Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
With the words, "then God remembered Noah and all the animals...in the ark," the narrative turns to the description of the gradual decrease of the water until the ground was perfectly dry. The fall of the water is described in the same pictorial style as its rapid rise. God's "remembering" was a manifestation of Himself, an effective restraint of the force of the raging element. He caused a wind to blow over the earth, so that the waters sank, and shut up the fountains of the deep, and the sluices of heaven, so that the rain from heaven was restrained. "Then the waters turned (ישׁבוּ i.e., flowed off) from the earth, flowing continuously (the inf. absol. ושׁוב הלוך expresses continuation), and decreased at the end of 150 days." The decrease first became perceptible when the ark rested upon the mountains of Ararat on the 17th day of the seventh month; i.e.,, reckoning 30 days to a month, exactly 150 days after the flood commenced. From that time forth it continued without intermission, so that on the first day of the tenth month, probably 73 days after the resting of the ark, the tops of the mountains were seen, viz., the tops of the Armenian highlands, by which the ark was surrounded. Ararat was the name of a province (Kg2 19:37), which is mentioned along with Minni (Armenia) as a kingdom in Jer 51:27, probably the central province of the country of Armenia, which Moses v. Chorene calls Arairad, Araratia. The mountains of Ararat are, no doubt, the group of mountains which rise from the plain of the Araxes in two lofty peaks, the greater and lesser Ararat, the former 16,254 feet above the level of the sea, the latter about 12,000. This landing-place of the ark is extremely interesting in connection with the development of the human race as renewed after the flood. Armenia, the source of the rivers of paradise, has been called "a cool, airy, well-watered mountain-island in the midst of the old continent;" but Mount Ararat especially is situated almost in the middle, not only of the great desert route of Africa and Asia, but also of the range of inland waters from Gibraltar to the Baikal Sea-in the centre, too, of the longest line that can be drawn through the settlements of the Caucasian race and the Indo-Germanic tribes; and, as the central point of the longest land-line of the ancient world, from the Cape of Good Hope to the Behring Straits, it was the most suitable spot in the world, for the tribes and nations that sprang from the sons of Noah to descend from its heights and spread into every land (vid., K. v. Raumer, Palst. pp. 456ff.).
Forty days after the appearance of the mountain tops, Noah opened the window of the ark and let a raven fly out (lit., the raven, i.e., the particular raven known from that circumstance), for the purpose of ascertaining the drying up of the waters. The raven went out and returned until the earth was dry, but without being taken back into the ark, as the mountain tops and the carcases floating upon the water afforded both resting-places and food. After that, Noah let a dove fly out three times, at intervals of seven days. It is not distinctly stated that he sent it out the first time seven days after the raven, but this is implied in the statement that he stayed yet other seven days before sending it out the second time, and the same again before sending it the third time (Gen 8:10 and Gen 8:12). The dove, when first sent out, "found no rest for the sole of its foot;" for a dove will only settle upon such places and objects as are dry and clean. It returned to the ark and let Noah take it in again (Gen 8:8, Gen 8:9). The second time it returned in the evening, having remained out longer than before, and brought a fresh (טרף freshly plucked) olive-leaf in its mouth. Noah perceived from this that the water must be almost gone, had "abated from off the earth," though the ground might not be perfectly dry, as the olive-tree will put out leaves even under water. The fresh olive-leaf was the first sign of the resurrection of the earth to new life after the flood, and the dove with the olive-leaf a herald of salvation. The third time it did not return; a sign that the waters had completely receded from the earth. The fact that Noah waited 40 days before sending the raven, and after that always left an interval of seven days, is not to be accounted for on the supposition that these numbers were already regarded as significant. The 40 days correspond to the 40 days during which the rain fell and the waters rose; and Noah might assume that they would require the same time to recede as to rise. The seven days constituted the week established at the creation, and God had already conformed to it in arranging their entrance into the ark (Gen 7:4, Gen 7:10). The selection which Noah made of the birds may also be explained quite simply from the difference in their nature, with which Noah must have been acquainted; that is to say, from the fact that the raven in seeking its food settles upon every carcase that it sees, whereas the dove will only settle upon what is dry and clean.
