Celestial navigation by day

[Pilot_76]

Historically (and nowadays as well) how can you navigate during the daytime using only the Sun as a reference? I've read a bit about the noon shot (finding your long. at noon, UTC difference, 15 degrees\4 min etc) but in one of the movies at youtube a "simulated" cel nav fix was done at sunrise and at sunset. What's this for?:hmmm::hmmm:

[kylania]

Someone linked a fantastic "Celestial Navigation Simplified (http://www.youtube.com/watch?v=cgeU_W9mFO0)" video here somewhere which explained the whole process. It relies on predetermined tables though - basically a list of the position of the sun at given times from given locations.

So you use the sextant to mark the sun's location at a specific time. You then estimate a location near to where you think you are given all the tables and stuff and figure out how far away from that point your assumed position is. This results in a line you could be along. Do it again an hour later (giving the sun time to move quite a bit) and measure it again, resulting in another possible position line. Where the two lines meet is where you are. At night you could simply take another reading from another star, but using the sun alone you need to triangulate via a second sun reading.

I guess the sunset and sunrise (and noon) checks are easiest since you're more likely to have knowledge of sunrise/sunset times then you are to have the multiple books of where the sun is at a certain time from the video?

[Pilot_76]

thanks for the info kylania, I am doing some experiments in SH4. BTW, do you apply Celnav in any SH? If yes what is your method?

[Rockin Robbins]

You can forget about the sky being anywhere near good enough in SH4 to do ANY celestial navigation from it. The only celestial navigation that can be done comes from knowing the position of the sub, putting those figures into a planetarium program and then doing the celestial navigation from there. SH4 was never meant to be a night sky simulation and it is not. It has a sky. That sky, especially solar system positions, is not accurate for date and time.

Typically, what is done with the Sun is meridian transit times. You have an accurate chronometer set for GMT. You get the time the sun passes its highest point. That is noon. The difference between GMT and your transit time is your longitude. This is not particularly accurate due to the size of the sun and the interpolation between precisely timed observations necessary to determine transit time. But it will get you in the same zip code.

[Arlo]

You can forget about the sky being anywhere near good enough in SH4 to do ANY celestial navigation from it. The only celestial navigation that can be done comes from knowing the position of the sub, putting those figures into a planetarium program and then doing the celestial navigation from there. SH4 was never meant to be a night sky simulation and it is not. It has a sky. That sky, especially solar system positions, is not accurate for date and time.

Typically, what is done with the Sun is meridian transit times. You have an accurate chronometer set for GMT. You get the time the sun passes its highest point. That is noon. The difference between GMT and your transit time is your longitude. This is not particularly accurate due to the size of the sun and the interpolation between precisely timed observations necessary to determine transit time. But it will get you in the same zip code.

I think I'm missing something here. What's the point of celestial nav in SH if the map is spot on?

[Sailor Steve]

I think I'm missing something here. What's the point of celestial nav in SH if the map is spot on?
The point is that the map is indeed spot on. It's impossible to be out of position by even one mile, whereas in real life navigation isn't perfect. It can be very close, but seldom perfect. The other half is that when you can't see the sky you have to use dead reckoning, which is the knowledge of your speed and course applied to your last known position, which is prey to currents you can't see. Some people want that uncertainty, and some want to actually navigate their boat.

[Arlo]

The point is that the map is indeed spot on. It's impossible to be out of position by even one mile, whereas in real life navigation isn't perfect. It can be very close, but seldom perfect. The other half is that when you can't see the sky you have to use dead reckoning, which is the knowledge of your speed and course applied to your last known position, which is prey to currents you can't see. Some people want that uncertainty, and some want to actually navigate their boat.

