From Tech Center Labs:
With Sextant for iPhone navigate with your GPS while you fake out the crew with this old style technology. Sight the horizon viewing along the top edge and tap horizon. Sight a star viewing along the top edge and tap celestial, it displays the angle. You can sight along the right edge for a little more accuracy. The animated sextant display remains pointing at the horizon as you rotate to shoot the star, the iPhone in the last image is actually rotated to the right to align with the star.