Question

You are on a boat which is crossing the prime meridian. the altitude of polaris is 50º.explain how you know the boat's location is 50º north latitude and 0º longitude.

Answer 1

The definition for the prime meridian is: The prime meridian with 0 degrees longitude runs through Greenwich. Polaris is the north star at 50 degrees above the horizon, which means 50 degrees latitude. 
If you observe Polaris  at 50 degrees of altitude, you are at latitude 50 North. You that's why you know that your boat's location is 50º north latitude and 0º longitude.