Tarshish is estimated to be a far west sea port perhaps in Spain or northern Africa. Though we don't know for certain where it was we know that it couldn't have been farther nor harder for for Jonah to get to. Jonah was going well out of his way in disobeying God.
These mariners certainly had seen many of tempest before enough to fear for their lives. Something about this tempest though told them that it was being caused by someone on this ship. Today we might be amused at simplicity and superstition of these seafarers, but, when it comes down to the facts they were precisely right.
The fish here may or may not have been a whale. People dispute this story often because of the uncertainty of this fish. But we are told that this fish was prepared by the Lord. It may have been been prepared as in being placed into the same locale, prepared as is being able to swallow without chewing, prepared as in having an air pocket provided if Jonah is alive inside this belly and not dead, prepared as in being something completely out of the ordinary beyond our experience or consideration.
We know that Jonah had intended to resist the will of God by not going to Nineveh. How Jonah came of mind that God would still be willing to help him even the belly of this judgment, I don't know. We should take it that even in our own lives when we think that we have blown any chance with God, God still has His chance with us. If we are honest and repentant, if we are like minded in faith, it is never to late for God to do a wonderful miraculous work.
Jonah had let his fear of what the Assyrians might say or do cloud his judgment as to what God needed him to do. He may have also been prejudice as to who was worthy of his prophetic gifts and who was not. It is apparent that God had been working long and hard in Nineveh to the point where when Jonah finally did speak to them they were willing and receptive to the message.
Jonah explains his reasoning for going to Tarshish, to change God's mind. Did he change God's mind or did Nineveh's repentance? Jonah is asked repeatedly 'doest thou well to be angry'? In the case of the gourd Jonah believes himself so. The Lord points out the difference of a anger/pity over a gourd and a great city of 120k lost gentile souls.
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