A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2015 Nov 9, 02:22 -0800
When it comes to achieving the correct time of shot, it’s amazing how the designers of the MkIX bubble sextant around 1938 (Nominally Mr P F Everitt at H Hughes & Son) thought of almost everything. The navigator wore his watch on the inside of his left wrist. In the right-hand handle there was a pea bulb shining inside the hollow handle. Light was directed via a clear plastic guide (Perspex??) to shine out of the bottom of the right-hand handle onto the navigators left wrist. He could take sneaky looks at his wrist with his left eye whilst observing the star through his right eye. The right-hand handle also had slots to shine light onto the coarse 10° setting, the 5° increase button, and the fine setting. There was also a rectangle of white plastic (melamine formaldehyde or celluloid??) upon which the navigator could write: object, start-time, finish time, and altitude in pencil. This was also lit from a slot in the right-hand handle. The light was operated by a switch on the left-hand handle. Why almost everything? Well, initially the sextant had no way of being hung in an astrodome. The hanger was added as an afterthought from 1941.
Whilst everyone wishes to be exact with their timing, it’s worth checking on the effect of a few seconds difference between the time the shot was calculated for and the actual time of the shot. This depends on the azimuth of the body and the observer’s latitude. It can be found in the Motion of the Body (MOB) tables in several publications such as HO249/AP3270. The worst case is at the equator with Zn = 090 or 270 when it’s 15 minutes of arc per minute of time. The least is at the poles or Zn = 180 or 360 (presumably that’s why Polaris hardly moves). If you know the time difference, you can correct for it from the tables. If you don’t, then at least you know how bad it was likely to be. I seem to remember being taught the mnemonic LBW. Not ‘leg before wicket’, but ‘late back west’. I assume this meant if the shot is late on pre-calculation, move the PL west, but I can’t see where the ‘back’ came in. Any ideas? DaveP