Q&A for How to Use a Sextant

Longitude requires you to know both local solar time and GMT. Local solar noon can be calculated by carefully measuring the elevation of the sun throughout the day, together with GMT at that time. Local solar noon is when the sun is highest. Knowing GMT requires you have an accurate clock set to GMT. Each hour difference is 15 degrees of longitude, and each second is 15 seconds of longitude.

Try YouTube or Google for tutorials on how to do this.

A skilled operator with a high quality sextant can obtain a position fix accurate to less than 1 nautical mile. 1-5 nautical miles is more typical, depending on the skill of the operator and the quality of the instrument. If sightings are being taken at sea, the movement of the vessel can also influence the quality of the navigational fix.

Small mirror regulation screw, frame, release levers, micrometer drum, Vernier scale, index plate, horizon glass, filter, telescope, telescope clamp, eyepiece, and the telescopic printing.

No you don't though you will need to divide your measurement by two and apply the previously mentioned corrections.