A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2015 Nov 18, 06:56 -0800
Chichester tried a bubble sextant in his Moth, possibly a Booth, and rejected it. He wrote during WWII that he used a "marine box sextant" but that normally means a small metal canister used by explorers or in lifeboats. I expect he used a conventional marine sextant stowed in a box on the front seat of his Moth, reached through a hole in the seat-back. I tried a Navy Mk V, and found slipstream a problem. I discounted my favourite A10 since it needs two hands, and my RAF Mk IXs have no horizon view. 2 box sextants were easy to shoot, but had vernier scales and no telescopes. My most convenient conventional model was a Frieberger Yacht, since its open type handle could be hooked to tubular fuselage longerons in a Stearman for stowage. I stuck a digital watch to the rear of the index mirror, and added a neck cord, and a writing surface. The mirrors and shades are vulnerable, but the index arm can be stowed pre-set.
In flight visibility from cockpit to horizon is all important. From a yacht cockpit, 3 miles is enough. From a substantial motor vessel, with a 50 ft bridge, 8 miles may do. CAVOK, the aviation term for cloud and visibility alright, means 10 km, which means only 35ft altitude. At 100 ft height 12 miles viz is needed. Flights at these heights are usually only permitted within licensed airfields. Chichester was sometimes in the troughs of the swell, so the viz must have been bad at times.
As it happens, at 200 ft, and a viz of 16 miles, semi-diameter, height of eye, and maybe refraction errors, pretty much cancel out. From 200 ft, a hill above 1000 ft high (Norfolk or Lord Howe) can be seen from 40 miles, viz permitting. As aircraft height increases, the height of eye correction will be off, but the increase in visible range of a hilltop will more than compensate for that. Chichester's altimeter was unreliable, but he reported seeing Lord Howe from 100 miles on his trip to Sydney. At jet altitudes, Mt Fuji, or Teide, can be seen from 200 miles. Flight at less than 140 knots is permitted 500 ft clear of people, buildings or structures. 500 ft vertically clear requires about 25 miles viz. Viz is crucial.
Chichester often needed to manoeuvre to get the sun and horizon visible at once, and often found himself in extreme positions. A tight cockpit and bulky clothing mean the right arm baulks against the side and rear when trying to sight a sextant to the right. You cannot see the horizon directly in front. The UK winter sun never comes above the upper wing. I just turned the aircraft until the sun was at the 9 o'clock position. Any offset from track is small compared to the 500 mile route. It is easy enough to fly with the left hand, and hold the sextant close to the right eye whilst looking left, as with taking a photograph. Watching the time come up, and bringing the pre-set sextant up to the eye to gather the horizon, then the sun, can be done, after practice higher up, quite readily, within 5 seconds.
If the sun and horizon are gathered, it is quite easy to judge the distance between sun and horizon using the sun as a scale of 32 minutes (miles) diameter, especially when the distance is small. Try it watching a sunset from a beach. If the sun was not sighted within 5 seconds, I abandoned that shot, and tried again, one minute later, adjusting the Hc by the amount shown on MOB extracts mounted on the Bygrave. There is no need to adjust the index arm of the sextant whilst sighting.
Next, Astro drift checks for the Landfall procedure.