Celestial Navigation in SH4

[I'm goin' down]

I would like to see a basic step by step tutorial, preferably a video tutorial, on how to navigate once the program is installed. I could not understand the instructions given some time ago.

[I'm goin' down]

What I intended to convey was this. I finally got the program installed with Don Reed's excellent assistance. However, after do a bunch of reading about celestial navigation, I could not follow the initial tutorial. I had no idea how to sight, and how that related to my position. I think it is something you have to do a few times, and then it sinks in. Anyway, if someone could do a break through tutorial, perhaps a video if that is feasible, I would jump in head first. Get aaronblood interested. He has the capability to do some amazing stuff.

[TorpX]

Originally Posted by I'm goin' down

I would like to see a basic step by step tutorial, preferably a video tutorial, on how to navigate once the program is installed. I could not understand the instructions given some time ago.

I agree. The tutorial isn't very helpful. The navigation mission in my setup isn't the same as in the tutorial and I had trouble using the USNO almanac. I think it would be better to use the pdf almanacs in the thread, and possibly construct a special navigation mission for this.

Otherwise, I found The Mathematics of the Longitude by Wong Lee Nah to be very helpful. I don't understand all the methods, but I think I can use the Intercept method well enough (for game purposes anyway).

One more thing, I wrote a simple program for my TI-85 to do the trig and number crunching. I highly recommend this approach.

[don1reed]

I really cannot speak for, Mike, when he and his son originally wrote the Python program for Stellarium; but, it was presumed it was for those of us that had previous knowledge and use of real life, celestial navigation. The genious of his application is that it is flexible enough to be modified along with each new version of Stellarium, either by design or accident.

Apparently it's beginning to take notice. A "build it and they will come" situation.

Sea going celestial navigators took five sights per day, like prayers to Allah, weather (wx) permitting.

1) Pre-dawn twilight--A round of three stars, moon, or planet. Make sure your chosen celestial bodies are spaced about 60〫apart.
2) Ante Meridian (AM) Sun.
3) Noon Sun--when coupled with the AM Sun forms a Running Fix (RFix).
4) Post Meridian (PM) Sun--when coupled with the Noon Sun forms a RFix.
5) Post Sunset twilight--A round of three stars, moon, or planet. Make sure your chosen celestial bodies are spaced about 60〫apart.

Twilight occurs when the Upper Limb (UL) of the Sun is between -12〫and -6〫. The (-) sign indicates that the Sun is below the horizon.

Sun Rise occurs when the UL of the Sun is rising and is precisely even with the Ocean's horizon--NOT when it's half way above the horizon as in the game.

Sunset occurs when the UL of the Sun is dipping and is precisely even with the Ocean's horizon--NOT when it's half way below the horizon as in the game.

When you decide to take a virtual sight while playing SH3/4, pause the game and do a save. Doing this causes the game to write your true geographical coordinates to the save games file.

Open Python enter the date and time, in Greenwich Mean Time (GMT) within their respective focus boxes. All time for navigation is done by Greenwich time. After entering the time, Python should automatically open Stellarium.

I'll continue this post when I get to a PC. I'm currently using an iMac and don't have my files readily avail.

[don1reed]

Taking a real life sextant sight can take anywhere from 30 seconds to 15 minutes...depending on the weather (wx). Under bad conditions, it's impossible.

What the navigator is trying to do is simultaneously view the horizon and the heavenly body. The celestial body is reflected into his field of view by two small mirrors while he adjusts the micrometer drum (the dial at the bottom of the index arm), precisely when it appears as if the body is resting on the horizon, he yells, "MARK", loudly if very windy. His assistant/time-keeper, annotates the time in the navlog by Hour:Minute:Second. The Nav then calls out the Angle of Elevation (read from the Arc at the bottom of the sextant), to the assistant, who jots that info into the log as well. After gathering enough sights, (Five or more for the Sun for an average), they head below or out of the wx to begin their sight-reduction.

