The altitude is the difference in the angular positions of the shadow and sighting vanes.
2.
The " sighting vane " on the arc is moved until it aligns with the view of the horizon.
3.
The instrument has three vanes, a " horizon vane " ( A in " Figure 8 " ) which has an opening in it to observe the horizon, a " shadow vane " ( B ) to cast a shadow on the horizon vane and a " sighting vane " ( C ) that the navigator uses to view the horizon and shadow at the horizon vane.
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