A sextant is a navigational instrument that is used to measure the angle between two visible objects, typically the horizon and a celestial body such as the sun or a star. The sextant is typically used to determine a vessel’s position at sea by measuring the altitude of the celestial body at a specific time.

Here are the basic steps for using a sextant to determine a vessel’s position at sea:

1. Level the sextant by adjusting the bubble vials until the horizon is visible through the index mirror.
2. Locate the celestial body you wish to observe, typically the sun or a star, and center it in the field of view of the telescope.
3. Adjust the micrometer drum to bring the image of the celestial body into alignment with the horizon in the index mirror.
4. Read the angle of the celestial body from the scale on the sextant arm. This is known as the “sextant altitude.”
5. Use a nautical almanac, a table of the position of the celestial body at a specific date and time, to determine the vessel’s position.
6. Repeat the process with a second star to get a cross-fix.

Note: It is important to practice using the sextant in the daylight first before using it at night.