Immediately after sunrise, the form of tho luminary appears some what elliptic, the horizontal diameter being longer than the vertical one. This is the effect of lizritAmos, which varies so rapidly near the horizon that the upper end of the vertical diameter is leas elevated than the lower end by a sensible quantity, while the two emits of the horizontal diameter are equally elevated. The same phenomenon omers with the moon, when rising at the full, and would also be seen in the planets, if they were large enough in appearance. It must also be noted that both sun and moon appear larger when near the horizon; but this, as to the sun, is delusion, eine° when measured with instru menta its apparent diameter is the same at all parte of the day. It is true that both bodies, when in the zenith, are nearer to the spectator than when in the horizon, by what may be called without error a semi diameter of the earth ; the moon is near enough to show the effect of this in instrumental measurements of its diameter, but it is not ae with the sun.
Before looking at what we know of the physical appearances of the sun, its distance from the earth must be mentioned, to which we may add at once the other elements of its orbit. Its equatorial horizontal l'AnAi.c.Ax, at its mean distance, is 8"-5776, and its apparent semi diameter 16' It is then distant from the earth by 24,000 semidiameters of the earth, or about 95,000,000 miles. Its diameter is 111 times as great as that of the earth, or upwards of 830,000 miles; and its bulk is 1,800,000 times as great as that of the earth. But its mass, as determined from its action on the planets, is only 355,000 times as great as that of the earth ; whence its averago density is only one-quarter of that of the earth. But this last result takes into the body of the sun all that is seen of it ; if the surmise presently to be mentioned, of its having a luminiferous atmosphere of considerable extent, be well founded, the real body of the sun may have as much density as the earth, or more. It revolves on its axis in 251 of our mean solar days ; according to Delambre, in 25-01154 days : the axis being inclined to the ecliptic at an angle of 82°.
The ecliptic is the circle in which the sun appears to move, in common language. In strictness, however, the earth does not move round the sun in a true plane, though it does so very nearly. The centre of gravity of the earth and moon (a point near the earth) does much more nearly describe a plane; that is, a spectator situated at that point would more nearly see the sun move in a great circle than we do. But to us, the sun is sometimes on one side and sometimes on the
other of this mean ecliptic, and therefore generally has some latiGule, though a very small one: the column entitled " the Sun's latitude " iu ` The Nautical Almanac' is a puzzle to those readers whose astronomy is drawn from the usual elementary writings : in truth, it is only a fraction of a second, and the sun crosses the mean ecliptic twice in every lunation. The obliquity of the mean ecliptic, for January 1, 1360, is 23° 27' 27"•33, and it is diminishing yearly by 0"•457: thus this same obliquity for January 1, 1842, is 23° 27' 35"-60. If this diminution could go sufficiently far, it would in time bring the ecliptic and equator to coincidence, or equalise days and nights all over the world ; and if it could be held to have continued long enough, would entitle us to presume that the poles were once in the plane of the ecliptic, or that every part of the earth went through all gradations in a year from equal days and nights to a polar day and n polar night. But the cause of this diminution of the ecliptic is known from the theory of gravitation; and it is also known that, under existing causes, it cannot be permanent, but must diminish in quantity and finally turn into an increase before its effect has amounted to many degrees. Persons acquainted with the phenomena of the heavens, but not with the results of the theory of gravitation, remembering the fact of tropical productions being found buried in high latitudes, sometimes imagine that they can look back to the time when the poles were so uear the ecliptic, that these same high latitudes were within the tropics. This, however, is a pure fancy, and they had better imagine another cause; the one they think of will not do.
The mean longitude of the sun, at Greenwich mean noon on the let of January of the year 1800+t, may bo determined from 230° 53' + + '00012218050-14' f, where f is the remainder of I divided by 4, or 4 if the remainder be 0 ; that is, the number of years after the Julian leap-year, or after the Gregorian leap-year for all years after 1804. The mean motion in longitude iu a mean solar day is 59' The excentricity of the run's orbit is (according to Laplace, .01635318), or the greatest and least distances of the sun from the earth are in the proportion of to -933, or as 30 to 29, or more nearly as 91 to 88. The greatest equation of the centre [Timm] is 1° 55' 27"1. The exccntricity diminishes by in a century.