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How To Calibrate A Sextant

October 18th, 2010

Brass Sextant Pen Holder

The sextant is a fairly complicated device. It relies on the use of a telescope for which one uses to spot the horizon and superimpose a celestial body onto it to determine one’s location. This is done by way of two opposing mirrors, one of which is attached to a moving scale that allows the light from a celestial object to be reflected onto the image of the horizon. However, if the sextant is not properly calibrated then the location the navigator perceives will be incorrect. This can be a very deadly fault, especially when at sea and without any other means of calculating location. This is why it is important to have the sextant that is intended for repeated use calibrated as much as needed.

Calibration of the sextant should be done at a facility that specializes in such. For instance, a United States Air Force aircraft bubble sextant (very different from a nautical sextant) should only be calibrated in a proper military-maintenance facility. This is done by propping the sextant on a calibration device, timing the mechanism’s average, and setting the elevation wheel to angles of 0, 30, 45, and 60 to check the HS reading. This will then have to be examined against preset marks to determine the correct calibration of the sextant.

Furthermore, you can calibrate a sextant by setting it on a table a certain distance from a wall and checking its elevation as opposed to pre-measured marks on the opposing wall. For example, if the sextant is about 5 feet away from the wall, then marking five feet higher than the height of the sextant’s eyepiece should show an elevation of 45 degrees. An improper reading will be considered the index error, and should be taken into account accordingly.

When using the marine sextant, calibration can be done by positioning the alidade to the 0 degree mark. Next, you will locate the horizon through the eyepiece. The image in the mirror should be aligned with the horizon. If not, then you will need to use the adjustment screws to position the mirror so that the image is calibrated correctly. You will know proper calibration when the two images are perfectly aligned with one another.

It is important that the sextant is calibrated before each use in order to make sure that the location you are getting is as accurate as possible. You must also take into account errors that may occur due to certain conditions. Calibrating the sextant will mean the difference between accurately knowing where you stand, and being totally lost. Luckily with the advent of a global positioning satellite, those who have a hard time calibrating a sextant, will be able to utilize GPS to their advantage. Though, GPS is extremely unreliable for its limited power source and constant loss of satellite reception.

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