The INTERCEPT:

The difference between Ho and Hc, the intercept is labeled "a". Assuming our DR & sextant sight are fairly accurate and we've reduced the sight correctly, the "Intercept" is usually < 30!0. If not readjust your assumed position (AP).

I usually use the major intersection of closest whole Lat/Long degrees as my assumed position. It's less confusing and doesn't clutter up the plotting sheet as much as AP's all across the nearest Lat line.

When navigating in SH3, I've found that the D=SxT formula doesn't hold true when traveling east or west. Whenever I use this formula to determine my DR position at a future time, I find that when that time arrives, my prediction of DR position is always way ahead of where I really am. To compensate for this "glitch" I shorten my distance run with the Longitude minutes x cos Lat formula. This usually puts me in the general proximity of where my boat really lies, and as a result my fixes are fairly accurate. BTW: I don't have this problem when using SH4.

Earlier it was asked if plotting was done on paper or the F5 screen...well...I'm finding more and more I've been using paper due to the fact that you cannot write notes on the F5 screen like we could with SH2. With plotting on paper its no problem to label each and every sight, so you know at a glance what each drawn line represents. It can become a real mish-mash on the F5.

Thanks for the info, Don.

If I get ambitious I may see if I can iterate another couple of columns for sunrise/set across the latitudes, for now I just have sunrise/set at the equator.

I thinkk the almanacs are now fixed. The old ones had all N decl on star sha's. I also added the equatorial sunrise/set and meridian passage, now you can take noon sights.
Almanacs are at http://hosted.filefront.com/6sj7gt/2231663

When I get the main package updated maybe we can get a [REL] tagged onto this thread title.

Also, I have been playing with the next leg of the tutorial, but may have to redo it to include a noon sight. Messing with a tutorial has been a good way to understand what i know and don't know about Celnav. (Now if I could just get my star sights a little closer with the aircraft sextant i've been playing with):doh:.

Cheers,
Mike

I would be very appreciative of more worked examples if you don't mind as I'm getting very inconsistent results when I plot my LOPs. I've spent so much time experimenting and reading about celestial navigation online that I feel like my head is going to explode. I feel like I'm really close to getting it but there's a fundamental step I'm missing somewhere.

When plotting the azimuth of your first star line you always start from the center of your plot and not your dead reckoning position? In this example the author starts with Rigil Kentaurus with an azimuth of 147 deg but his initial line doesn't start from the center nor does it pass through 147 deg on the compass.

Again, I'd appreciate some examples expressed in layman terms if you're up for it. I'm seriously thinking of taking a navigation class offered by my local Coast Guard auxiliary as I'm truly interested in this stuff but I feel like I"m spinning my wheels without having a guiding hand to assist me.

JMR,
I found that example when I started looking at celnav. I'll try to go through his sight as I understand it.

Hs: the reading on the sextant
ie: index error; a correction for errors of the sextant itself. You can check this if you have a sharp horizon. If your sextant error is 0 then your reading will equal your correction for height of eye.
dip: the correction for the height of your eye above waterline.
Ha: Height Apparent; Hs with above corrections.
corr: the correction for atmospheric refraction from a table in the nautical almanac.
Ho: Height Obsreved; Ha plus/minus corr. NOTE: when taking "sights" for the sub position in Stellarium you don't have any of the above corrections.

The next section: (* = degree sign)

GHA (of Aries for 23:00) 166*58.3
corr: (GHA of Aries for minutes and seconds of time, not the same as the above corr) 10*30.7
SHA: hour angle of Rigil K 140*06.3
GHA: of Rigil K, the above added together. (For other sights you may have to subtract 360 if the result is over 360 deg).

