The technical definition of nautical twilight will include wording about the center of the sun being 12 degrees below the horizon.  What that means practically is that movement is particularly dangerous, as the horizon and the stars will be clearly visible, but not much else.  That being the case, nautical twilight is the ideal time for being still and for mariners to take their readings by the stars, by which they will navigate, once friendly beacons of truth are lost from sight.

We are taught in 2 Corinthians 5:7, among other places, that those who belong to the Lord "walk by faith, not by sight."  The implication in this verse is that there will be times when what we see will appear to contradict what God has said.  We can see throughout scripture where the saints believed God in direct contradiction to what their eyes told them.  One of my favorite instances comes from 2 Kings 6, when Elisha's servant woke to the sight of the Aramean army surrounding them.  Panicked, he ran to Elisha who responded:  "Do not fear, for those who are with us are more than those who are with them."  Then Elisha prayed:  "Lord, I pray, open his eyes that he may see."  Verse 17 goes on to tell us that the servant's eyes were opened and that he saw that the mountain around Elisha was "full of horses and chariots of fire."  

The Bible does not explicitly say so, but I believe that Elisha never saw the horses and chariots of fire with his eyes as the servant did (nowhere does the scripture say that Elisha saw them).  He didn't need to.  Elisha had already taken his readings.  He knew the truth, regardless of what was visible.  After all, faith is the assurance of things hoped for, the conviction of things not seen (Hebrews 11:1).