That doesn't appear to be a point as much as a desire. Celestial navigation isn't needed in SH4 and wasn't part of the game's realism settings. I'd like to be able to explore my submarine from stem to stern but there's no point in it, game-wise, because it's not necessary. Now, immersive role-play wise, it'd be cool but if it's not practical to mod or implement then there's no point in asking for it as much as there is in daydreaming about it with other sub-simmers here. :DL

If it does become a mod, well, honestly, there's a few desires I have that outweigh shooting an azimuth in between missing targets with my torpedoes. But to each their own and I'm not one to spray on anyone's celestial parade. :)

[Rockin Robbins]

Arlo, I wouldn't mind the ability to do celestial navigation if it were possible in-game. What's being done now is that people have turned off their position indicator in the nav map. Then they feed the game position to a separate planetarium program. Apparently they even have an in-planetarium sextant so you can actually simulate taking a sight. Then, after doing the calculations to obtain longitude and latitude for your sub, you go back into SH4 to plot the position.

Unfortunately, you also give up the ability to track target positions relative to your boat because there is no indicator for your boat's position. You just end up giving away three times more authenticity than you gain.

If you're not playing the game as such and just want to learn celestial navigation it's a great practice tool.

But in-game our sky isn't good enough to navigate from. I've demonstrated that moon phases and positions are totally wackazoid. You see, I have some of those whiz bang planetarium programs, Maurice Chevalley's Sky Charts (highly recommended and free!) and The Sky for Windows by Software Bisque.

[Arlo]

Arlo, I wouldn't mind the ability to do celestial navigation if it were possible in-game. What's being done now is that people have turned off their position indicator in the nav map. Then they feed the game position to a separate planetarium program. Apparently they even have an in-planetarium sextant so you can actually simulate taking a sight. Then, after doing the calculations to obtain longitude and latitude for your sub, you go back into SH4 to plot the position.

Unfortunately, you also give up the ability to track target positions relative to your boat because there is no indicator for your boat's position. You just end up giving away three times more authenticity than you gain.

If you're not playing the game as such and just want to learn celestial navigation it's a great practice tool.

But in-game our sky isn't good enough to navigate from. I've demonstrated that moon phases and positions are totally wackazoid. You see, I have some of those whiz bang planetarium programs, Maurice Chevalley's Sky Charts (highly recommended and free!) and The Sky for Windows by Software Bisque.

If someone can guru a mod that can make it come to pass, I'm good with it. Alas, I'm also good with no mod of this nature surfacing (currently - ya never know what I may get a hankerin' for the next day). It's pretty skies (or stormy, which, well, as long as I'm not really cold and wet, is pretty too) and water breaking over the prow and I'm good with that. What would really float my boat, so to speak, would be the ability to do some shore fun - both as part of a party sent ashore in combat or on liberty. And it would be fun to have the ability to form and join a boarding party (on those oh so rare occasions that a sub got to board a combatant).

We all got dreams and I support all of em ... just mine more. But until I become a modder all I am is a voice in the peanut gallery. Well, I guess even if I manage to mod, I am. ;)

[kylania]

thanks for the info kylania, I am doing some experiments in SH4. BTW, do you apply Celnav in any SH? If yes what is your method?

I don't use it in Silent Hunter 4, but Silent Hunter 5 has a very robust Real Navigation system. I'm still not that good with finding my location via sextant, so I rely on the navigator, but it's still a fun experience.

[Arlo]

Well, I may reconsider. Anything with the word sex-tent in it is worth exploring if there's a co-ed crew (that was bad but then I'm not known for the best puns). :03:

[Sailor Steve]

That doesn't appear to be a point as much as a desire.
Absolutely true, but also true of manual targetting and a host of other options. Some people want to do it.

What I would like to see in a sub sim is semi-manual everything. When the weather is clear you follow your listed waypoints. When it's not so fine there is a chance the boat will get off course, but the chart will still show you on track, until the next clear day and then you find out where you really are, and lay new waypoints. Same with targetting. You get the range and bearing yourself from the periscope, and the target's estimated position is drawn on the map. You do it again, and it's more accurate. The more you do it the better the solution is. Of course, as with navigation, you might still be off.

