24 hours is 360 degrees of earth spin
so time west of Greenwich England = time at Greenwich - (24/360) deg here
so if it is 8 pm at Greenwich where longitude is zero
and you are at 70 degrees west of Greenwich
your time is 8 pm - (24/360) 70 = 8 - 4.7 pm about 5 hours earlier (lock time depends on local time zone of course)
Therefore if you have a good clock you can tell how far west of Greenwich you are.
(Google Harrison longitude)
How to convert time to degree in longitude term
2 answers
By the way, quicker response if you labeled your question Geography-Math-Navigation