I must go down to the sea again
To the lonely sea and sky
And all I ask is a tall ship
And a star to steer her by
- Sea Fever by John Masefield
How does a captain determine a ship's position in mid-ocean? In our space age, this is easily done, by using the GPS system of satellites--the Global Positioning System (http://www.phy6.org/stargaze/Satell5.htm#q150). That network of 24 satellites constantly broadcasts its positions, and small hand-held receivers exist which convert those signals into positions accurate within at least 15 meters or about 50 feet.
Before the space age, however, it was not as easy. One had to use the Sun and the stars.