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)
