« A.U.C. 717 = 37 B.C. »
M. Terentius Varro, De Re Rustica I 28, written in A.U.C. 717 = 37, gives the dates of the equinoctes according to the Caesarian calendar, and names them as a.d. IX Kal. Apr. (24 March) and a.d. VI Kal. Oct. (26 September). The mean Julian dates should be 23 March and 25 September. This suggests that the Caesarian calendar was aligned one day early compared to the later Julian calendar.
This discrepancy has long been known. However, since the equinoctes vary by roughly six hours from year to year, allowing them to wander over two consecutive days, the match has usually been considered good enough.
Varro also shows that the Caesarian calendar had the same month lengths as the Julian calendar. He gives the following definition of the seasons based on the sun's motion through the zodiac, with the seasons starting when the sun is in the 23rd degree of Aquarius, Taurus, Leo and Scorpio, respectively, resulting in dates and durations as follows:
Spring: a.d. VII Id. Feb. (7 February) -- 91 days
- Summer: a.d. VII Id. Mai. (9 May) -- 94 days
- Autumn: a.d. III Id. Sex. (11 August) -- 91 days
- Winter: a.d. IV Id. Nov. (10 November) -- 89 days
These dates and durations match the Julian calendar. They do not match the Caesarian calendar proposed by Sacrobosco.
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