[Arlo]

Sounds interesting. But then it becomes a matter of what may or may not be a practical investment for those wonderful modders. Can it be done? Can it be done easier than a mod that'll let me play acey-deucey in the crew's mess? I may hafta put up or shut up and learn to mod my ownself. :cool: (But I'd download a 'celestial nav' mod to give it a go if someone else invested their time on one for the sake of the community. As long as it's TMO 2.2 compat.)

[Rockin Robbins]

I'm afraid all this stuff is hard coded in the .exe files. Modding those isn't modding, it's illegal hacking, so you have two choices as a modder.

Choice 1 is to turn off the sub position symbol on the nav map and attack map, find the sub position in the game files and feed that to an exterior planetarium file. That's what has been done by the navigation mod people for SH4. In doing so you give up a ton of realistic and useful things......like the ability to attack and sink opponents. This is not a workable scheme to my way of thinking other than as a proof of concept and a navigation trainer.

Choice 2 is not to do celestial navigation in SH4. That's been my path.

Or you could go to door #3 where the object of the game seems to be walking around your sub asking how the soup is for the ten thousandth time to build morale, while playing a bizarre Frankenstein poorly executed submarine meets horrible parody of the Sims monster. I'll pass on that one too. I could care less if Johan's girlfriend is pregnant with kittens.

[Arlo]

Or you could go to door #3 where the object of the game seems to be walking around your sub asking how the soup is for the ten thousandth time to build morale, while playing a bizarre Frankenstein poorly executed submarine meets horrible parody of the Sims monster. I'll pass on that one too. I could care less if Johan's girlfriend is pregnant with kittens.

Hehehe .... loud and clear. :know:

[TheDarkWraith]

Or you could go to door #3 where the object of the game seems to be walking around your sub asking how the soup is for the ten thousandth time to build morale, while playing a bizarre Frankenstein poorly executed submarine meets horrible parody of the Sims monster. I'll pass on that one too. I could care less if Johan's girlfriend is pregnant with kittens.

Or you could go to door #4 where the object of the game is whatever OHII has in the works, using whatever interface style you prefer (SH3/4/5), with unparalleled graphics, an AI that will test your abilities as captain, and real navigation (none of this fake stuff you're doing in SH4). That door #4 would be SH5 modded - a completely different game from the original ;) If you haven't tried it you are really missing out :yep:

[Arlo]

Or you could go to door #4 where the object of the game is whatever OHII has in the works, using whatever interface style you prefer (SH3/4/5), with unparalleled graphics, an AI that will test your abilities as captain, and real navigation (none of this fake stuff you're doing in SH4). That door #4 would be SH5 modded - a completely different game from the original ;) If you haven't tried it you are really missing out :yep:

Does SH5/OHII feature U.S. fleet boats in the mods yet?

[Sailor Steve]

Does SH5/OHII feature U.S. fleet boats in the mods yet?
No, but TDW wasn't really dissing SH4, he was responding directly to RR's comments on SH5. Responding to a single point sometimes causes trouble. I should know. :sunny:

[Arlo]

No, but TDW wasn't really dissing SH4, he was responding directly to RR's comments on SH5. Responding to a single point sometimes causes trouble. I should know. :sunny:

Didn't cause trouble as far as I'm concerned. It was a genuine question. If SH5 has more bells and whistles *and* I get to fleet boat it, I'm on board. I didn't think there was a mod that changed theaters for SH5 (I look, regularly) but that doesn't mean one doesn't exist somewhere (I also miss finding things, regularly).

So, if the Batfish wasn't cracked by the Okie earthquake, wouldja be interested in a road trip to visit that ol gal?