AP: Assumed Point; This is where you are having a problem. Ed Falk is using the HO229 tables where you want the LHA (difference of the GHA of Rigil K and AP) to be a whole degree. His DR longitude is 74*44.6W (-74*44.6), the closest point that gives a whole degree is -74*35.3 You plot this point on your plot sheet at 54S Latitude. (closest whole degree to your DR latitude of 53*38.4S. When you make your plot sheet note that in S latitudes the degrees advance DOWN the sheet, and W longitudes advance right to left. I got those mixed up a few times making plot sheets. The az 147* is measured from the AP. Place a ruler from the center of the plot 147* and you will see the line is parallel. (A parallel rule is nice for drawing these).

Here is a pic of the page from HO229;
http://users.ameritech.net/mjones004/Image1.jpg

And Ed's plot with a parallel line:
http://users.ameritech.net/mjones004/plot4.gif

If you are using the Ageton method or the online almanac the LHA does not have to be a whole degree, so you can use a whole degree for your AP to make plotting easier.

(how did I do, Don?)

Hope this helps,
Mike

Ok here's an example where I'm getting LOPs that don't make any sense to me.

On patrol in SH3.

Date: 4/23/1942
GMT: 08:59

Assumed Position: N 48* 00.0' W 11* 00.0'

Star shoot and accompanying data from Stellarium and USNO site:

Menkar @ 08:59:09
Hc 16* 00.1'
Ho 15* 32.6'
Dist -32.5
Zn 102*

Fomalhaut @ 08:59:25
Hc 11* 39.3'
Ho 11* 09.0'
Dist +30.3
Zn 172

Diphda @ 08:59:52
Hc 16* 30.9'
Ho 15* 55.5'
Dist -24.6
Zn 145

http://img206.imageshack.us/img206/9...plotwh6.th.jpg


Uggg what a mess. Stellarium shows me at N 48* 27' 33" W 11* 32' 50"

JMR,

Recheck your math, the sights are good. (hint; 16* > 15*, remember when you carry a dgree to minutes, it adds 6, not 10 to the 10min column. Made that mistake more than once).

I'll post a new plot sheet in a bit.

(Edit) Here is the plotsheet redone.
Mike

Lordy, what a brain fart! I knew I had to be close but there was a simple fundamental step or conversion I was doing wrong, and yup, there it is. :doh:

Thanks, 6SJ7GT!

What do you do if Hc is a whole degree less than Ho?

Example:

Hc 34* 33.0'
Ho 35* 08.0'

The difference between Hc and Ho I calculate as being 35mins (60-33 + 8). That's less than 1 degree. Or did I misunderstand you?

Howdy, Gents.

Determining the "INTERCEPT" (a), the difference between Observed Altitude (Ho) and the Calculated Altitude (Hc), is always either:
Ho - Hc = a (toward the body), or
Hc - Ho = a (away from the body).

If the resulting intercept is >45 min. of arc (45 nm), then one should recheck their math for an undetected error, or lacking math error, take another sight. (in real life)

In your case, jmr, your intercept is 35 nm which is totally acceptable.

Calm seas,

Thanks guys.

I've been digging around for tutorials on celestial navigation and I came across an old video by William F. Buckley Jr. called Celestial Navigation Simplified which amazingly is carried by Netflix. It's nice to have a video narrative on this subject as I've spent a lot of time reading about celestial navigation on the net and quite frankly I find a lot of it befuddling. Buckley's video taps lightly into mechanics of celestial navigation but the main focus is on how to find your position using a sextant and plotting tools. I need to watch it a few more times along with working out some practice problems to drive the point home but after one viewing it has already cleared up a lot of questions I had remaining after my readings on 'net. It's certainly worth a rental for other real nav neophytes who trying this mod for the first time.

Hey guys, since the in game clock doesn't show our time to the second, how do we use the Increments and Corrections table?

(Just speaking of time) What I do is go to python with the HH:MM and then enter Stellarium where the time in seconds is ticking off. I choose a celestial body to use and stop the clock by pressing the #6. I use the HH:MM:SS from where I stopped the clock in Stellarium.

It all works out fine.

