Celestial Navigation

[SkywalkerII]

After reading the board for some time, it's time to join in actively.

I decided to teach myself celestial navigation this winter. I've read several books, bought a sextant, nautical almanac, HO 249 sight reduction tables, artificial horizon, and off I went. My first round of sights averaged 5 nautical miles from mile actual position (my backyard).

I have a question that I have been unable get answered and I was wondering if someone could help. HO 249 has three volumes. Volumes II and III are permanent solutions to spherical triangle solutions and do not go out of date. Volume I (the stars) is time sensitive. Are there mathematical corrections to earlier Epochs of Volume I or do they become unusable once the time frame has passed?

Thanks.
Skywalker II

[sailingdog]

IIRC, the nautical almanac ephemeris data for the stars basically repeats every four years... not exactly, but in a pinch it'd get you in the ball park. No mathematical corrections that you'd want to learn how to do that I know of.

[SwiftCloud]

If you can get your hands on a 1981 edition of Bowditch you will find a partial answer to your question. Basically, and while the ephemeral data for the fixed stars repeats (approximately) each leap year cycle, there needs to be (i) an addition to GHA Aries of 1.84 minutes of arc/leap year cycle after the base (to account for the slow westward movement of the first point of Aries), and (ii) small annual corrections to SHA and Dec (different for each of the navigational stars).

The 1981 edition of Bowditch gives the latter corrections for 38 of the navigational stars to a precision of +/- 0.005 minutes of arc. Given the numbers involved mental interplation for parts of a year is good enough for small boat purposes.

If you can't find an early edition of Bowditch send me a message with your email address. In reply I'll attach some tables from my book (base period 2000-03) which shows how the whole catastrophy works.

[sailingdog]

They also do sell multi-year almanacs, that basically have a corrections table that uses corrections like the one SwiftCloud describes. I have one that is good for 50 years.

[openwater]

Skywalker.......I too have recently gotten into CelNav as a hobby...and, as you, have been reading many books on the math and mechanics of it all. After much searching, I have been trying to find people who are as interested in it as I am. Hope to talk to you guys soon.

[Boasun]

Originally Posted by SkywalkerII

After reading the board for some time, it's time to join in actively.

I decided to teach myself celestial navigation this winter. I've read several books, bought a sextant, nautical almanac, HO 249 sight reduction tables, artificial horizon, and off I went. My first round of sights averaged 5 nautical miles from mile actual position (my backyard).
Skywalker II

Your first round of sights will have a lot of personal error in them. As you practice you will slowly reduce the personal error down to naught or a known constant that can be applied to the reduction. Also try to have up todate almanac and I use H.O. 229 myself. But 249 is good also.
The preference depends on what you trained with.

[openwater]

I did the same. I am not a sailor per se, but I work on a deepwater drilling rig and am responsible for the dynamic positioning system we use for navigation. It uses all Mil-Spec equipment that costs ridiculous amounts of money in a many-tiered redudant system. I am a "math romantic" and I guess that is why the old ways facinate me so much. What type of sextant did you buy?

[openwater]

Skywalker....also some info for you regarding Volume 1 of the Sight Reduction Tables......Technically speaking Volume 1 is good for 10 years....5 years prior to and 5 years after the epoch it is labeled for. This change from edition to edition is to compensate for precession and nutation, however, in the back of Volume 1 you should find some incremental corrections to apply after the labeled epoch for the edition that will ultimately make your values correct and render the "older" edition still usable.

[Boasun]

What is the sextant that we own & use; I have a C-Plath. Which is a very nice one.

[Flybyknight]

Greetings,
I too have a C. Plath, and use the Nautical Almanac Site Reduction Method, although I have the full set of H.O. 229, 214, and 204. I earned the grade of "N" in the U.S. Power Squadron in 1993 in Northern New Jersey. There are no Power Squadrons close to my retirement home in lower Delaware.
Would certainly like to brush up with anyone nearby who may be interested
in taking sites,
Dick