NavList:

Gary LaPook you wrote: HO 249 volumes two and three provide computed altitudes as low as negative seven and a half degrees to allow observations of the sun at a sextant altitude of negative 4 degrees from B-52s flying at 55,000 feet .......

I don’t think you’d see many B52s shooting celestial at 55,000 ft Gary.  Lower than 50,000 might be more typical.  The Vulcan, and Victor, could exceed 55,000ft easily, but crews were generally restricted to 50,000ft, because a caring Government worried that in the event of rapid decompression, the pressure difference between oxygen regulator pressure and cabin pressure might cause our lungs to explode.  44Sqn had a commitment to go higher, 56,000 is a figure that comes to mind, and were provided with pressure jerkins, which they were required to try out occasionally.  We used to see them climbing into their crew coaches dressed up like Tweedledum and Tweedledee. https://capri-kyoto.ocnk.net/product/4365 . 

The highest I managed was 54,000 (naughty without a pressure jerkin, but we’d committed ourselves by then) on the way to Offutt, Nebraska.  The airline traffic was giving the controllers a hard time asking for routing around Cbs, so our pilot offered to make their lives easier by climbing over the tops. 

Aircraft such as the SR71, and even Concorde, have regularly operated above 55,000ft, but those which use celestial usually have star trackers, which I understand can see the stars 24/7 at those sorts of height, so they don’t need to rely on a horizon scraping Sun.  DaveP