Cheers,

I'm sure the Silenthunter (3 or 4) clock doesn't matter at all for taking sights. It does for the start of a new deadreckoning track though, but rough minutes are ok for that. Only the location from the savegame is carried over to Stellarium and that remains fixed the whole time. It's not really realistic for our purposes, but something we simply need to except, as astronomical telescopes don't have a tendency to go wandering off (hopefully anyway) like ships do. As long as the times used in your calculations match those taken in Stellarium you should be fine and get the correct save-location. The time (corrected for timezone) only needs to be the same in SH and Stellarium (roughly) because it would be an immersion killer if you saved SH at nighttime, and did Stellarium shoots plus calculations with a dusk/dawn/day-time horizon. But for getting a location fix in you could use sights taken any time of the day since you are not moving.

My first excercise in celnav was last week when I tried to fix the position of my home using those web-almanac and altitude/azimuth calculator pages mentioned in the tutorial. Stellarium was set to the long/lat coords where I lived. I took 3 sights of stars, 2nd star about 10 mins later than the first and the 3rd more than a whole hour because I went overboard on time accelleration. I chose the stars to be in 3 distinct directions on the horizon (E,S,W) so the plot-lines cross clearly. After the calculations and plot were compared with an atlas, and Google Earth (the 8th World Wonder in my book!!!), I found out I did everything correctly, except for using the correct lat/long coords in Stellarium. My house wasn't underneath it, but a different area of my town. But that location still matched the coordinates I had set Stellarium to, give or take halve a nm. No errors, so I still made a victory dance. :)

On another note, I asked 6SJ7GT last week to make the almanacs for the beginning years of the war ('39, '40) for Sh3 (which I play exclusively as Sh4 is too tardy on the framerate) So I hope he reads this message and gives a heads-up.

I need help again =\

I'm working all of this out manually and my 1st shot and calculated results turned out fine but my second shot is giving me trouble.

AP N48* W05*

Date/Time (both GMT) 05/24/1942 03:17:01


Antares
Ho: 7* 14.1'

Dec: S 26* 18.3'

GHA Aries: 287* 4.2'
+ time corr 4* 15.3'
+ SHA 113* 31.7'
-360* 00.0'
-----------------
44* 51.2'
+/- AP 5* 51.2'
------------------
LHA: 39* 00.0'
SIGHT REDUCTION HO-229 tables (latitudes 45 - 60)

Ok, I need clarification here. Both the Sun and myself are in the same hemisphere so I should use LATITUDE SAME NAME AS DECLINATION, correct?

If I do that, I go find my LHA of 39*, Dec 26* and get . . .

Hc 52* 28.9' d +43.0 Z 111.8
I check with the USNO website and that Hc is way out of the ballpark so obviously I shouldn't have used LATITUDE SAME NAME AS DECLINATION, right?

So I then go look up under LATITUDE CONTRARY NAME TO DECLINATION

Hc 8* 08.5' d – 54.4 Z 145.2

Ok, that looks a bit better, but what about Z? If I'm using LATITUDE CONTRARY NAME TO DECLINATION do I still abide by:
N. Lat. {L.H.A. less than 180°.............Zn=360°–Z
{L.H.A. greater than 180° .....Zn=Z

If I do, then I subtract 145 from 360 and get a Zn of 215 which confirms what the USNO site says.

Continuing with sight reduction . . .

Hc 8* 8.5'
+d 16 <---------- from Correction to Tabulated Attitude for Minutes of Dec
----------
=Hc 8* 24.5'
-Ho 7* 14.1'
----------------
1* 10.4'

That's way off. Checking with the USNO site I get an Hc of 7* 32.1' which when subtracted from my original Ho gives me a distance of 18nm

When plotted against my other star (Capella) the LOP intersect at around my actual position according to Stellarium.

It's 1:20 am and my brain is fried. Please help :huh: Sorry the formatting couldn't be any cleaner.