Concise Bible Dictionary:

1. The lights in the firmament of the heaven are said to be for “signs” as well as for “seasons” (Gen. 1:14). A false application has been made of this passage in using the varied positions of the sun, moon, and planets as a means to foretell events. Of what then are they signs? it may be asked. This is perhaps answered in Psalm 19:1-6. The stupendous distance and marvelous regularity in the movements of the heavenly bodies are a sign of the glory of the One that created them, as is stated of God in Romans 1:20: “The invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead,” or divinity.
2. Signs were wrought by Moses, first to convince the children of Israel that God had sent him; and then to attest and enforce on the Egyptians God’s demands upon Pharaoh to let the Israelites go that they might serve Him (Psa. 78:43). So in other cases, signs were given to show the finger of God.
3. When Christ was on earth He wrought miracles, wonders, and signs, but the Pharisees and Sadducees demanded of Him “a sign from heaven” (Matt. 16:1), and it is added that they did this “tempting Him.” He was Himself God’s sign, according to Isaiah 7:14, as the manna was the sign of God to Israel in the wilderness. As they had not eyes to see God’s signs, they should have no other sign than that of Jonah (Matt. 12:39-40), that is, of a man who was cast into the overwhelming judgment of God, and found deliverance from Him. Christ’s death and resurrection is God’s way of deliverance. In Matthew 16:4 the Lord does not mention the type being fulfilled in Himself for them, but we have the dreadful words, “He left them and departed”: they were left in the judgment of Jonah. See MIRACLES.