sextant



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    sextant


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    sextant is an instrument used
    to measure the angle between
    any two visible objects. Its
    primary use is to determine the
    angle between a celestial object
    and the horizon which is known
    as the object's altitude. Making
    this measurement is known as
    sighting the object, shooting the
    object, or taking a sight and it is
    an essential part of celestial
    navigation. The angle, and the
    time when it was measured, can
    be used to calculate a position
    line on a nautical or aeronautical
    chart. Common uses of the
    sextant include sighting the sun
    at solar noon and sighting Polaris
    at night, to find one's latitude (in
    northern latitudes). A sextant
    can also be held horizontally to
    measure the angle between any
    two landmarks which allows for
    calculation of a position on a
    chart. A sextant can also be used
    to measure the lunar distance
    between the moon and another
    celestial object (e.g., star, planet)
    in order to determine Greenwich
    time which is important because
    it can then be used to determine
    the longitude.
    The scale of a sextant has a
    length of ⅙ of a turn (60°);
    hence the sextant's name
    (sextāns, -antis is the Latin word
    for "one sixth"). An octant is a
    similar device with a shorter scale
    (⅛ turn, or 45°), whereas a
    quintant (⅕ turn, or 72°) and a
    quadrant (¼ turn, or 90°) have
    longer scales.
    Sir Isaac Newton (1643–1727)
    invented the principle of the
    doubly reflecting navigation
    instrument (a reflecting quadrant
    —see Octant (instrument)), but
    never published it. Two men
    independently developed the
    octant around 1730: John Hadley
    (1682–1744), an English
    mathematician, and Thomas
    Godfrey (1704–1749), a glazier in
    Philadelphia. John Bird made the
    first sextant in 1757. The octant
    and later the sextant, replaced
    the Davis quadrant as the main
    instrument for navigation.sextant

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