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In 1700 Issac Newton designed a concept sketch of an instrument for angle measurement with the help of mirrors. But it was uncared till 1742, the year of his death. In order to 1730 John Hadley, an English astronom and mathematician and Thomas Godfrey, an optician and inventor in the British colony of America, invented independently, the first sextant and turned in their sketches to the Royal Society. The construction of Hadley was an octant, this proved as more useful and was the precursor of all further sextants. The name sextant comes from the angle scala that 60° (the sixth part of a circle) comprises. It consits of a telescope, the turning mirror and a rotation arm that can be adjusted to left or right in an area from 0-60 degrees. Thus, the turning mirror, that is atached on the rotations arm, can be adjusted that the sunbeams become guided through the ocular. In the ocular view the sun is associated on the highness of the horizon. So the angle is found under that the sun stands to the horizon.  For optical reasons the actuall angle of the sun is the double of the value of the angle scale of the sextant. It is not only a navigation instrument, for example, the sextant is equally used by researchers, surveyors and all persons that calculate the distance with the help of angle measurement.

This sextant and another one are donations of the former Seefahrtschule Grünendeich.

01.        Index Mirror
02.        Telescope
03.        Aliddade
04.        Graduated arc
05.        Quick release
06.        Micrometer drum
07.        Shades
08.        Horizon glass
09.        Shades
10.        Screwed cap (for battery)
11.        Index mirror adj. screw
12.        Horizon mirror adj. screw
13.        Horizon mirror adj. screw


schematic structure of a sextant

The sextant serves for measuring the angle between two materials (for example the sun a and the horizon c). As for the use no solid account is necessary, it becomes on the sea for measuring the sun highness above the mean sea level used, for determine the geographic latitude. The principle: Around the centre of a circle part of something more than 60° spins a pointer which is in the centre of rotation of the mirror b, with which is non-switched. An other mirror d is fixed parallel on the frame, so stands with b, if the pointer shows to the zero point. A telescope e is as well connected with the part circle and on the mirror d aligned. At the highness measurement of the sun is first the horizon line c of the sea through the telescope, as well as trough d located. This is possible because the mirror d is semipermeable. The angle a between the two mirrors at turning the pointer is variable, so it is possible any high objects, the sun for example above the two mirrors as well in the telescope to mirror, so that two picture superpose. Now the horizon line and the sun have to be exactly coordinate. If this is successfull the pointer can be fixed and the searched angle on a scale to read of. For optical reasons the heigh of the sun corresponds the double of the angle between the two mirrors. Therefore reaches the degree scale from 0 till 120. a    =    sun
b    =    mobile mirror (fixed on the locator)
c    =    horizon
d    =    semipermeable steady mirror
e    =    telescope
Astrid Zscheyge und Nadja Striesche, VT 02, 19.06.03