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Jackson, You wrote:
"What is this sextant, and how is it used?"

The auction title is actually correct. It's called a "box sextant". These are real, diminutive sextants that have been arranged so that all of the parts fit inside a small cylindrical "box". They work just like standard nautical sextants though it seems that they were popular primarily with surveyors on land. There are two mirrors, one of which can be rotated and is attached to an index arm. There is a knob that turns the index arm (rather than moving the arm directly as in a traditional nautical sextant). You use it just like any nautical sextant, bringing the Sun or other body to the horizon and swinging the arc, etc. These are usually (probably always) vernier sextants, and since the scale is quite small, even with a magnifier, they are tough to read accurately. It's certainly possible to take observations with an accuracy of +/-3' (in the 1 s.d. sense), and with a little skill and practice reading the vernier, an accuracy of +/-1' can be achieved. And that's pretty darn good for an inobtrusive, portable instrument. Note that this error is in addition to all other sources of error.

-FER

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