When you look into the eyecup of a sextant, you see an image cut in half in the direction of the height. On the left side, you can see straight ahead. The right side is formed by an image in a mirror. It is in the reflection of the mirror that one will aim at the height of the star to be measured.
1- To start taking a measurement, set the alidade of your sextant to zero and aim at the horizon. At this stage, the reflection of the mirror gives you the same image as the one on the left.
2- By gradually tilting the alidade forward, the mirror image starts to scroll down. It continues slowly, always in the heading of the star to be measured.
3- Once the star appears, it is gradually brought to touch the horizon. Here we measure the height of the sun.
4- Once the celestial body is on the horizon, the adjustment is refined by using the fine adjustment screw and by rotating the sextant sideways (and not from front to back). We will then "see" the star following a curve above the horizon. If it is well "set" above the horizon, we can then note the hourly top.
