Online sextant

This could be the sun at solar noon, so the instrument has dark glass shades to permit the safe viewing of the sun's position.

Sextants were used to determine latitude through the determination of the angle between a known celestial object and the horizon.

With a marine sextant a navigator can measure altitudes of the Sun and stars.

The large ring in front of the telescope is half clear glass and half mirror.

The navigator sees the horizon through the clear glass and a reflection of the observed body in the mirror.

When the two are in line, the altitude is read from the scale (degrees) and micrometer (minutes).

With this data and the exact time of observation, the position can be found.