Thursday, 27 November 2008

Math/Social Studies: Sextant and how it tells latitude

Antique sextant

  • A sextant is a device the measures the angle of a celestial body. Ex. planets, the moon, the sun or other stars.
  • Polaris (the North Star) is above the north pole at 90*.
  • The mariner’s quadrant is a device similar to the sextant. The quadrant is one quarter of a circle.
  • The quadrant can be traced back to 1200s, but it was around 1450 that the quadrant came into widespread use.
  • During the 1400’s, Portuguese explorers were traveling south along the coast of Africa searching for a route to the orient. As a seafarer nears the equator heading south, Polaris disappears below the horizon. So, in southern seas, mariners had to have a different way of finding their latitude. Under orders from the Portuguese Prince Henry, The Navigator, by 1480, Portuguese astronomers had figured out how to determine latitude using the position of the sun as it moved north and south of the equator with the seasons, what we now call its "declination."
  • The quadrant had two major limitations: On a windy, rolling deck, it was hard to keep it exactly vertical in the plane of a heavenly body. And it was simply impossible to keep the wind from blowing the plumb bob off line.
  • In about 1731 John Hadley in England and by Thomas Godfrey independently and almost simultaneously, thought of the fundamental idea is to use of two mirrors to make a doubly reflecting instrument—the forerunner of the modern sextant.
How it tells latitude: For example, if the sun is at 0* (the equator) and you are at 90* (the north pole) the sun will be on the horizon. As you head toward the equator the angle between the horizon and the sun will increase, from 0* (at the north pole) to 90* (at the equator).

Illustration. One line points to the sun. The sextant works in a similar way giving the exact angle.

The History of the sextant



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I am a grade nine student living in Mexico. This is my home school education blog. I post the things I learned during the week on this blog. I hope you can learn things from this too.

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