Fig. 8. Second Year, normal. New Moon not far from Pleiades on the 1st of Nisan.
It takes the sun 365 days to return to the same place among the stars, but the Babylonian year of 12 lunar months (each of 29 or 30 days) was 11 days short of this: therefore on the 1st of Nisan in this year the sun had still 11 days’ march before him ere he returned to the position of [Fig. 7]. This is equal to about 11°, so the young moon was also about 11° west of her former position, near the Pleiades. But as she travels about 13° eastward every day, she would be near the Pleiades on the following evening, the 2nd of Nisan, so this year was also counted normal.
Fig. 9. Third Year, “full.” New Moon distant from the Pleiades on the 1st of Nisan.
The sun is now 2 × 11 = 22 days’ march, or about 22°, short of his first position, and the young moon consequently about 22° west of the Pleiades, so she will not come up with them until the 3rd Nisan, after travelling 2 × 13 = 26°. The year was therefore “full,” that is an extra month of 29 days was added, which is more than the 22 days needed to enable the sun to reach his first position by the 1st of Nisan in the fourth year.
It appears, therefore, that the extra month must have been added once in three or four years.
Several lists of stars and star-groups indicating the months in this way have been found, the early lists containing only a few, the later twelve. If our zodiac originated with the Babylonians, there is little doubt the idea took its rise from these monthly stars, but it is not possible, with our present knowledge, to say when these old astronomers first linked the isolated stars into a continuous series of twelve star-groups and connected the idea of the month with the invisible group among which the sun was known to be shining, instead of with the stars seen east or west of him, or in conjunction with the crescent moon.[21]