The length of ships was still
determined by the same length of trees that could be grown.
Sailing ships were still vulnerable to a lee shore. Latitude was
easy to determine using an octant and later a sextant with mirrors
and a small telescope to measure the angle between a celestial
body such as the sun or north star and the horizon. But longitude
could not be determined with any degree of accuracy. One method
relied on accurate predictions of the future position of the moon
as observed from a fixed reference point, such as Greenwich. By
precisely observing the local time of the moon's occultation of a
known star at a particular place, and looking up in a table the
predicted time of the event at Greenwich, one could approximate
the time difference of the place from Greenwich. There were so
many shipwrecks on this account that the government offered a
reward to anyone who found a way to measure longitude accurately.
In 1763 carpenter and clockmaker John Harrison made the
chronometer to do this with an accuracy of 21/2 seconds per month,
and received 5,000 pounds. He was promised 10,000 pounds to
explain the principle of his timekeeper and build three more. The
chronometer kept time with extreme accuracy and was mounted to
remove the effect of the ship's motion. To find a ship's position,
a navigator noted the time and measured the positions of certain
stars. He compared these positions with tables that showed the
stars' positions at Greenwich mean time, and then calculated the
ship's position.
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