Regular calculations of latitude and longitude have been the cornerstone of ocean navigation for about 300 years. A sextant and an accurate sea-going clock were needed to calculate both. Sextant: This instrument was once an essential aid to navigation. With it, a sailor could accurately measure the angles of celestial bodies above the horizon. To take a reading, the observer looks through the telescope and moves the sextant’s arm until an image of a star or the sun lines up with the horizon. The angle is then read off the scale. Magnetic compass: The magnetic compass contains a magnetized needle which, due to the Earth’s magnetic field, points roughly toward the North Pole. Determining latitude: To determine latitude is to find out how far north or south you are from the equator. In the northern hemisphere, latitude is measured with reference to Polaris. Using a sextant, you measure the angle of Polaris above the horizon, and this gives you your latitude, expressed in degrees. If Polaris is directly overhead, you must be at the North Pole (90 degrees north latitude); if it is the horizon, you must be at the equator (0 degrees). Determining longitude: To determine longitude is to find out how far east or west you are from Greenwich, England. To do this, while you are still in your home port, you set your sea-going clock for noon when the sun is at its highest point. Then, while you are at sea, you check the clock again when the sun is at its highest point. If the clock says 3 P.M., then you must have traveled 3 hours west of the port. Since the sun moves 15 degrees per hour, 3 hours corresponds to 45 degrees west.