NavList:

Chris you wrote:

oops!  I use center of the body when using my bubble sextants - I should try a lower and upper limb sight.

Chris in VT

For a bubble sextant, putting the centre of the body in the centre of the bubble, or in the case of the Sun, leaving an equal overlap of bubble all around the Sun, is almost universal.  I have heard an air navigator suggest that for a sickle shaped Moon on a tilt, attempting to centre an obvious upper or lower edge might be advantageous, but I’ve never tried it, and he might have been talking about a pendulous reference sextant in any case.

With a Smiths pendulous reference sextant, the graticule is in the shape of the old London Underground sign, so you can use the circle for centring the Sun, or the horizontal line with a small gap in the centre for aiming at stars or for edge shots on the Sun or Moon.  However, realising the lower order of accuracy of bubble and pendulous reference sextants and the need for simplicity, most air navigators stick with centre shots.  In the air when everything’s moving around, trying to keep a body centred is definitely easier than attempting to line up an edge.  The Kollsman periscopic reference sextants are much the same except, if I remember correctly, the graticule doesn’t have a circle; you just get a vertical and a horizontal line with a tiny bit missing at the very centre, so you can see the star. 

Last night was minus three centigrade outside in Lincolnshire, but crisp and clear enough to see beyond the light pollution.  Inside the “Astrovan” it was nice and warm, and I got some useful calibration shots on Vega, Deneb, Aldebaran, and even, with a certain amount of hope and imagination, on Polaris with my Smiths RAF Mk 2C, which now includes 24V for the mounting, the pendulous cell heater, and the gold film window heater.  They pull slightly less than 5 Amps in all. DaveP