To find Longitude you need a device that was a highly classified and occulted secret for millennia. Protected by the world’s great navigators. Fortunes and empires were made by use of this device. That super secret highly guarded and “classified” tool was…a stopwatch. Called a Chronometer. It measures 24 hours accurately. This allows you to take readings from your sextant at EXACTLY the same time each day to see how the location of the sun is different in the sky. With this you can calculate longitude.
Latitude is how north or south you are from the equator. You can calculate this using a Sextant (they only work on open water with “flat” view of the horizon). But with latitude alone you ONLY know how far above the equator you are. (For example, Boston, Massachusetts and Barcelona, Spain are at the same latitude.) Using only latitude, you could be anywhere along that “ring” around the planet, it doesnt give you your exact location. For that you need another coordinate, called Longitude.
Some of the most historically important wars were won by use of highly accurate highly-classifed secret stopwatches. It allowed pre-planned strikes to happen precisely simultaneously from two sides of the enemy without direct communication between the flanks. Those supplying these secret devices to military leaders built fortunes, like Patek Phillipe who supplied Napoleon et al.
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supplementary image showing use of a sextant:
Using a Sextant