Before the invention of accurate clocks, it was nearly impossible for sailors to find their longitude. This did not stop them from trying, however. Columbus made two attempts in his lifetime to measure his longitude. Both results were pretty bad, even by the standards of his day.
The only practical method for determining longitude in the fifteenth century was the well-known method of timing lunar eclipses. This method had been in use since ancient times, but since eclipses are rare, it is of limited use. A recent suggestion (Molander 1992) that Columbus used planetary conjunctions to determine his longitude on the first voyage has been shown to be incorrect (Pickering 1996).