A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Sep 15, 15:36 -0700
Davis Smith, you asked:
"When the horizon has land in the distance, as in your first photo, how can we be absolutely sure that we are sighting the true horizon and not just the beach? "
We need three things: distance to the island, observer's height of eye, and enough meteorological data to have some confidence that refraction is not abnormal. With the height of eye you calculate distance to the sea horizon. Then if the distance to the island is greater than that, and if meteorological conditions are not exotic (specifically with no temperature inversion in the lowest layers of the atmosphere), then you can confidently say that the sea horizon is in front of the land, like a hill of water in front of you with the island in the valley beyond.