YEAR. Much connected with this article is to be found in KALEN. Dale, PERIODS OF REVOLUTION, Moox, SUN, CHRONOLOGY, TIME, &e, We here confine ourselves to matters of useful reference connected with the length and subdivisions of the year, omitting discussion of points of history, which do not directly bear upon chronological ockoning.
The year is, roughly speaking, the period of time in which the SUE melee the circuit of the heavens, and the seasons of agriculture rue their course.
A sidereal year is the period in which the sun moves from a star tc the same again ; that ia, the interval between the two times when the sun has the same longitude as a given star. The mean period is mean solar days, or 365 Gi 9" 9."6.
A tropical or civil year is the time in which the sun moves from the vernal equinox. to the vernal equinox again ; and its mean length it mean solar days, or 365's 48' 49'7.
The anomalistic year is the time in which the sun moves from its perigee (or nearest point to the earth) to its perigee again ; and its length is mean solar days, or tib 13' 49..3.
The tropical year is shorter than the sun's actual orbital revolution, or the sidereal year, because the equinox moves slowly backwards [Piteeessiox], and therefore the sun meets it again before it arrives at the point at which it met it last. The anomalistic year is longer than the sidereal year because the perigee moves forward, and the sun is not nearest to the earth until it has passed the longitude at which it was nearest to the earth before. The tropical year is the year, when no distinctive term is applied ; for the passage of the sun from the southern to the uorthemi side of the ecliptic is the positive pheno menon on which the seasons depend, though it may not be correct to say that it is then that the succession of seasons begins.
The anomalistic year does not, and from the theory of gravitation most probably cannot, vary by any quantity which the human senses could appreciate; but the sidereal and tropical years vary very slowly in length. The reason is twofold. In the first place, the amount of the yearly precession of the equinoxes is slowly increasing ; so that the part of the orbit by which the equinox moves backwards to meet the sun becomes greater, or the duration of the year less. In the second
place, the gradual motion of the equinox, combined with that of the perigee, brings the part of the orbit which the sun is saved from per forming by the recession of the equinox into different places with respect to the perigee in successive years; so that the excepted portion is in different years what would have been described in different times. The second consideration affects the sidereal year as well as the tropical ; but since in both cases the abet is very small and slow, a few seconds in a thousand years, there is no occasion to do more than point it out in an article like the present. Laplace makes the tropical year to be 13 seconds shorter than it was in the time of Ilipparehus.
The excess of the tropical year over 365 days has been given by different astronomers as follows :— Whether the present length of the tropical year can be said to be determined within a second, we cannot collect from the writings of astronomers. The method of determining this length is by carefully observing solstices or equinoxes (that is, times when the sun is in the solstices or equinoxes) at distant periods, and taking the mean year from the whole interval elapsed. Unless that interval were a whole revolution of the solar perigee with respect to the equinox, the real mean tropical year could not be determined, from observation alone, so well as it might be.
The civil year must, for convenience, begin with a day, and contain an exact number of days. But any exact number of days would have the disadvantage of the old Egyptiau year Peition], namely, that the seasons would be thrown into all parts of the year iu suc cession. Those who lived in the intense heats of March (when that month is near the autumnal equinox) would read old poets who describe the spring as about to arrive in that month, or allude to the past winter, and that before the poets would have become properly ancient : this alone would' be worth avoiding. Of the mode of doing it we shall presently say more; but in the meanwhile we have to observe, that it has always been the greater source of difficulty to combine the revolutions of the moon with those of the sun.