The practice refers to inverting the nominal observation. In CN, we typically take an observation at a (relatively) unknown location, but at a known time. This helps us to resolve our location
For this observation, we take the observation at a known location, so as to "rate the chronometer". Frank Worsley famously performed this observation from Elephant Island prior to his Southern Ocean crossing to South Georgia Island. Since we know where we are, any error we find in observed altitude from the expected (desired) altitude can be attributed to an error in time in our chronometer. Thus, we rate the chronometer.
I think but am unsure, that the "harbor marker" part of your question refers to a specific monument or place near the harbor, but those more knowledgeable than will need to confirm this. From the description, it certainly sounds like it. The more accurate the known location, the better rating of the chronometer. Hence a harbor marker would provide an excellent reference.
To be honest, while the observation is possible, there are many, far more accurate, methods of recovering time.
I came across this item in an article on celnav on the website tiititudorancea.com.
" Traditionally, a navigator checked his chronometer from his sextant, at a geographic marker surveyed by a professional astronomer".
Could the members throw some light on the method referred to here and its history. The marker is also referred to as a "Harbor marker"
Do any of these markers still exist?
53.3° N 06.3° W
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