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To the lonely sea and sky And all I ask is a tall ship And a star to steer her by |
![]() ![]() Longitude ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
How does a captain determine a ship's position in mid-ocean? In our space age, this is easily done, by using the GPS system of satellites--the Global Positioning System. That network of 24 satellites constantly broadcasts its positions, and small hand-held receivers exist which convert those signals into positions accurate within at least 15 meters or about 50 feet. Before the space age, however, it was not as easy. One had to use the Sun and the stars.
Finding latitude with the Pole StarImagine yourself standing at night at point P on Earth and observing the pole star (or better, the position of the north celestial pole, near that star), at an elevation angle h above the horizon.The angle between the direction of the pole and the zenith is then (90°–h) degrees. If you continue the line from zenith downwards (see drawing) it reaches the center of the Earth, and the angle beween it and the Earth's axis is also (90°–h). Therefore (as the drawing shows) h is also your latitude.
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Finding latitude with the Pole StarImagine yourself standing at night at point P on Earth and observing the pole star (or better, the position of the north celestial pole, near that star), at an elevation angle h above the horizon.The angle between the direction of the pole and the zenith is then (90°–h) degrees. If you continue the line from zenith downwards (see drawing) it reaches the center of the Earth, and the angle beween it and the Earth's axis is also (90°–h). Therefore (as the drawing shows) h is also your latitude.
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Finding latitude with the noontime SunIf you are sailing a ship in mid-ocean, you can get the same information from the noontime Sun--probably more accurately, since at night you might not see the horizon very well.Noon is when the Sun reaches the highest point in its journey across the sky. It then crosses the north-south direction--in the northern hemisphere, usually south of the observer. Because the axis of the Earth is inclined by an angle e = 23.5° to a line perpendicular to the ecliptic, the height of that point above the horizon depends on the season. Suppose you are at point P. We examine 3 possibilities:
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| (1) Suppose the date is the winter solstice, around December 21, when the north pole is inclined away from the Sun. To find your latitude λ (Note: Navigators on the ocean may use a different notation!) you measure the angle a between the direction of the noontime Sun and the zenith.
Look at the drawing and imagine you could rotate
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(2) Half a year later, at the summer solstice (June 21), the north pole is inclined towards the Sun, not away from it, and now (if λ is larger than e)
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| (3) Finally, suppose you are at equinox, around March 21 or September 21. The inclination of the Earth's axis is now out of the plane of the drawing--away from the paper, if this were a picture in a book. The direction to the Sun is in the plane of the equator, and we get
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Next Stop: #5b. The Cross-Staff (and how to build your own)
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 9-17-2004