Nautical Almanac

[RWJ76]

Underneath the column for the GHA of Aries is the time of Meridian Passage of Aries. I cannot think of a use for this. Does anyone have an answer for why this would be useful?

[Tyler Szwarc]

Hi RWJ76,

I haven’t heard of any specific uses for the time of Meridian Passage of Aries, but I can think of a potential use for it. You can use it to identify stars or planets that are crossing the meridian at twilight, simply by subtracting the time of sunrise (or sunset) from the meridian passage of Aries. The result will give you the SHA of anything crossing the meridian at twilight; all you have to do is look in the Almanac for objects with a similar SHA, and they should be approximately on the meridian at that time. This could be useful if you wanted to find a star that was crossing the meridian at twilight, so that you could use it to quickly determine your latitude (if Polaris wasn’t visible or you were in the southern hemisphere). For example, if I wanted to find a star near the meridian today (4/4/22) at latitude 40N at sunset, I just look up the time of meridian passage of Aries (1105) and subtract the time of sunset (1828), add 24 (since it’s negative) and I get 1637. If I multiply this by 15, I get an SHA of 249. Browsing the list of stars in the Almanac, I can see that Procyon is pretty close to this (SHA of 245). So at sunset at 40N, I can predict that Procyon is very near the meridian and can provide a nice E-W LOP to include in my round of sights.

I’m sure there are other uses for it, but that’s the first one that came to mind. It also conveniently fills the space at the bottom of the Aries column, so maybe that is its primary purpose!

Tyler

[Tyler Szwarc]

I want to clarify in my example that you should choose a time between civil and nautical twilight to use in the calculation, not the time of sunset itself. It will be too bright at the moment of sunset to see the star!

[RWJ76]

Thank you Tyler. You describe a well-reasoned use for what I think is a somewhat vestigial holdover from the Nautical Almanac’s dim past. I did find in my own research (aided by others as well) that it is there to predict the Meridian Passage of a star. But I had not yet taken it to the practical aspect that you’ve done. It may also be true that when the NA gave star positions not with SHA & Declination but with Right Ascension & Declination there was a simpler “quick scan” to be had (still working on this). Do you mind if I forward your explanation (with credit to you of course) to my circle of celestial navigation friends who are also researching this?
Rick Jacks

[Tyler Szwarc]

Hi Rick,
Sure thing, feel free to forward. Please let me know if you come across any other answers to your question!
Thanks,
Tyler

[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

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