A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2014 Dec 13, 01:57 -0800
This is getting more complicated than I imagined.
The problem for an Atlantic crossing was that it might not be possible to correct the altimeter sub scale for changes in surface pressure when out of contact with watch ships, so there was a chance of the airship flying into the sea at night or in cloud or fog. Therefore, opportunities had to be taken to update the sub-scale if a shadow was visible. As the rays from the sun are essentially parallel by the time they reach the Earth the length of the shadow will always be the same to an observer on the ground, irrespective of the height of the airship. It’s just the angle the shadow makes at the sextant in the airship that changes with the height of the airship and the position of the sun. You could reduce the position of the sun complication by turning into sun before taking the sextant reading. In a similar way, when over land, you could work out your groundspeed from the time the known length of shadow took to cross a feature on the ground – a sort of Dutchman’s log of the air.