[Rockin Robbins]

Or you could go to door #4 where the object of the game is whatever OHII has in the works, using whatever interface style you prefer (SH3/4/5), with unparalleled graphics, an AI that will test your abilities as captain, and real navigation (none of this fake stuff you're doing in SH4). That door #4 would be SH5 modded - a completely different game from the original ;) If you haven't tried it you are really missing out :yep:

I'll continue to "miss out" thank you. I have no desire to feed the monster. My money stays in my pocket until something better appears.

[Sailor Steve]

Didn't cause trouble as far as I'm concerned. It was a genuine question. If SH5 has more bells and whistles *and* I get to fleet boat it, I'm on board. I didn't think there was a mod that changed theaters for SH5 (I look, regularly) but that doesn't mean one doesn't exist somewhere (I also miss finding things, regularly).
No, SH5 doesn't even include u-boats other than various versions of the Type VII. There's a very good reason for that, but it still is what it is. That said, depending on what the modders who actually program can figure out, it certainly may be possible in the future.

So, if the Batfish wasn't cracked by the Okie earthquake, wouldja be interested in a road trip to visit that ol gal?
I'm always interested in meeting people I've talked with at Subsim. At this time a road trip means a bus trip, but once I have a car (and then a bike) I'll be a lot more mobile.

[Arlo]

No, SH5 doesn't even include u-boats other than various versions of the Type VII. There's a very good reason for that, but it still is what it is. That said, depending on what the modders who actually program can figure out, it certainly may be possible in the future.


I'm always interested in meeting people I've talked with at Subsim. At this time a road trip means a bus trip, but once I have a car (and then a bike) I'll be a lot more mobile.

Well, there's a few boats closer to ya but the travel logistics remain the same. I've been tempted to fly up to see them, however, and I'd hafta get a car rental anyhow. If I can arrange it with work and the ol lady I'll give ya a head's up before we start finalizing the plans.

What I'd really like to do is arrange a large SubSim meet closer to here, though. I thought the Batfish would be a perfect backdrop and we have a singing group here in the DFW area known as 'The New Andrews Sisters' that love to perform at reenactments and WWII reunions. Dunno what they charge but they're into it. I'm picturing luau and dance with whatever facility near the sub that can kinda fake a 'Grand Hawaiian' setting.

Yeah, as usual, I think in grandiose terms. :03:

[Pilot_76]

You can forget about the sky being anywhere near good enough in SH4 to do ANY celestial navigation from it. The only celestial navigation that can be done comes from knowing the position of the sub, putting those figures into a planetarium program and then doing the celestial navigation from there. SH4 was never meant to be a night sky simulation and it is not. It has a sky. That sky, especially solar system positions, is not accurate for date and time.

Typically, what is done with the Sun is meridian transit times. You have an accurate chronometer set for GMT. You get the time the sun passes its highest point. That is noon. The difference between GMT and your transit time is your longitude. This is not particularly accurate due to the size of the sun and the interpolation between precisely timed observations necessary to determine transit time. But it will get you in the same zip code.

I am currently testing SH4 with Stellarium as some users have posted. My curiosity is what can I do with an apparent angle of zero (sunrise/set) to help my DR?

[Pilot_76]

I think I'm missing something here. What's the point of celestial nav in SH if the map is spot on?

Well, I think I can put it this way....

When I started playing SH series and was introduced to the TDC I was puzzled at first. After using auto-target I got bored. It was simple point-and-shoot. Just lock, wait for at least a yellow or preferably green triangle on the target and fire the fish. After some patrols the game kind of lost its glamour to me. After joining subsim and reading a lot about manual targeting, I've decided to perform a simple target interception in real time. After lot of plotting and some math I fired and scored 2 hits out of 3. Then ran for my life from 2 DDs. That took at least 2 hours of depth charge attacks and praying not to be hit. The self-pride I had from doing everything on my own plus the successful escape (plus my almost breakdown from the charges) were things that I hadn't experienced in a long time from a PC game. So I've kind of felt what those submariners had to endure for real.

