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History of Navigation:
Measurements for Navigation |
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On the Earth
Measurement is fundamental to navigation. To find our position on the Earth, we need a frame of reference to fix our position. In a classroom, we can describe our location using tiles on the floor: so many tiles in one direction from the door and so many tiles in another direction from the wall. On the Earth, we locate ourselves using latitude and longitude.
Latitude measures location north or south of the equator. Latitude lines are parallel to the equator starting at 0°. The north and south poles are at 90° north or south. Each minute of latitude equals one nautical mile, 6080 feet. Sixty nautical miles are in a degree.
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Longitude is measured in degrees east and west of an arbitrary reference line. Since 1884 the longitude line running through Greenwich, England near London has been that reference, called the prime meridian. Like the slices of an orange, longitude lines converge at the poles.
Navigators use time to measure speed and to determine longitude. As there are 360° in a circle, and the earth rotates once every 24 hours, the earth turns 15° every hour and 1° every four minutes.
Direction is indicated by compass bearing, measured in degrees. The direction to the North Pole is zero. Magnetic compasses point towards the magnetic North Pole, located in the Canadian Arctic islands. This was the only type of compass available from medieval times to the mid 20th century. The magnetic North Pole moves a small amount annually, changing the angle between the magnetic North Pole and the true North Pole. This angle is called variation. In Penobscot Bay, the variation, or difference between true north and the magnetic north pole is 18° West. This information is found on the compass rose on most nautical charts.
Large ships have gyroscopic compasses that use the earth’s rotation around its north-south axis to determine true north. A GPS system can also show directions as true north. Magnetic compasses are used to back up gyro compasses and are still the main type used on smaller boats
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In the Heavens
Astronomers have a grid similar to latitude and longitude to identify the positions of the sun, moon, planets, and stars. The celestial sphere is cut with latitude-like parallels called declination, which locate a star’s position north or south of the celestial equator. The celestial equivalent to longitude is called right ascension. The celestial poles are directly over the earth’s poles, and the celestial equator runs over the earth’s equator.
The ecliptic is the line the sun follows through the year, ranging from 23 1/2° North declination to 23 1/2° South declination, in keeping with the angle of the earth’s axis to the plane of the solar system.
Astronomical measurement corrects for small time errors caused by seasonal variation in the earth’s rotational speed around the sun, using the equation of time.
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