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The bubble octant and bubble sextant are air navigation instruments. Although an instrument is called a "bubble sextant", it may actually be a bubble octant.
Ships had long used sextants for navigation, but sextants had problems in aircraft navigation.
A ship at sea is on a relatively flat surface and can use the horizon to measure the altitude of celestial objects.
An aircraft may not have the sea's horizon as a flat reference surface. It may be flying over land where the horizon is formed by mountains of unknown height.
A solution to the problem was to use a bubble to determine the reference plane.
The bubble in an airplane is subject to the plane's acceleration. If the plane is in sharp turn, the bubble will be displaced. Consequently, when the navigator is using a bubble sextant, the pilot tries to fly the plane straight and level.
Even when flying straight and level, the plane is subject to accelerations from density and wind changes. Consequently, many readings are taken and then averaged for a more accurate result. Some bubble octants have accessories to make the averaging simpler.
The octant was a further improvement. It could measure altitudes of up to 90° above the horizon.
The sextant became the standard navigation instrument for ships. It could measure altitudes of up to about 120 degrees. That allowed the navigator to sight the horizon in front of him and measure the altitude of a star that was behind him.
The first bubble instruments were bubble sextants; they were copying the features of an ordinary sextant. However, a bubble instrument does not look at the horizon, and so it never needs to measure an angle more than 90°. So the modern instruments are technically bubble octants, but they may be labeled bubble sextants.
Byrd developed the first bubble sextant.
- Amelia Earhart. Her navigator, Fred Noonan, used a bubble octant.
- Eddie Rickenbacker. A damaged bubble octant caused a navigation error resulting in a ditching at sea.
- Warner, Deborah (2005). "Celestial navigation aloft: aeronautical sextants in the US" (PDF). In Finn, Bernard S.; Hacker, Barton C. (eds.). Materializing the Military. Artefacts series: studies in the history of science and technology. 5. London: Science Museum. pp. 95–119.
- https://amhistory.si.edu/navigation/type.cfm?typeid=11 Aircraft Sextant at Smithsonian institution
- Ifland, Peter (1 November 1998). Taking the Stars. Celestial Navigation from Argonauts to Astronauts. Newport News: Krieger. ISBN 978-1575240954.
- Bud, Robert; Warner, Deborah, eds. (17 March 1998). "Sextant, Aircraft". Instruments of Science. An Historical Encyclopaedia. Garland Encyclopedias in the History of Science. New York & London: Routledge. pp. 532–534. ASIN B00FAZ0CKI.
- https://amhistory.si.edu/navigation/object.cfm?recordnumber=123452 "Bubble sextant", Bausch and Lomb type 23
- US 1531615, Hunt, Franklin L. & Karl H. Beij, "Aircraft sextant", issued 31 March 1925
- US 1674550, Hunt, Franklin L. & Karl H. Beij, "Liquid level", issued 19 June 1928
- US 1703705, Beij, K. H., "Means for Determining Altitude"
- US 2080851, Gallasch, G. B. & H. F. Kurtz, "Sextant"