So, what do navigators need in order to find their position on the earth’s surface when they don’t have a GPS?
They need an almanac, prepared by the astronomers to forecast precisely where the heavenly bodies: the sun, the moon, planets and navigational stars, are going to be, hour by hour, years into the future, relative to the observatory that prepared the almanac - in modern times Greenwich, England.
They need a chronometer or some other means of telling the current time at the observatory that was the reference point for the data in the almanac.
They need charts, and it is the cartographer’s job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference to landmasses or the hazards of rocks and shoals.
They need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart.
Finally, they need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference.
Notes from the Fram Museum, Oslo.