Two things needed for Sight-Reduction:

Current Almanac and Sight-Reductions Tables. In RL that could be HO Pub 229, HO Pub 249, or HO Pub 211. Or, if mathematics is your game, by the following two formulae:

Hc = asin [(sin Lat sin Dec) + (cos Lat cos Dec cos LHA)]
Z = acos [( sin Dec - sin Lat sin Hc) / (cos Lat cos Hc)]

What is needed to make your sight in the game:

1) Select a body, pause Stellarium program, jot down the HH:MM:SS and it's Altitude.
2) Resume Stellarium, Select another body, pause Stellarium, jot down theHH:MM:SS and it's Altitude.
3) Resume Stellarium, Select another body, pause Stellarium, jot down theHH:MM:SS and it's Altitude.
4) Exit Stellarium,

From Almanac:

Get:
Sidereal Hour Angle (SHA) of Aries and Declination (Dec) if using Stars,
Greenwich Hour Angle (GHA) of Sun, Moon, & Planets.
Minute/Second Increments
Add them up by xxx° xx.x'
Add East Longitude (Assumed Long)
Subtract West Longitude (Assumed Long)
Answer = Local Hour Angle (LHA)

Use Angle of Elevation from Stellarium as the "Ho" (Observed Height)
(In RL several corrections are needed to discern the true Ho) We won't need to make corrections when using Stellarium.

From the Sight-Reduction Tables:

Get by Assumed Lat, LHA and Dec:

Hc d Z
Hc = the scientifically calculated Altitude of the body.
d = declination correction.
Z = Azimuth of the body.

If Ho > Hc, then subtract Hc. The difference is the distance in nautical miles your line of position (LOP) is in the direction toward the observed body

If Hc > Ho, then subtract Ho. The difference is the distance in nautical miles your line of position (LOP) is in the direction away from the observed body.


From an Assumed Position (usually an intersection of the Assumed closest Lat/Long) construct a intercept line the length in Nautical Miles toward or away from your Zn of the body. At the end of this intercept line, construct a perpendicular line which will represent your LOP.

Continue this process until all your celestial bodies are done this way. When finished, all your LOP will cross in one position or pretty close to one spot. This spot is your Sight-Reduced position.

In theory that's how it's supposed to work. Your results may vary....

[don1reed]

Purist may complain that I've paused both my game and the Stellarium clock in order to obtain sight reduction.

Consider this before passing any judgement:

1) In RL the skipper was not the navigator aboard the Sub/U-boat. In RL the clock does not stop for anyone. So, if you want to simulate the navigator's chores, you must stop the game because the two programs are not synchronized., unlike doing your own tax return, small arithmetic errors may cause death, both simulated and in RL.

2) Cheers and have a great Holiday everyone.

[TorpX]

Thanks Don for your technical help and expertise.

I am starting a new career and am determined to rely on the stars come what may. I may get lost and cruise around in circles aimlessly, which, come to think of it, happens a lot anyway. After all, what is the worst that can happen?
Anyway, we are due to shove off Dec. 2, 1941, from Cavite. We are taking on a full load of torpedos, but the Admiral assures us this is just a precaution................ the Japanese wouldn't dare attack.


Merry Christmas!

[Pisces]

Originally Posted by don1reed

...
If Ho > Hc, then subtract Hc. The difference is the distance in nautical miles your line of position (LOP) is in the direction toward the observed body

If Hc > Ho, then subtract Ho. The difference is the distance in nautical miles your line of position (LOP) is in the direction away from the observed body.
...

You forgot to mention that mnemonic you figured out: Ho-Mo-To-Wo, or Ho More To-w(o)ard. It's very catchy, I'll never forget. It has gotten stuck in my head, only being surpased by saying "Jawohl, Herr Kaleun!" to myself all the time ... for no appearant reason.

[don1reed]

TorpX, you might want to try some local Chicken Adobo before shoving off.
Cheers


Pisces: I almost put it in my explanation...but thought, Nah, they wouldn't understand....LOL. They taught you that phrase too...small world.

Happy Holidays

[6SJ7GT]

Greetings, It's been a while, but I see things are picking up again on this thread.

Looks like Don laid out the procedure very well. About the only difference is that I leave stellarium running, and consider the small differences as the randomization that was talked about earlier in the thread. My readings are a minute or 2 apart, and at 9 kts the sub would only move .9 NM.

Also, I record the ALT and time, with both clicking away I sometimes get a little error from this, which I chalk up to rough seas. Just got a new computer and its been fun getting things running again. Mike.

[Pisces]

Originally Posted by don1reed

TorpX, you might want to try some local Chicken Adobo before shoving off.
Cheers


Pisces: I almost put it in my explanation...but thought, Nah, they wouldn't understand....LOL. They taught you that phrase too...small world.

Happy Holidays

No, you did. My knowledge of CelNav started here on this forum, and from what I already knew about the sky from my interest in astronomy. I've never held a real sextant in my hands, unfortunately.