Davis Artificial Horizon 
Answers the need of every sextant user who cannot depend on a natural sea horizon. Use for backyard practice, finding exact positon when inland, and making exact sextant shot on foggy mornings.
To use, fill with water and read the angle of the reflected sun with your sextant. Half your sextant reading is the sun's angle above the true horizon. Windproof and corrosion resistant, the reflecting surface is completely enclosed.
Comes with two sun shades, a lid and full directions.
Compact 6" x 4" x 1-1/2" (15 x 10 x 4 cm).
Owner's Manual (pdf)