic, and are parallel to the equator, are called the Polar Circles.

{{ti|1em|The ecliptic has been divided by astronomers, from time immemorial, into twelve equal parts, called Signs, each of which consequently contains 30 degrees. The names and symbols by which they are characterised are as follows:—

North of the Equator. Aries, <y> Taurus, y Gemini, n Cancer, 03 Leo, ft Virgo, T1J South of the Equator. Libra, b Scorpio, TT|^ Sagittarius, / Capricornus, j.-^ Aquarius ~^ Pisces, >(

In each of these signs the ancients formed groups of stars, which they denominated the Zodiacal constellations ([ Greek ], animals), not confined to the ecliptic, but included within an imaginary belt, extending 9° on each side of it, to which they gave the name of Zodiac ([ Greek ], circle or zone of the animals). The term sign is now employed only to denote an arc of 30°, and will probably soon be banished entirely from astronomical tables. It is now seldom used even for tables of the planets. For merly, to denote that the longitude of a planet is 276° 12′, it was usual to write 9? 6° 12′; or even to employ the characteristic symbol, and to write [ symbol ] 6° 12′, meaning that the planet was 12′ in the 6th degree of Capricornus, or the tenth sign. The latter inconvenient practice is now laid aside, and the signs, when they are employed, are simply distinguished by the ordinal numbers.

As the greater part of the celestial phenomena connected with the planetary system take place either in the ecliptic or in planes not greatly inclined to it, it is found to be most convenient to refer the positions of the planets, and frequently those of the stars also, to that plane. The first point of Aries, which is the technical expression for the intersection of the ecliptic and equator, or the place of the sun at the vernal equinox, is assumed as the origin from which the degrees of the ecliptic, as well as of the equator, are counted from west to east, or in the direction of the sun s annual motion. The angular distance of the sun from this point is called his Longitude, and the longitude of a star is the arc intercepted on the ecliptic between the same point and a great circle passing through the star per pendicular to the ecliptic. The arc of this circle inter cepted between the star and the ecliptic, or, which is the same thing, the complement of the star s distance from the pole of the ecliptic, is called the Latitude of the star ; so that longitude and latitude bear the same relation to the ecliptic that right ascension and declination bear to the equator.

The sun s motion along the ecliptic is found not to be strictly uniform, a circumstance into which we shall have to inquire more particularly farther on. In this place, let it suffice to notice that the sun is found to move more quickly in winter than in summer, the rate of motion changing from its maximum nearly in mid-winter to its minimum nearly in midsummer, and thence to its maxi mum again. But at no time does the motion differ greatly from its mean rate of very nearly 59 in a sidereal day. If we call the mean rate 10,000, then the greatest and least rates of motion are represented by the members 10,336 and 9664 respectively.

The direction in which the sun travels round the ecliptic, and in which longitude is measured, is from west to east, that is, it is contrary to the direction in which the star- sphere rotates.

Chapter IV.—The Solar Day—Equation of Time—The Seasons.

Since the sun travels thus around the celestial sphere, it ia manifest that the successive returns of the sun to the meridian cannot recur after the same interval of time as the successive returns of any given star. If on any day the sun s centre when he is crossing the meridian has a particular position on the star-sphere, then when that point of the star-sphere next returns to the meridian, that is, one sidereal day later, the sun has travelled about from that point, moving along the ecliptic in a direction opposed to that in which the star-sphere rotates. The star-sphere must, therefore, rotate a little further round before the sun will be on the meridian. As a convenient first approximation to the actual effects, let us make the supposition that the sun moves along the equator instead of the ecliptic, and that he moves exactly 1 in a sidereal day. In this case he would be exactly 1 from the meridian when the point he had occupied on the meridian the day before had reached the meridian. That point on the star-sphere would have completed the full circuit of 360 of rotation while the sun had completed only 359, and his diurnal motion being therefore only =359/360 of the star-sphere s, it fol lows that the solar day (or the interval between the sun s successive returns to the meridian) would be greater than the sidereal day in the ratio of 360 : 359. Therefore, the solar day being divided into 24 x 60 minutes, the sidereal day would manifestly be 4 min. shorter.

But as the sun moves in a circle inclined more than 23 to the equator, and as the sun s motion is slightly variable, and the mean rate less than 1 per sidereal day, these relations are not exactly presented.

Let us, as a next approximation, suppose the sun to

move uniformly round the equator once in the course of a

year of 365J days, and determine the length of a solar day