Sidereal year 365.25636=365 days 6 hours 9 min 9 sec Tropical year 365.24220=365 days 5 hours 48 min 46 sec Since the recurrence of the seasons depends on the tropical year, the latter is the one to be used in forming the calendar and for the purposes of civil life generally. Its true length is eleven minutes and fourteen seconds less than three hundred and sixty-five days and a fourth. It will be seen that the tropical year is about twenty minutes shorter than the sidereal year. (3) The time it takes the earth to pass from its perihelion point around to the perihelion point again is called the _anomalistic year_. This year is about four minutes longer than the sidereal year. This is owing to the fact that the major axis of the earth's orbit is slowly moving around to the east at the rate of about ten seconds a year. This causes the perihelion point _P_ (Fig. 97) to move _eastward_ at that rate, as indicated by the short arrow. The earth _E_ is also moving eastward, as indicated by the long arrow. Hence the earth, on starting at the perihelion, has to make a little more than a complete circuit to reach the perihelion point again. [Illustration: Fig. 97.] 83. _The Calendar._--The _solar year_, or the interval between two successive passages of the same equinox by the sun, is 365 days, 5 hours, 48 minutes, 46 seconds. If, then, we reckon only 365 days to a common or _civil year_, the sun will come to the equinox 5 hours, 48 minutes, 46 seconds, or nearly a quarter of a day, later each year; so that, if the sun entered Aries on the 20th of March one year, he would enter it on the 21st four years after, on the 22d eight years after, and so on. Thus in a comparatively short time the spring months would come in the winter, and the summer months in the spring. Among different ancient nations different methods of computing the year were in use. Some reckoned it by the revolution of the moon, some by that of the sun; but none, so far as we know, made proper allowances for deficiencies and excesses. Twelve moons fell short of the true year, thirteen exceeded it; 365 days were not enough, 366 were too many. To prevent the confusion resulting from these errors, Julius Cæsar reformed the calendar by making the year consist of 365 days, 6 hours (which is hence called a _Julian_ year), and made every fourth year consist of 366 days. This method of reckoning is called _Old Style_.