To test for index error, zero the index arm and observe the horizon. Index Plate The index plate is a large polished plate reflecting objects onto the horizon glass. The biggest problem for sailors in the 18th century was finding their position in the middle of the oceans during long journeys. Bring the two stars into coincidence either to the left or the right of the field of view. Some say they are more accurate because one's hand trembles less.
Small Mirror Regulation Screw The small mirror regulation screw is used to adjust the horizon mirror. Sextants with aluminum frames and brass arcs have also been manufactured. Learn More in these related Britannica articles: About the Author Mark Slingo has been a professional writer since This sextant was made in England at the end of the 18th century. Adjustable on board ship. Most sextants also have filters for use when viewing the sun and reducing the effects of haze. The filters usually consist of a series of progressively darker glasses that can be used singly or in combination to reduce haze and the sun's brightness. Navigators hold the sextant by its handle in the right hand, avoiding touching the arc with the fingers. For solar observations, filters allow direct observation of the sun. The standard frame designs see illustration are supposed to equalise differential angular error from temperature changes. Sextants helped them to find out both of these things. Navigators' sextants were primarily used for ocean navigation. It is used for measuring angles so it is labelled with degrees. All of these instruments may be termed "sextants". As the name suggests, in vernier sextant, it is read by the vernier whereas in Micrometer sextant, it is read on the micrometer screw. Professional sextants can mount an artificial horizon in place of the horizon-mirror assembly. Some navigators mount a light-amplifying monocular to help see the horizon on moonless nights. Electronic sextants are also put into missiles because they allow the missiles to know their position without needing to rely on external radio signals from satellites, which might be jammed by the enemy. It is necessary to flip back the horizon mirror shade to be able to see the horizon, and then the fine adjustment screw on the end of the index bar is turned until the bottom curve the lower limb of the sun just touches the horizon. Most sextants mount a 1 or 3-power monocular for viewing. Index Plate The index plate is a large polished plate reflecting objects onto the horizon glass. The other important parts of a sextant are a telescope, a piece of glass which is half see-through and half mirror called the horizon glass , and a moving arm which has another mirror fixed to it. Releasing the index bar either by releasing a clamping screw, or on modern instruments, using the quick-release button , the image of the sun can be brought down to about the level of the horizon. Used vertically, a measurement of the angle between the lantern of a lighthouse of known height and the sea level at its base can also be used for distance off.
Video about types of sextant:
SEAMANSHIP AND NAVIGATION
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