time. Symbol a, annum. Any of the various periods equated with one passage of Earth about the Sun, and hence of roughly 365 days. The familiar calendar has a mixture of 365- and 366-day years, reflecting the fact that the time for one complete passage takes about 365¼ days; the precise value for this figure depends on the manner of defining the year.
As discussed under time, there are various lengths of year (and of day). The obvious year is the interval between successive mid-summers or other seasonal mark. Because of particulars of astronomical motions, this interval varies from one passage to the next, and progressively over time. Throughout historical times and continuingly, the underlying fluctuations are most pronounced around the time of the solstices, so the astronomers bury most of these variations within the year by regarding a year as beginning at an equinox. Occurring when the conceptual line of intersection between Earth's equatorial plane and the plane of the ecliptic (the plane in which Earth travels) passes through the Sun, equinoxes are also inherently easier to recognize for a given level of accuracy. The northern spring (or vernal) equinox is the chosen one, with the marker being the noon position of the Sun as it crosses the Equator (termed the First Point of Aries), and the year beginning at that point is called the equinoctial year or tropical year (atrop); the mean value is 365.242 19~ mean solar days. This is the true year of the seasons; any discrepancy from it in the average year of any calendar results in progressive creep of the seasons, the factor that produced the drastic change of the European calendars from Julian to Gregorian, a few centuries ago. (But the Julian year of precisely 365.25 days remains in use in astronomy.)
Because Earth's axis itself is turning relative to the stars (‘precessing’), in a manner contrary to Earth's movement about the Sun, the successive vernal equinoxes occur about 20 minutes earlier each orbital passage, so the tropical years understate the time for a complete orbit by that much. The effect of precession on the registration of time can be avoided by demarcating the year at a point on the elliptical orbit regardless of seasons and axial tilt. The point of closest approach to the Sun is the chosen point, a unique one because the Sun lies at a focus of the ellipse, not the centre. This point is called perihelion, which marks the anomalistic year (aanom). Currently perihelion occurs in early January, (and its opposite, aphelion, early in July) bringing in some of the variations around the solstices, but the average is a truer representation of the time for one orbit, and obviously much more appropriate than the tropical year for orbital calculations. However, this year too is discrepant in some sense, for the ellipse itself is slowly changing position relative to the wider context; the axes of the ellipse are rotating relative to the stars, in the same direction as Earth about the Sun, making aanom about five minutes greater than the true time for one revolution.
The truest measure of one revolution is the sidereal year (asid), the time for one passage measured relative to the stars; this is the most stable year, but this too, because of overall motion of the solar system within the Galaxy, is not absolutely fixed. The Gaussian year (agauss) is the theoretical year derived from Kepler's law.
The values of these various years (in mean solar days) at the opening of the year 2002 were
[The Astronomical Almanac for the Year 2002 (Washington: US Government Printing Office and London: HMSO, 2001), also Explanatory Supplement to the Astronomical Almanac (Mill Valley, CA: University Science Books, 1994).]
| atrop | = 365.242 189 7~ days | = 365 days 5 h 48 min 45.19~ s |
| asid | = 365.256 363~ days | = 365 days 6 h 9 min 9.8~ s |
| aanom | = 365.259 635~ days | = 365 days 6 h 13 min 52.5~ s |
| agauss | = 365.256 90~ days | = 365 days 6 h 9 min 56.~ s |
The original Besselian year, being a period over which the right ascension of the Sun increases by 360°, was essentially the same as the tropical year; the name astronomical year seems to be applied to either. The term Besselian year now refers to the Julian year.
The calendar year, in contrast to the above years, is necessarily a whole number of complete days. See calendar for further discussion.
By natural extension, the term year is used, appropriately qualified (and often parenthesized), for the equivalent periods for other planets, e.g. the Martian year. See also eclipse year.
