ECLIPTIC

ecliptic, in astronomy. the plane of the ecliptic is that plane in or near which the centre of gravity of the earth and moon revolves round the sun. the ecliptic itself is the great circle in which this plane meets the celestial sphere. it is also defined, but not with absolute rigour, as the apparent path described by the sun around the celestial sphere as the earth performs its annual revolution. owing to the action of the moon on the earth, as it performs its monthly revolution in an orbit slightly inclined to the ecliptic, the centre of the earth itself deviates from the plane of the ecliptic in a period equal to that of the nodal revolution of the moon. the deviation is extremely slight, its maximum amount ranging between 0.5' and 0.6". owing to the action of the planets, especially venus and jupiter, on the earth, the centre of gravity of the earth and moon deviates by a yet minuter amount, generally one or two tenths of a second, from the plane of the ecliptic proper. owing to the action of the planets, the position of the ecliptic is subject to a slow secular variation amounting, during our time, to nearly 47" per century. the rate of this motion is slowly diminishing. the obliquity of the ecliptic is the angle which its plane makes with that of the equator. its mean value is now about 23 deg. 27'. the motion of the ecliptic produces a secular variation in the obliquity which is now diminishing by an amount nearly equal to the entire motion of the ecliptic itself. the laws of motion of the ecliptic and equator are stated in the article precession of the equinoxes. attempts have been made by laplace and his successors to fix certain limits within which the obliquity of the ecliptic shall always be confined. the results thus derived are, however, based on imperfect formulae. when the problem is considered in a rigorous form, it is found that no absolute limits can be set. it can, however, be shown that the obliquity cannot vary more than two or three degrees within a million of years of our epoch. the formula for the obliquity of the ecliptic, as derived from the laws of motion of it and of the equator, may be developed in a series proceeding according to the ascending powers of the time as follows: we put t, the time from 1900, reckoned in solar centuries as a unit. then, obliquity = 23 deg. 27' 31.68" - 46.837" t - 0.0085" t^2 + 0.0017" t^3. from this expression is derived the value of the obliquity at various epochs given in the following table. the left-hand portion of this table gives the values for intervals of 500 years from 2000 b.c. to a.d. 2500 as computed from modern data. for dates more than three or four centuries before or after 1850 the result is necessarily uncertain by one or more tenths of a minute, and is therefore only given to 0.1'. b.c. 2000; obl. = 23 deg. 55.5" a.d. 1700; obl. = 23 deg. 28' 41.91" 1500 " = 23 52.3 1750 " = 23 28 18.51 1000 " = 23 48.9 1800 " = 23 27 55.10 500 " = 23 45.4 1850 " = 23 27 31.68 0 " = 23 41.7 1900 " = 23 27 8.26 a.d. 500 " = 23 38.0 1950 " = 23 26 44.84 1000 " = 23 34.1 2000 " = 23 26 21.41 1500 " = 23 30.3 2050 " = 23 25 57.99 2000 " = 23 26.4 2100 " = 23 25 34.56 2500 " = 23 22.5 (s. n.) eclogite (from gr. [greek: ekloge], a selection), in petrology, a typical member of a small group of metamorphic rocks of special interest on account of the variety of minerals they contain and their microscopic structures and geological relationships. typically they consist of pale green or nearly colourless augite (omphacite), green hornblende and pink garnet. quartz also is usually present in these rocks, but felspar is rare. the augite is mostly a variety of diopside and is only occasionally idiomorphic. the garnet sometimes forms good dodecahedra, but may occur as rounded grains, and encloses quartz, rutile, kyanite, and other minerals very frequently. the hornblende is usually pale green and feebly dichroic, but, in some eclogites which are allied to garnet-amphibolites, it is of dark brown colour. among the commoner accessory minerals are kyanite (of blue or greyish-blue tints), rutile, biotite, epidote and zoisite, sphene, iron oxides, and pyrites. the rutile is invariably in small brown prisms; the kyanite forms bladed crystals, with perfect cleavage; felspar, if present, belongs to basic varieties rich in lime. other minerals which have been found in eclogites are bronzite, olivine and glaucophane. the last mentioned is a bright blue variety of hornblende with striking pleochroism. the eclogites in their chemical composition show close affinities to gabbros; they often exhibit relationships in the field which show that they were primarily intrusive rocks of igneous origin, and occasionally contact alteration can be traced in the adjacent schists. examples are known in saxony, bavaria, carinthia, austria, norway. a few eclogites also occur in the north-west highlands of scotland. glaucophane-eclogites have been met with in italy and the pennine alps. specimens of rock allied to eclogite have been found in the diamantiferous peridotite breccias of south africa (the so-called "blue ground"), and this has given rise to the theory that these are the parent masses from which the kimberley diamonds have come. (j. s. f.)