Just to get the Longitude thread on topic: Were sundials used, or proposed to be used at sea to compare with chronometers? Mayall and Mayall picture a gimbaled Japanese noon mark dial designed for use at sea opposite page 153 of the paperback edition.
I realize that it would be necessary to read time to less than a minute because the apparent sun moves one degree in 4 minutes. One degree of longitude can be 60 nautical miles at the equator. Thus it would be necessary to provide careful alignment and application of the EOT. The latter implies the need for an almanac similar to what was used for celestial navigation with the sextant. As Chris Lusby Taylor has explained, use of just one body, like the sun, gives only a single line. At a noon fix, both a East West line of position from the altitude of the sun and a longitude can be read. The first required the development of a good almanac for the position of the sun. What was needed was a good record of the sun's declination by the hour of the day. Altitude measurements must read in minutes of arc since one minute of altitude can mean one nautical mile in position. The advent of satellite technology has made all this very much a historical question. Is it true that US Naval midshipmen no longer study use of the sextant? You can only hope that the GPS or radio in your lifeboat has good batteries. Claude Hartman
