Reply To: Nautical Almanac

[RWJ76]

Hello Tyler,
I’ve been studying your method of finding the SHA of the Local Meridian at Sunset from the Meridian Passage of Aries. I ‘ve come at it from what I think is a different (historical) perspective that utilizes Right Ascension (in time units) rather than SHA. My RA when converted to SHA matches your work. I have a reprint of a 1916 Bowditch which I referenced for my approach to this. The first thing I noticed in the NA extracts was that star positions were given in RA & Dec rather than SHA & Dec. So a computation of the RA of the Local Meridian at the time of the event (Sunset) allows you to look at the star list to make a direct comparison to see what star might be on your meridian at that time. The best way to “see” this (and all celestial computations) is to draw a Time Diagram. An aside: In modern celestial navigation we still refer to a “Time Diagram” when in actuality we draw our arcs in angular measurement, not time. Back in the day Time Diagrams really did show Time! I will do my best to describe this algebraically as I can’t here draw the Time Diagram. Using your Sunset example on 4 APR 22, a quick glance shows Aries ahead of the Sun by approximately 1 hr. The time of Sunset is 18-28-00. By subtracting the Mer. Pass. of Aries (11-05-06) from the time of Sunset we find Aries to be 07-22-54 ahead of our Local Meridian at Sunset. But this time interval is Solar Time and Aries moves at Sidereal Time so a correction must be made. This correction (there’s a slick table in the 1916 NA for this) is 1m 13s which is added to the time interval bringing it to 07-24-07. This is the RA of our Local Meridian at the time of Sunset and can be directly compared to our historical NA list of star RA’s. Subtracting this RA from 24 h yields the SHA in Time. Converting Arc to Time yields an SHA of 248* 58.3’ so both our computations yielded an SHA of approximately 249*. Trying to understand the evolution of celestial navigation as seen through the eyes of the navigators in their day is a great passion of mine. Hey, if your findings are different I’d love to see them as I’m no expert at all and I enjoy the inquiry process.
Rick