So manual target is still a learning tool. I'm not 100% there but I'm on the way. Same thing for Cel nav. I was only doing Navigation with our zero error navigator and I felt that it was too perfect even for real life with GPS. After doing some research again I decided to try it out with Stellarium and I was offset just a few miles from my DR. So I am more immersed now in the sim than before. I am currently doing the cel nav calculations using an iPad app but in the future will try to do it by hand. The good thing is that Cel nav is optional. I am even complaining that in Dangerous Waters you can't align your SINS. The SINS is always 100% accurate.

[Pilot_76]

I don't use it in Silent Hunter 4, but Silent Hunter 5 has a very robust Real Navigation system. I'm still not that good with finding my location via sextant, so I rely on the navigator, but it's still a fun experience.

kylania, how's the cel nav mod working in SH5 today? I'm currently concentrating on SH4 only. Is there a MOD to implement a sextant sight? I know that a MOD has a navigator plotting the DR position (no more moving sub icon) but to take the sights yourself would be much better.

[Pilot_76]

I'm afraid all this stuff is hard coded in the .exe files. Modding those isn't modding, it's illegal hacking, so you have two choices as a modder.

Choice 1 is to turn off the sub position symbol on the nav map and attack map, find the sub position in the game files and feed that to an exterior planetarium file. That's what has been done by the navigation mod people for SH4. In doing so you give up a ton of realistic and useful things......like the ability to attack and sink opponents. This is not a workable scheme to my way of thinking other than as a proof of concept and a navigation trainer.

Choice 2 is not to do celestial navigation in SH4. That's been my path.

Or you could go to door #3 where the object of the game seems to be walking around your sub asking how the soup is for the ten thousandth time to build morale, while playing a bizarre Frankenstein poorly executed submarine meets horrible parody of the Sims monster. I'll pass on that one too. I could care less if Johan's girlfriend is pregnant with kittens.

I am with Stellarium 11.0. Modified the pyhton script to work with 11.0. I will post this as a new topic after some more testing.

[Pilot_76]

Does SH5/OHII feature U.S. fleet boats in the mods yet?

May I ask what is OHII?

[ETsd4]

Open Horizon II, a campaign for SH5.

[ETsd4]

how's the cel nav mod working in SH5 today?
Calculating 1 star for a 3-star position fix in SH5:
http://i714.photobucket.com/albums/ww143/snDf1/Test01.jpg
Mod: SH5 true navigation = SH5 real navigation without any AI

[Pilot_76]

Calculating 1 star for a 3-star position fix in SH5:
http://i714.photobucket.com/albums/ww143/snDf1/Test01.jpg
Mod: SH5 true navigation = SH5 real navigation without any AI

OMG!! Is this from TheDarkWraith's mod?? Can't get any more real than that...

[ETsd4]

OMG!! Is this from TheDarkWraith's mod?? Can't get any more real than that...

The basic is TDW's real navigation. For additional tools look here under "equipment" => http://www.subsim.com/radioroom/showpost.php?p=1712325&postcount=110 Also you need stellarium v11 and higher, which emulates the real refraction of stars and planets.

[Rockin Robbins]

Is the difficulty with using a sextant on a submarine simulated? Using a sextant on a moving boat is another world from using it on dry land. How about atmospheric refraction and scintillation? Scintillation alone can make a 1º position difference in a star, forget about measuring minutes and seconds!

Thankfully, over the open water, the atmosphere tends to be much steadier than over land but scintillation and refraction are variable effects having different importance for each individual observation.

But that ability to actually do the mechanics of reducing observation to position is an incredibly valuable experience.

Now, in SH5 are the navigational map and tactical plot combined as in SH4 or do they have completely separate navigation and tactical plots? Because in SH4, introducing manual positioning destroys your tactical ability. In reality they were completely separate functions.

Edit: Yup! The refraction correction is there! Well done. Is that built into the game or did you introduce it with your mod?

