The Differences Between a Magnetic Compass & an Astrolabe

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The compass and astrolabe both have a long history; however, they function in very different ways and provide different types of information. Furthermore, the mariner's astrolabe is another, entirely unique, device that is different from both the compass and the traditional astrolabe. The compass is a directional guide for navigation, the mariner's astrolabe is a positional guide for navigation and the true astrolabe is a temporal guide, combining the functions of a clock and calendar.

History of the Compass

  • The Chinese developed the compass during the Han dynasty, beginning with the development of a rudimentary, spoon-shaped compass made from lodestone. By the T'ang dynasty, the Chinese had devised a method for magnetizing iron needles, leading to the development of wet and dry compasses. By the Sung Dynasty, A.D. 1000, Chinese sailers were using the compass for oceanic navigation. The compass was introduced to Europe during the twelfth century, either by way of the Silk Road or the Middle Eastern trade routes.

How a Compass Works

  • The Earth is like a giant magnet, surrounded by a magnetic field. This field has two magnetic poles located near the geographic North and South poles. This field causes a freely suspended, magnetized pointer, or needle, to swing into alignment with the poles, visually indicating the horizontal component of the Earth's magnetic field. This enables a navigator to maneuver over a selected course or fix his position by taking visible bearings.

History of the Astrolabe

  • Around A.D. 150, Claudius Ptolemy wrote extensively on the concept of stereographic projection, a geometric function that projects a sphere onto a plane, and Theon of Alexandria wrote a treatise on the astrolabe around A.D. 390. Historians believe that Synesius of Cyrene constructed the first device that was, arguably, a form of an astrolabe between A.D. 378 and 430. By 800, astrolabes had become highly developed in the Islamic world and were introduced to Europe by the early 1100s. The Astrolabe became the most popular astronomical instrument until around 1650, when it was replaced by more specialized and accurate instruments.

How an Astrolabe Works

  • Astrolabes are astronomical computers that are used to solve problems relating to time and the position of celestial bodies. They consist of two components: fixed and rotating. The fixed parts represent time scales and the stereographic projection of the sky. The rotating parts simulate the daily rotation of the sky. Astrolabes are used to determine the time of day or night, determine the time of a celestial event, such as sunrise or sunset, and determine the celestial positions for a given time.

The Mariner's Astrolabe

  • Ancient navigators had no means for determining longitudinal position; however, by measuring the altitude of the noon sun or a star of known declination, they could determine the latitude. By maintaining this latitude and traveling east or west, a ship could arrive at a predetermined point. They called the device they used to perform this task the mariner's astrolabe, composed of a ring with degree marks. In the traditional sense, mariner's astrolabes are not a true astrolabes.

References

  • Photo Credit Jupiterimages/Photos.com/Getty Images Jupiterimages/Comstock/Getty Images Photos.com/Photos.com/Getty Images Hemera Technologies/PhotoObjects.net/Getty Images
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