Lat = declination - altitude +90° (or)
L = D + (90-A)
You can measure the alt. of the sun off the horizon at noon w. a sextant, and look in a table for the declination of the sun. (sun's declination changes daily!)
Horizon = perpendicular to the earth radius.

Things to know:
Historical people
Geography
Tides
etc.

Things to bring:
Calculator! (but don't forget to show your work)