NavList:

Howard
You were asking about celestial on land.  Unless you were doing super accurate scientific work when you might use a vertical reading theodolite, the standard method around the time you’re talking of was to use a cast-iron tray into which you poured mercury.  As a back-up you might also take a mirror equipped with spirit levels in case your mercury freezes, as Amundsen did.  Hg freezes at -39 degrees C, whereas 100% ethanol doesn’t freeze until -114 degrees C.  

Most polar navigation takes place in the summer when you might have 24-hour daylight, plus you’re not moving very far in a day, so you could take Sun lines (if visible) at convenient times and at different angles throughout the day, as Scott and his navigator Bowers* did.  Alternatively, very close to the pole you could use the method suggested by Hinks, as Amundsen and his team did**.  If you were stuck over winter, you had the stars of course.

You also mentioned Chichester’s Gipsy Moth flight over the Tasman Sea.  It’s now believed he used a ‘box sextant’, which is a two-mirror sextant like a marine sextant, but the mirrors are contained within a brass drum, which fits in the palm of your hand.  One handed operation would have been essential to Chichester, because the Gipsy Moth wasn’t particularly stable at the best of times, and fitted with borrowed, wrong-sized, and possibly leaky floats might have been even worse.  Brian Walton who’s tried operating a box sextant from a biplane believes that if you set an expected value before you start and judge the gap between the Sun’s lower edge and the sea in terms of the sun’s semi-diameter, you can make a fair estimate of the Sun’s height.  You can simplify the mental arithmetic even further by flying at a height where dip cancels out semi diameter and atmospheric refraction.

That’s not to say that true marine sextants weren’t used in aircraft.  Airships Such as the R34 and the Graff Zeppelin certainly used them if they had a good sea or stratus cloud horizon.  The dip tables work up to great heights.  The problem was knowing your exact height above the sea or stratus cloud mid-ocean with no Met actuals.  The R34 crew got round this on the first Atlantic crossing in 1919 by using the sextant to measure the angle subtended by their shadow on the surface and applying appropriate trigonometry.  Coutinho, flying with Cabrai, also used a marine sextant on the first aircraft crossing of the South Atlantic.  It’s true he had a spirit-level attached as a back-up, but come the flight, he didn’t need it, because he had a good sea horizon all the way.

You mentioned drift sights.  Good, weren’t they.  We had one in the Varsity, and was good fun and a chance to stretch your legs, so I always tried to fit one in.  Charles Lindbergh believed strongly in flying an accurate course plus the use of drifts.  I’ve always wondered where his drift sight was in the Spirit of St Louis.  Professor Google is strangely deficient on the matter.  Between his legs would have been ideal.  Perhaps his forward-looking periscope could be tilted down.   DaveP

* As there were questions over whether Peary had reached the exact North Pole.  It’s thought that Scott made sure he had a second qualified navigator in his team to confirm his own navigation to the South Pole.

** See https://southpolestation.com/trivia/igy1/hinks1944southpolepaper.pdf