John Hadley 1682


John Hadley was the man who made accurate navigation at sea a possibility.


Born April 16, 1682, in Hertfordshire, England, Hadley was a mathematician and inventor, an optician by trade, who improved upon Newton’s design of the reflecting telescope. Hadley’s models were more powerful, yet less cumbersome.

Hadley and the Sextant

In 1730, Hadley invented a “quadrant” for measuring the altitude of the Sun or a star above the horizon to pinpoint a location at sea. He later fixed a level to his quadrant so that measurements could be taken when the horizon was not visible. This device later evolved into what we know as the sextant.

The sextant, in conjunction with the compass, was the basic navigational tool for more than two centuries. In fact, it was used to help map the American West.

When Lewis and Clark were preparing for their expedition, they wanted to take along the most sophisticated navigational equipment available. They realized they could determine latitude if they knew the position of the sun in the noon-day sky. To track the distance of the noon sun from the horizon, Lewis and Clark used a sextant during the winter, and an octant in the summer.

*On a personal note, I can attest that learning to use a sextant takes more skill than I had. I was pretty much a navigational flop!

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