NavList:

Dale L. you wrote:
"At one point it is indicated that they are at an altitude of 650' Accurate enough Eye Height for a regular sight reduction?"

Yes! Or no? Just how much accuracy do you need? For a zeppelin, +/- one mile was surely sufficient (excluding bombing missions in the First World War but that was usually done visually, sometime by lowering a guy in a pod below a cloud deck, as below).

Suppose your height was 650 feet, give or take 25 feet, then the dip of the horizon would be between 24.2' and 25.2'. So not a problem. Aboard those airships, they had options. At low enough altitude, below about a thousand feet and over the ocean, they could use the sea horizon, and at higher altitude, especially above about 2000 feet where the horizon is often indistinct, that "Coutinho" [coa-teen-you!] bubble attachment provided a surrogate local vertical. The bubble wasn't as accurate as the sea horizon, but navigators realized within a few years, that the difference had little practical significance.

There's a feature of a bubble horizon that those of us who first learned marine celestial navigation often miss at first: it works everywhere and at all hours. Flying over unknown or featureless land, gliding along thousands of feet high above a cumulus cloud deck, sailing over the ocean with surface fog, or cruising a few hundred feet high above a frozen sea of ice... the bubble still works. And also in all of these places, the bubble works all night long.

An observer using the natural horizon is generally limited to twilight for star sights. We don't give it much thought, but that's a huge limitation. If you have an alternative to the natural horizon, a source for your vertical that is inside your ship, like a bubble or a modern inertial device, then the stars are available and enable accurate navigation with multi-star fixes for hours and hours straight through the night. It's a different world...

Frank Reed

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