How to Use a Sextant to Find Latitude

Align the sextant's telescope with the horizon. Locate the North Star (Polaris) with the arm and clamp the arm in place. Alternatively, align the protractor's 90 degree mark with the horizon, then find the north star above the horizon, lining up the ruler with the angle on the protractor. Mark the angle with a thin piece of tape.

Move the North Star's position (on the sextant's arm) downward so that it aligns with the horizon. Allow the weighted plumb line to hang. Alternatively, move the taped angle on the protractor downward so it lines up with the horizon and let the plumb line hang. Read the degree mark that lines up with the plumb line (example: 60 degrees). Note that this gives the degrees above the horizon.

Find latitude by subtracting the angle reached in Step 2 from 90 degrees. Example: 90 degrees (the horizon) minus 60 degrees (above the horizon) equals a latitude of 30 degrees.

Find the solar declination -- the angle of the sun above the horizon (example: 30 degrees) -- in the same basic manner as finding the North Star. Be careful not to look directly at the sun. Determine the angle in the same manner as Step 2, by allowing the plumb line to dangle. Note that this is the solar declination for the current date and time locally. Find the solar height for noon of the current date (example: 75 degrees) at this location (local noon) in an almanac.

Find latitude by subtracting the local noon degrees from 90 degrees then adding the solar declination degrees. Example: 90 degrees (horizon) minus 75 degrees (local noon) plus 30 degrees (solar declination) equals 45 degrees latitude.