A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2016 Feb 5, 18:34 -0500
I am puzzled here. The sun has a maximum rate of change in declination of 0.1 minutes of arc per hour. That is the d correction at the bottom of the daily page of the NA.
0.1 minutes of arc is the minimum increment used in practical navigation.
Thus, I understand that over the course of an hour, the declination of the sun will change by, at most, by the minimum increment observed.
What do you mean then, that culmination will be different than transit "by minutes", when the sun's variable declination is accounted for? Are we not at the minimum increment of observation?
Please treat this as an opportunity to educate me. I'd like to understand more.
You are quite right to spot a problem here. Of course, it is trivial to say that meridian transit of any given body happens at the same time for all observers on the same meridian. But for the navigator, the observable event is culmination, not transit. For a body with variable declination such as the sun, the time of culmination is indeed affected by the observer's latitude - to the extend that the method of the classical noon observation will break down near the pole. North of 89 deg, the time between transit and culmination can exceed many minutes, and thus also the time between the observed culminations by two observers on the same meridian. Besides the theoretical problem you will also face the practical one that at some point the observable will get lost in the noise.