![]() ![]() Perpendicularity error is caused when the index mirror is not perpendicular to the plane of the instrument. Correctable sextant errorsĬorrectable errors are ones that you should aim to remove every time you take sights with your sextant. Sextant errors and correctionsįor a sextant to work to its maximum accuracy, proper care needs to be taken to account for its potential errors.įor a complete breakdown of the errors and corrections in a sextant, you can read this article: Sextant Errors & Corrections.įor now, we can summarise them by considering correctable and non-correctable errors. It can either be made from metal or plastic, depending on the construction material of the sextant you have purchased. The frame is the solid part of the sextant that all other parts are mounted on. ![]() When looking at a celestial body, you release the clamp to bring the horizon mirror down to the horizon while still keeping the body lined up in the index mirror. The micrometer is mounted on the end of the index arm and is used to take a precise measurement of the altitude of the body you are observing. For full accuracy, you use the micrometer in combination with the increments along the arc. Using the arc alone will only provide accuracy to the closest degree. This is because it tells you the deflection of the beam of light rather than the deflection of the index mirror.Īlong the arc, you read whole degrees. You’ll notice that the arc spans 60°, but shows readings up to 120°. The numbers along the arc tell you the altitude of the body you are measuring, so you do not need to worry about understanding the double reflection principle. The arc is the curved part of the sextant frame which is etched with numbers to read off the deflection of the index mirror. It is attached to the top of the frame with a bearing, allowing it to move smoothly.Īt the other end, it is controlled precisely using the clamp and micrometer to position it along the arc. The index arm and the index mirror are connected rigidly to each other. The index arm is the movable part of the sextant, used to alight the index mirror. ![]() When using the sextant during twilight, you will often find that shades are not necessary.ĭo you want to know why twilight is the best time for sights? You should read this article: Best Times To Use A Sextant Index Arm Then, as you bring the instrument down to the horizon, you can remove the horizon shades, if necessary to see the horizon clearly. When initially lining up the sextant on the sun, you should have all the shades in place. You use different combinations of shades to protect your eyes when looking at bright objects like the sun. The shades on my sextant, a Davis Mk15 ( Read my review of this sextant) ![]()
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