Noah waited some time, and then, on the first day of the first month, in the 601st year of his life, removed the covering from the ark, that he might obtain a freer prospect over the earth. He could see that the surface of the earth was dry; but it was not till the 27th day of the second month, 57 days, therefore, after the removal of the roof, that the earth was completely dried up. Then God commanded him to leave the ark with his family and all the animals; and so far as the latter were concerned, He renewed the blessing of the creation (Gen 8:17 cf. Gen 1:22). As the flood commenced on the 17th of the second month of the 600th year of Noah's life, and ended on the 27th of the second month of the 601st year, it lasted a year and ten days; but whether a solar year of 360 of 365 days, or a lunar year of 352, is doubtful. The former is the more probable, as the first five months are said to have consisted of 150 days, which suits the solar year better than the lunar. The question cannot be decided with certainty, because we neither know the number of days between the 17th of the seventh month and the 1st of the tenth month, nor the interval between the sending out of the dove and the 1st day of the first month of the 601st year.
The first thing which Noah did, was to build an altar for burnt sacrifice, to thank the Lord for gracious protection, and pray for His mercy in time to come. This altar - מזבּח, lit., a place for the offering of slain animals, from זבח, like θυσιαστήριον from θύειν - is the first altar mentioned in history. The sons of Adam had built no altar for their offerings, because God was still present on the earth in paradise, so that they could turn their offerings and hearts towards that abode. But with the flood God had swept paradise away, withdrawn the place of His presence, and set up His throne in heaven, from which He would henceforth reveal Himself to man (cf. Gen 9:5, Gen 9:7). In future, therefore, the hearts of the pious had to be turned towards heaven, and their offerings and prayers needed to ascend on high if they were to reach the throne of God. To give this direction to their offerings, heights or elevated places were erected, from which they ascended towards heaven in fire. From this the offerings received the name of עלת from עולה, the ascending, not so much because the sacrificial animals ascended or were raised upon the altar, as because they rose from the altar to haven (cf. Jdg 20:40; Jer 48:15; Amo 4:10). Noah took his offerings from every clean beast and every clean fowl - from those animals, therefore, which were destined for man's food; probably the seventh of every kind, which he had taken into the ark. "And Jehovah smelled the smell of satisfaction," i.e., He graciously accepted the feelings of the offerer which rose to Him in the odour of the sacrificial flame. In the sacrificial flame the essence of the animal was resolved into vapour; so that when man presented a sacrifice in his own stead, his inmost being, his spirit, and his heart ascended to God in the vapour, and the sacrifice brought the feeling of his heart before God. This feeling of gratitude for gracious protection, and of desire for further communications of grace, was well-pleasing to God. He "said to His heart' (to, or in Himself; i.e., He resolved), "I will not again curse the ground any more for man's sake, because the image (i.e., the thought and desire) of man's heart is evil from his youth up (i.e., from the very time when he begins to act with consciousness)." This hardly seems an appropriate reason. As Luther says: "Hic inconstantiae videtur Deus accusari posse. Supra puniturus hominem causam consilii dicit, quia figmentum cordis humani malum est. Hic promissurus homini gratiam, quod posthac tali ira uti nolit, eandem causam allegat." Both Luther and Calvin express the same thought, though without really solving the apparent discrepancy. It was not because the thoughts and desires of the human heart are evil that God would not smite any more every living thing, that is to say, would not exterminate it judicially; but because they are evil from his youth up, because evil is innate in man, and for that reason he needs the forbearance of God; and also (and here lies the principal motive for the divine resolution) because in the offering of the righteous Noah, not only were thanks presented for past protection, and entreaty for further care, but the desire of man was expressed, to remain in fellowship with God, and to procure the divine favour. "All the days of the earth;" i.e., so long as the earth shall continue, the regular alternation of day and night and of the seasons of the year, so indispensable to the continuance of the human race, would never be interrupted again.