[Pilot_76]

The basic is TDW's real navigation. For additional tools look here under "equipment" => http://www.subsim.com/radioroom/showpost.php?p=1712325&postcount=110 Also you need stellarium v11 and higher, which emulates the real refraction of stars and planets.

Speaking of Stellarium a realistic way of simulating a sextant sight in there would be getting the apparent alt. instead of geometric when you click on any celestial body correct? If I'd like to have more errors as in real life would it be more realistic by using the angle tool and measuring myself?

[Rockin Robbins]

Speaking of Stellarium a realistic way of simulating a sextant sight in there would be getting the apparent alt. instead of geometric when you click on any celestial body correct? If I'd like to have more errors as in real life would it be more realistic by using the angle tool and measuring myself?
That's what ETsd4 means when he says "which emulates the real refraction of stars and planets." It is the lens-like refraction of the light from astronomical bodies that shifts their positions, varying mostly by altitude above the horizon, but also in a lesser way by temperature and atmospheric pressure. In practice, you only need to consider the effect of altitude because positional shifts caused by the other two are smaller than the variations in your ability to measure.

[Pilot_76]

BTW, regarding finding your position with the sun other than using the "LAN" or noon shot, I found this:

http://www.oceansail.co.uk/Downloads/SightReduction.html

[Pilot_76]

That's what ETsd4 means when he says "which emulates the real refraction of stars and planets." It is the lens-like refraction of the light from astronomical bodies that shifts their positions, varying mostly by altitude above the horizon, but also in a lesser way by temperature and atmospheric pressure. In practice, you only need to consider the effect of altitude because the other two are smaller than the variations in your ability to measure.

The thing is that when getting the geometric alt which in my understanding the the most accurate and using the apparent I get a huge difference from my assumed position (DR). In real life what is a good margin of error acceptable between actual position and Assumed position? I kind of find that using the geometric (once I was only 700 yards apart!!) is cheating.

[Rockin Robbins]

No matter when you shoot the sun its position is beastly difficult to determine due to its position. The thing is big, half a degree in diameter and it heats the air up something fierce, making it swim all over the place like a blob of jello.

All that uncertainty can be boiled down into an error deviation in your position that would be quite large. That's not to say that it isn't better than having absolutely no idea!:D Navigators sometimes grab anything available in their quest to avoid COMPLETE ignorance....

Even in a three star computation done on dry land you end up with a triangle drawn, and your position is "somewhere in this here triangle." And from repeated use, you get an expectation of how large the triangle should be. If it's way too large, you start looking for computational or observational errors. If you find none, you do the whole blasted thing over again. "Hey Cap, can we surface again?"

[Pilot_76]

From I've seen from a video is that you get the sun's declination, than compare it using your LHA or GHA then with GMT you can kind of guess which latitude you are near. Is this correct? I just don't want to be limited in noon shots during the day...

[Rockin Robbins]

Yes it is true that you can get some idea of your longitude by measuring the altitude of the sun at a given time. Since the sun moves 15º per hour through the sky (360º / 24 hours), by knowing GMT and knowing the sun's altitude you can determine how many degrees you are from the zero meridian, within limits.

The reason noon sights are useful is that by repeated sightings before and after noon you can interpolate the sun's position exactly at noon local time. It just gives a more accurate number than using a randomly chosen time of observation and being a prisoner of one shot.

[Pilot_76]

Yes it is true that you can get some idea of your longitude by measuring the altitude of the sun at a given time. Since the sun moves 15º per hour through the sky (360º / 24 hours), by knowing GMT and knowing the sun's altitude you can determine how many degrees you are from the zero meridian, within limits.

The reason noon sights are useful is that by repeated sightings before and after noon you can interpolate the sun's position exactly at noon local time. It just gives a more accurate number than using a randomly chosen time of observation and being a prisoner of one shot.

