פירוש על ברכות 5:10

Tosafos D''H "L'man D'gani." "and if you say, doesn't Rebbi Eliezer hold (later on Daf 9b) "until one recognizes between [the color] of techeilet and karti (a dark green)" [is what is required in order to recite] the Kriyat Shema of the morning? In any case, since one knows when Amud HaShachar occurs, [by the time] one arises and prepares himself [for the morning prayers,] the time will have already arrived." It is clarified from Tosafos that he reasons that Kriyat Shema of the morning is dependent of the time of "m'sh'yakir," and "m'sh'yakir"prevents, even after the fact, that if one recites it before "m'sh'yakir"

Windows were rare in houses, especially of the poor. Cf. T. A. I. p. 42, and p. 347 n. 539.

And if you'll ask, we know that Rabbi Eliezer holds that in order to read Krias Shema it must be light enough to recognize between blue and green? We can answer that since he knows when the rising of the sun is, by the time he gets up and gets ready it will be the right time.