The next major developments in navigation were the magnetic compass and
the sextant. The needle of a compass always points to the magnetic North
Pole, so it tells you your “heading,” or the direction you're going. Mariner’s
maps in the Age of Exploration often depicted the headings between key ports
and were jealously guarded by their owners.

The sextant uses adjustable mirrors to measure the exact angle of the stars,
moon, and sun above the horizon. From these angles and an “almanac” of the
positions of the sun, moon and stars, you can determine your latitude in clear
weather, day or night. Sailors, however, were still unable to determine their
longitude. When you look at very old maps, you sometimes find that the
latitudes of the coastlines are accurate, but the longitudes are off by hundreds
of miles. This was such a serious problem that in the 17th century the British
government formed a special Board of Longitude consisting of well-known scientists.
This group offered 20,000 British pounds—equal today to about $32,000 but worth a lot more back then—to anybody who could find a way to determine a ship’s longitude within 30 nautical miles.