There are ways to calculate that my friend. For instance there are a few birds in the game. Let's assume a pigeon who has an average flight speed I don't know off the top of my head and you could at the least calculate an incoming ship on earth not sure if there are birds in the tower but if so then there ya go. What about the fog on the tower? That's slow enough moving to measure or even guesstimate pretty easily but fast enough To see in comparison to a ship in the tower which you could then calculate using the fog as your basis. Or have someone walk in the tower since we know the average humans walking speed is between 3-5 mph