Could you give me a step by step as how you would perform a "sun shot" other than noon ? I am going nuts about this. I know noon is better but I gotta do something during the patrol in daylight other than noon. Keep busy. me real time. me no like fast forward time :salute:

[Rockin Robbins]

Let's do one for sunset. Atmospheric refraction is about 34' at the horizon, raising the position of the sun by almost exactly one diameter. In addition, when the limb of the sun touches the horizon, the position of the sun (measured at the midpoint of the disc) is 15' above the horizon. (Yeah I know the radius of the sun changes a bit with the time of year and isn't exactly 15' and........go away, my head hurts already!:timeout:)

So the moment the sun's lower limb touches the horizon, the true position of the sun must be corrected 15' higher (correcting for the diameter) plus another 34' lower (because refraction raises the apparent position). So your true altitude is 0º -34' + 15' or -11 minutes.

Now let's have fun! There are two kinds of minutes and two kinds of seconds, those of arc and those of time. And we're going to translate minutes of arc to seconds of time. Why? We know that at the exact instant of sunset our local time is 6:00 pm.

Because we are measuring something we can more easily detect, first contact of the sun with the horizon, instead of something quite difficult to observe, the time the center of the sun is coincident with the exact position of the true position of the horizon in the sky (uhhhhh......that's 34' above the horizon--no point of reference there) we have to determine our exact local time when we see that limb touch the water.

Basic math: the sun moves 15º through the sky in an hour, 360º divided by 24 hours. That means it moves one degree in 4 minutes of time. (60 minutes divided by 15 degrees gives you 4 minutes per degree).

Now it gets messy. That means 4 minutes time for the sun moving 60 minutes of arc. Let's change that to seconds of time: 240 seconds of time for 60 minutes of arc. We can further reduce that to 4 seconds of time per one minute of arc. Do you follow okay?

So we can apply our correction. The time we measured was not 6:00 pm, but the time when the sun was 11' of arc lower, later in time. We know that 1 minute of arc takes the sun 4 seconds to move, so 11 is 44 seconds later. Our local time was 18:00:44. Hope you recorded exact GMT at the instant of observation because you need it!

I would subtract GMT from local time to calculate the time difference. Express that in decimal hours instead of hours:minutes:seconds and multiply by 15º. That is your number of degrees east of the prime meridian. Convert to deg:min:sec. Translate for E and W longitude by subtracting 360 if the number is greater than 180º (it then becomes west longitude, expressed as a negative number) and there's your longitude!

I'm doing this off the top of my head with no reference materials so if I've made a mistake somebody whack me and get the right info out there!:salute:

[Pilot_76]

Let's do one for sunset. Atmospheric refraction is about 34' at the horizon, raising the position of the sun by almost exactly one diameter. In addition, when the limb of the sun touches the horizon, the position of the sun (measured at the midpoint of the disc) is 15' above the horizon. (Yeah I know the radius of the sun changes a bit with the time of year and isn't exactly 15' and........go away, my head hurts already!:timeout:)

So the moment the sun's lower limb touches the horizon, the true position of the sun must be corrected 15' higher (correcting for the diameter) plus another 34' lower (because refraction raises the apparent position). So your true altitude is 0º -34' + 15' or -11 minutes.

Now let's have fun! There are two kinds of minutes and two kinds of seconds, those of arc and those of time. And we're going to translate minutes of arc to seconds of time. Why? We know that at the exact instant of sunset our local time is 6:00 pm.

Because we are measuring something we can more easily detect, first contact of the sun with the horizon, instead of something quite difficult to observe, the time the center of the sun is coincident with the exact position of the true position of the horizon in the sky (uhhhhh......that's 34' above the horizon--no point of reference there) we have to determine our exact local time when we see that limb touch the water.

Basic math: the sun moves 15º through the sky in an hour, 360º divided by 24 hours. That means it moves one degree in 4 minutes of time. (60 minutes divided by 15 degrees gives you 4 minutes per degree).

