So Jonah arose, and went unto Nineveh, according to the word of the LORD. Now Nineveh was an exceedingly great city of three days' journey around it.
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Augustine of Hippo
Why, then, are we asked what was prefigured by the prophet being swallowed by that monster and restored alive on the third day? Christ explained it when he said an evil and adulterous generation seeks a sign, and a sign shall not be given to it, but the sign of Jonah the prophet. For as Jonah was in the whale’s belly three days and three nights, so shall the Son of man be in the heart of the earth three days and three nights. … So then, as Jonah went from the ship into the belly of the whale, so Christ went from the tree into the tomb, or into the abyss of death. And as Jonah was sacrificed for those endangered by the storm, so Christ was offered for those who are drowning in the storm of this world. And as Jonah was first commanded to preach to the Ninevites but his prophecy did not come to them until after the whale had vomited him out, so the prophecy made to the Gentiles did not come to them until after the resurrection of Christ.
Journey. By the computation of some ancient historians, Ninive was about fifty miles round: so that to go through all the chief streets and public places, was three days' journey. (Challoner)
Diodorus (iii. 1.) says Ninive was 150 stadia or furlongs in length. It must have been therefore 480 round; and as each furlong contains 125 paces of 5 ft. each, the compass would be "60 Italian miles, (about 50 English) "which would employ a person three days to go through the principal streets. (Worthington)
Ninive "was much larger that Babylon. "(Strabo xvi.)
Hebrew, "a great city of God", denoting its stupendous size.