The sextant like the one above is a tool developed in the year 1735. A sextant is kind of like a fancy astrolabe. It helps navigators figure out the angles between the sun and other heavenly bodies and the horizon. Then, by using the sextant, navigators can calculate their latitude and their position. To find their longitude, navigators use clocks.
Lines of longitude run up and down the world from the north pole to the south pole.
A place called Greenwich, England, is at 0 degrees longitude. Greenwich is called the Prime Meridian. There are 180 degrees west and 180 degrees east of the Prime Meridian. Then the navigator looks at the time in Greenwich and notes the difference in time between local noon and Greenwich time. The earth rotates on its axis 360 degrees in one day or 15 degrees in an hour.