Learning celestal navigation

[ron]

Yup, and an investigation into the celestial object and how they show in VS has been done by yours truly a while back.
The celestial projection in VS is off by time? Ilan has the ambition to correct it in VSF.
My own numerous VS position fixes, using the sextant, told me the projection is off by some 40 minutes (I think it is 44), explained in the tutor I wrote. Still using the minutes correction my actual position in VS against the sextant position, was off by 1.5 miles. This tells me that besides a minute correction (44) there might also be seconds involved.
Still, out in the open, a sextant obtained fix off by 1.5 NM is rather good and I left it at that.
BTW:
I knew right away that when you'd alter the "height measured" (Ho, height obtained/observed) you would be "messing" with time and azimuth?
The issue with Ilan's projection (the celestial map he's using) is just time, which I think I have proven.

a PS:
If you are game I'm willing to go into an experiment with you, let's take the scenery Gibraltar Central, freeze time, take the stars heights using the sextant and create a fix. You name the date and time and your anchored position. I like anchored, not to add the complexity of having to progress the lops (using assumed speed against time).

[laukejas]

Well, ron, I have red your tutorial many times, but when I try to apply this minus 44 minutes correction of yours, I get my fix far far from where I am. Take a look at my second test (in my previous post, sight 2). With all procedures like in real life, I get position which is 15nm off. To get accurate position I have to add 57 seconds to GMT. But if I would minus 44 minutes, as you suggest, my position would be hundreds of miles off.
Maybe celestial sky is out of sync differently for every scenario? Or maybe it changed after 7.1 patch (I'm using it)?

What do you think?

[ron]

I think we should test, I'm going to post a scenario, give me some time.
Not that it will do any good to VS, it is not in developement mode, so anything we find will make it's way into VSF, not VS.
The topic is dear to me so let's collaborate and identify the issue...
More to come shortly,

So, I think how to do the first test is to be on lattitude zero and longtitude zero, this is where I will position my boat and anchor it (to avoid guess sailing the lops).

I'm here, place this in yout VS root.. (right click, save as.. it is a "cruise" to load).
http://www.virtual-sailor.nl/zeroNW.def

[laukejas]

Thank you for your help.

I opened the scenario. What next?

[ron]

Okay, I have selected 4 stars to shoot, for our first calculation we are going to use the VS time, I have my session frozen at 18:38:00 (press F10 to freeze your setup, so you have all the time in the world).

These stars are (must be comfortable to you, they are amongst the most common ones):
Altair, which is at alt 61:23,9 azimuth 289
Antares, at alt 11:05,0 azimuth 243
Deneb, at alt 42:51,1 azimuth 346
Fomalhaut, at alst 55:06,5 azimuth 150

Now I'm going to shoot these 4 in VS and see what I come up with...... (I should be way off as I didn't allow for the time correction).

To be continued so..

[laukejas]

Hey, I can't find either of these stars on these azimuths and altitudes! I looked with sextant at the 18:38:00 VS time at these azimuths, turned knob to these altitudes, but no star is even close.

[ron]

Not sure what you are not doing right,

I shot at 18:38:00 such:
Altair at 70:55.2 azimuth 299
Antares at 20:11.0 azimuth 242
Deneb at 44:30.3 azimuth 356
Fomalhaut at 48:48.0 azimuth 139

Remember we are trying to find fault!

So these values obtained from VS at 18:38:00 don't jive with the time. When correcting the time to 17:56:42 (so less 41 minutes 18 seconds) I get good lops which puts me at 00:07N and 000:02E, which I think is pretty darned good considering I've not tried exactly (where to place the star on the horizon).
I do know the VS celestial projection is off by some 40 minutes, (just like I mentioned before).

[laukejas]

Hm, you're right, I have recreated this, and I get pretty much the same results.

Could you please recreate my tests with sun sighting? This is the data:

Date: 2011 01 01

13:19:17 GMT

23:00:10S
14:08:30W

My results:
intercept off by +52.4' (52.4nm) (need to add this amount to get accurate fix)

sight 2:

Date: 2011 01 01

19:25:21 GMT

23:00:10S
14:08:30W

My results:
intercept off by +12.6' (12.6nm) (need to add this amount to get accurate fix)


This is without minus 44 minutes correction. Could you try please, at these coordinates at that time?

[laukejas]

Or take a look at this scenario: I'm sighting polaris (pause the game just as scenario loads) http://www.filedropper.com/test1_12

Polaris is at 035°52.6' altitude, 16:33:47 (GMT shown on sextant) 18.10.2011, and LOP is -13.4nm away from star. If I change GMT minus 44 minutes, as you suggested, LOP is 5.8nm off. This isn't too accurate... I don't understand. With your scenario, it worked, with mine, it doesn't.

[ron]

Hi, this is good, I'll try and tell you what I find, mind you we are in fault finding mode so if it doesn't work it only confirms which is what Ilan is going to correct in VSF.
Remember, from a development point of view, VS is a dead end, you're stuck with what you have.

Later,

Edit: your file can't download..
Edit2: with this exercise I've been fine tuning, the offset (as per me) is 41 min 18 secs, which in the case of Polaris would matter somewhat, but not much... (the height of Polaris above horizon should be your lattitude).
Still, only 6 miles off? I think that is pretty darn good (not knowing how you placed Polaris on your horizon., top, middle, bottom?)