NavList:

Gary wrote:
"Yes, a fix using only stars results in a fix with the coordinates of latitude and LHA Aries. That latitude is completely immune to errors in time. You only need time to calculate GHA of Aries which then allows you to find longitude. To make this obvious just look at HO 249 volume one."

Exactly. From the perspective of modern LOP navigation methods, the fact that a pair of star sights yield a latitude even if GMT is uncertain is entirely obvious. Even trivially obvious. For a star fix, if you change GMT by almost any amount (less than some weeks), that just slides the fix around on a circle of latitude on the globe.

The interesting thing about these early methods (from about 1820 onward) is that they PRECEDED most LOP concepts. It's the same outcome, of course, but their point of view and method of analysis was very different. This idea of getting a latitude from the altitudes of two stars observed nearly simultaneously arose (at least once) as an extrapolation of the problem of latitude by two altitudes of a single star at different times (which was extremely popular with mathematicians of nautical astronomy starting way back in the mid-18th century, though not so much among practicing navigators). In fact, it makes good sense. There's really no difference between observing two altitudes of a single star at different times to get latitude and observing altitudes of two stars at the same time to get latitude, except that the former case has the convenience of no difference in declination. These methods usually turned out to be computationally involved. They didn't catch on. But they were leading steadily towards the graphical, plotting methods which we know today. You might say that Sumner's method was "in the air" by 1838 when he invented it. And maybe that, too, is why Sumner's approach wasn't seen as something really revolutionary, and worth study, for decades yet.

-FER

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