CASSENS & PLATH BOBBY SCHENK Sextant Handhabung Seite 21

BOBBY SCHENK Sextant
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How a Sextant Works
Star
Horizon
Horizon mirror B
The principles of measurement may be outlined as follows:
Index mirror A reflects the light from the star to the internal surface of horizon mirror B. This
reflects the light into the objective of the telescope. Thus the navigator observes the star through
the telescope doubly reflected.
As the horizon mirror also allows the light from the horizon to transmit this will be seen directly
through the telescope by the navigator.
So one has to distinguish between direct vision of horizon and doubly reflected vision of the
celestial body.
The index mirror A can be adjusted by moving of the alidade.
If the index of the alidade points to zero of the limbus, mirrors A and B are parallel. So the
observer sees two images (the directly seen one and the doubly reflected one) of the same object.
Both should be exactly superimposed if the mirror positions are error free. If these two images do
not exactly coincide along the vertical, then there is an index error (either + or -), which requires
correction in all measurements. If these two images do not coincide along the horizontal, then
there is a tilt error which also requires correction. Usually one will find a combination of both errors.
To measure the vertical angle between the horizon and a star the observer should move the
alidade along the limbus until the reflected image of the star is superimposed on the direct image
of the horizon. The method of sighting will be explained in detail later. By the law of light reflection
the angle which the index mirror has been turned, is half the angle between star and horizon. For
this reason the limbus of the instrument has been graduated so as to allow the user to take a
reading without having to make a conversion. The total measuring range of a sextant is 120°, so
that the angle of the arc is 60° only. This gives the sextant the name because 60° is the sixth part
of a complete circle.
Instructions
Index mirror A
Scale "0"