Info The animation illustrates the use of a marine sextant at sea, for measuring the altitude of the Sun above the horizon. This information, coupled with the knowledge of the exact time and the position of the Sun in the celestial sphere at the moment of the observation, allows the determination of a line of position, with an accuracy of about 1-2 nautical miles. Created and nominated by Joaquim Alves Gaspar.
Neutral - nice explanation of the theory of using a sextant, or on land, with the sextant fixed on a stable tripod. How does one obtain 1-2 nautical mile accuracy in practice? (see typical actual situations, right) - MPF14:01, 4 April 2007 (UTC)[reply]
Info - The marine sextant cannot be fixed to a tripod, it is designed to be held with the right hand (the handle can be see behind the index bar, in this picture). This way, the ship's pitch and roll don't affect its use very much (unless you feel really sick or just can´t stand...). The accuracy of a celestial line of position at sea depends mainly on the accuracy of the altitude measurement, which is affected by the instrument itself (the precision of the sextant is about 0,5' of arc), the observer personal error, the accuracy of the available time (the celestial sphere moves, with the sun, 15' of arc in each minute of time) and the effect of atmospheric refraction. The accuracy value of 1-2 nautical miles for each line of position (about 1,85 to 3,7 km) is a little bit optimistic. On land, celestial observations are (were, as a matter of fact) normally made with an astronomical theodolite and the accuracy of the measurements is much higher (of the order of 100 times better). By the way, the sea states shown in the images are not, fortunately, very typical. Alvesgaspar14:29, 4 April 2007 (UTC)[reply]
Comment - Thanks; though I still don't see how it can be used in a practical situation. I've never used a sextant, but have frequently used binoculars on board boats (both large ships and small fishing boats, for birding), and know how impossibly difficult it is to keep the horizon (or a bird) steady in them with engine vibration and even just slight rolling. Even with a calm sea, engine vibration transmitted through the body and arm makes the horizon vibrate up and down by a degree or two. The two pics may be a bit more than normal, but it is extremely rare for there to be no swell in the open oceans (look at e.g. the difficulties of landing on oceanic islets like Rockall, where 1-2 metres swell is normal even in 'calm' weather), and on an average day (in the North Sea at least, where I have the most experience) wave heights very typically make the horizon 'lumpy' by several degrees as seen from the deck of a small ship. One degree of error from a lumpy horizon, or vibration, is equivalent to about 60 nautical miles. - MPF14:58, 4 April 2007 (UTC)[reply]
Comment The beauty of the sextant is that it is not relevant if you are pointing exactly to the horizon or not to make an accurate measurement (as long as you see it through the glass). The important thing is that the heavenly body is put tangent to the horizon, which is reflected in a correct position of the index bar. The waves can indeed affect the process but only when the observer is quite close to the surface and the sea is rough. With an elevation of 3 m (1,5 for the observer's eye plus 1,5 for the boat's deck), the visible horizon is about 3,5 nautical miles away. Alvesgaspar15:40, 4 April 2007 (UTC)[reply]
Support Very good. Now the difference has been pointed out, I prefer the left version. BTW, in caption 4, for 'clump' read 'clamp'. --MichaelMaggs19:34, 4 April 2007 (UTC)[reply]
SupportSupport the image and the information associated to it. I feel like I understand the principle of a sextant now :) Benh08:18, 7 April 2007 (UTC)[reply]
Info -- In this version the rotation of the sextant around the optical axis of the telescope (which may be considered less than perfect) is not visible. Alvesgaspar09:01, 4 April 2007 (UTC)[reply]
Info The animation illustrates the use of a marine sextant at sea, for measuring the altitude of the Sun above the horizon. This information, coupled with the knowledge of the exact time and the position of the Sun in the celestial sphere at the moment of the observation, allows the determination of a line of position, with an accuracy of about 1-2 nautical miles. Created and nominated by Joaquim Alves Gaspar.
Support --Alvesgaspar 22:56, 3 April 2007 (UTC)
Neutral - nice explanation of the theory of using a sextant, or on land, with the sextant fixed on a stable tripod. How does one obtain 1-2 nautical mile accuracy in practice? (see typical actual situations, right) - MPF 14:01, 4 April 2007 (UTC)
Comment - Thanks; though I still don't see how it can be used in a practical situation. I've never used a sextant, but have frequently used binoculars on board boats (both large ships and small fishing boats, for birding), and know how impossibly difficult it is to keep the horizon (or a bird) steady in them with engine vibration and even just slight rolling. Even with a calm sea, engine vibration transmitted through the body and arm makes the horizon vibrate up and down by a degree or two. The two pics may be a bit more than normal, but it is extremely rare for there to be no swell in the open oceans (look at e.g. the difficulties of landing on oceanic islets like Rockall, where 1-2 metres swell is normal even in 'calm' weather), and on an average day (in the North Sea at least, where I have the most experience) wave heights very typically make the horizon 'lumpy' by several degrees as seen from the deck of a small ship. One degree of error from a lumpy horizon, or vibration, is equivalent to about 60 nautical miles. - MPF 14:58, 4 April 2007 (UTC)){kind=link}
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