Charles Ellicott Commentary Genesis 7:11

"In the six hundredth year of Noah`s life, in the second month, on the seventeenth day of the month, on the same day were all the fountains of the great deep broken up, and the windows of heaven were opened." — Genesis 7:11 (ASV)

In the second month. —This refers to the civil year, which began in Tishri, at the autumnal equinox. The flood thus started towards the end of October and lasted until the spring.

The ecclesiastical year, which began in Abib (or April), was instituted in remembrance of the deliverance from Egypt (Exodus 12:2; Exodus 23:15) and therefore is not relevant here.

The year was evidently the lunar year of 360 days, because the waters prevailed for 150 days (Genesis 7:24) and then abated for 150 days (Genesis 8:3). Now, since the end of the first 150-day period is described in Genesis 8:4 as the seventeenth day of the seventh month, while the flood began on the seventeenth day of the second month, it is clear that the 150 days constitute five months of thirty days each. But see Genesis 8:14 for further proof.

The fountains of the great deep broken up (Hebrew, cloven), and the windows (lattices) of heaven were opened. —This is usually understood by commentators as a description of extraordinary torrents of rain, related in language consistent with the popular ideas of the time and of the narrator himself. The rains poured down as though the floodgates that usually shut in the upper waters were thrown open, while from the abysses of the earth, the subterranean ocean burst its way upwards. But the words at least suggest the idea of a great cosmic catastrophe, by which some vast body of water was set loose.

Without some such natural convulsion, it is very difficult to understand how the ark, a vessel incapable of sailing, could have gone against the current up to the watershed of Ararat. Since the annual evaporation of the earth is also a comparatively fixed quantity, the concentrated downpour of it for forty days and nights would scarcely have produced a flood as vast as Noah's deluge evidently was. It is therefore probable that, besides the rains, there was some vast displacement of water that helped produce these terrific effects.

We will later have occasion to notice the exactness of the dates (Genesis 8:14). Tradition might have handed them down correctly for a short time, but they must soon have been committed to writing, or confusion would inevitably have crept in.