Albert Barnes Commentary Matthew 12:40

"for as Jonah was three days and three nights in the belly of the whale; so shall the Son of man be three days and three nights in the heart of the earth." — Matthew 12:40 (ASV)

For as Jonas was three days, etc. See Jonah 1:17. This event took place in the Mediterranean Sea, somewhere between Joppa and Tarshish, when he was fleeing from Nineveh. It is said that the whale seldom passes into that sea, and that its throat is too small to admit a man. It is probable, therefore, that a fish of the shark kind is intended. Sharks have been known often to swallow a man entire. The fish in the book of Jonah is described merely as a great fish, without specifying the kind. It is well known that the Greek word translated whale, in the New Testament, does not of necessity mean a whale but may denote a large fish of any kind.

Three days and three nights. It will be seen, in the account of the resurrection of Christ, that he was in the grave but two nights and a part of three days. See Matthew 28:6. This computation is, however, strictly in accordance with the Jewish mode of reckoning. If it had not been, the Jews would have understood it and would have charged our Saviour as being a false prophet; for it was well known to them that he had spoken this prophecy, Matthew 27:63. Such a charge, however, was never made; and it is plain, therefore, that what was meant by the prediction was accomplished.

It was a maxim, also, among the Jews, in computing time, that a part of a day was to be received as the whole. Many instances of this kind occur in both sacred and profane history. See 2 Chronicles 10:5–12; Genesis 42:17–18.

Compare Esther 4:16 with Esther 5:1.

In the heart of the earth. The Jews used the word heart to denote the interior of a thing, or to speak of being in a thing. It means, here, to be in the grave or sepulchre.