1
The vernal equinox is day in spring when the daytime hours equal the nighttime hours.2
Inferred from Yerushalmi (Avodah Zarah 1:2), see Bleich, Bircas HaChammah, p. 57, n.15.
3
Eruvin 56a.
4
This number too is not an absolute constant!
5
Indeed, it is highly problematic for one to bless the sun at any other time, for such is considered to be idol worship (Tosefta Brachot ch. 6, see Encyc. Talmudit, Birkat Hahama).
6
See Bleich, Bircas HaChammah, p.58.
7
So too R. M. M. Schneerson (http://www.chabad.org/library/article_cdo/aid/820926/jewish/In-Depth.htm). Indeed another anomaly inherent in the calculations is that the first equinox falls out 7 days BEFORE creation itself (Smag Aseh 47; Yesod Olam 4:14)! As a result, the Chazon Ish (O.H. 138:2,9) states that the Birkat Hahama is simply a reminder "zicharon".
8
See Bleich, Bircas HaChammah, p.77, n.11.
9
Rosh Hashana 10b.
10
R. Y. Abarbanel (Ex.12:1) explains that the time from Tishrei to Nisan is the "pregnancy" upon which the "birth" takes place.
11
Rosh Hashana 27a.
12
See Metzudat David (Tehilim 104:19).
13
Here the intent of "unchanging" is "as perceived by man", for recent science has succeeded in noting a cyclical nature to the sun wherein every 11 years it goes through cooling and heating.