12.
The year of the European nations is the tropical year, that is, the time, which elapses between two successive passages of the Sun through very nearly the same point of the equator. This year is regulated only by the Sun's course, which determines the seasons without any regard to the Moon. When the Sun crosses the equator in the direction from South to North, spring commences, when from North to South, autumn begins; in the exact middle term between these two moments, when the Sun stands farthest from the equator to the North, summer commences, and when farthest to the South, winter. This holds good for the northern Hemisphere; for the southern the order of the seasons is inversed.
The uninterrupted series of these years begins with the year of Christ's birth called 1 A.D. (Anno Domini i.e. in the year of the Lord) or +1. The year immediately before 1 A.D. is called 1 B.C. (before Christ) or −1. The nth year after the beginning of this era is called n A.D. or +n and the nth year before n B. C. or −n, so that the standard-scale of the European (Christian) chronology has the following form:
| −(n+1) | or | (n+1) | B. C. |
| −n | or„ | n | B.C. |
| −1 | or„ | 1 | B.C. |
| +1 | or„ | 1 | A.D. |
| +n | or„ | n | A.D. |
| +(n+1) | or„ | (n+1) | A.D. |
The difference between two successive figures of the table (9) is always 1, excepting only the difference between the years −1 (1 B.C.) and +1 (1 A.D.); which is 2, therefore between 1 B.C. and 1 A.D. is a discontinuity or a leap.
The years are named as current years and not as elapsed years. Hence, when two dates are given either before or after Christ, for instance:
| 1 January 500 B.C. or −500 |
| 1 January„ 200 B.C. or −200 |
and
| 1 January 500 A.D. or +500 |
| 1 January„ 200 A.D. or +200, |
we find the years elapsed between them by subtracting the earlier epoch from the later in the first case −200 − (−500) = 300, in the