Now it gets messy. That means 4 minutes time for the sun moving 60 minutes of arc. Let's change that to seconds of time: 240 seconds of time for 60 minutes of arc. We can further reduce that to 4 seconds of time per one minute of arc. Do you follow okay?

So we can apply our correction. The time we measured was not 6:00 pm, but the time when the sun was 11' of arc lower, later in time. We know that 1 minute of arc takes the sun 4 seconds to move, so 11 is 44 seconds later. Our local time was 18:00:44. Hope you recorded exact GMT at the instant of observation because you need it!

I would subtract GMT from local time to calculate the time difference. Express that in decimal hours instead of hours:minutes:seconds and multiply by 15º. That is your number of degrees east of the prime meridian. Convert to deg:min:sec. Translate for E and W longitude by subtracting 360 if the number is greater than 180º (it then becomes west longitude, expressed as a negative number) and there's your longitude!

I'm doing this off the top of my head with no reference materials so if I've made a mistake somebody whack me and get the right info out there!:salute:

Wow Robbins, that was too much of a crash course on celnav. Too fast too much info for me :dead::dead::dead::dead: :D

Anyway thanks for the info I'll save it and will try to do it live when I get my sextant in a couple of weeks (Davis Mark 15 or 25? doubts doubts...)

OK. Going back coming out from Warp speed.

From your last post I recognized that you were doing the Longitude by noon sight/GMT but instead of noon it was during sunset. I'm still trying to sort out LHA/GHA and d' (I started to read celnav only 10 days ago). Well, my question is that in what sort of way does making this "sun shot" during sunset and sunrise helps? i.e. Having the Sun at zero degrees from the sextant...

Other one: Can I still find my latitude with the so called "reduction sight tables"? Match the Sun's declination with my GMT at the nearest Latitude on the table? This being done with a noon/sunrise/sunset shot.

Last one (I promise :DL)

The SH4 nav pack file comes with info such as GHA and Sun's declination from 1939-45 plus meridian passage and sunrise/sunset. Is that all it takes or I need more info from the almanacs?

Sorry to ask these I do not intend to have you present a full lecture on celnav...

I am doing some experiments with Stellarium 11.0 and SH4 and will post a separate topic regarding this once I am done.

I was just curious with the instructions located here: http://www.oceansail.co.uk/Downloads/SightReduction.html

at the bottom of the page.

[Pilot_76]

The basic is TDW's real navigation. For additional tools look here under "equipment" => http://www.subsim.com/radioroom/showpost.php?p=1712325&postcount=110 Also you need stellarium v11 and higher, which emulates the real refraction of stars and planets.

ETsd4, I think I understand now. The apparent altitude takes account the refraction and other "real" factors from the real declination. Using the geometric alt. in Stellarium would be cheating or having a super sextant with a bionic eye attached taking accurate readings...So using the apparent altitude is much more realistic.


In one of the forums at stellarium's page they state that the altitude that you set is to simulate the refraction and atmosphere. Not the actual altitude of the observer. Since in SH you are always at sea setting zero or 2000m won't make a difference in the sextant reading.

[Sailor Steve]

In real life what is a good margin of error acceptable between actual position and Assumed position?
Sextants measure the angle between the sea horizon and a celestial body. These angles are measured in degrees and minutes of arc (1/60th of a degee). Measuring this angle to an accuracy of 1 minute of arc (1') will result in a positional accuracy of 1 nautical mile. Accurate sextants can measure this angle to an accuracy of 0.2'. This means that theoretically one can determine their position to 1/5 of a mile.
http://www.cse.yorku.ca/~amana/personal/navigation/

That's roughly 1200 feet, or 400 yards, or 365 metres. Very accurate for the open sea, not so good in a harbor, which is why harbor navigation is done by eyeball. :sunny:

[Rockin Robbins]

In practice, at sea, in a small craft or a submarine, you're doing incredibly well to get within 20 miles of your true position. Larger errors than that are very common, leading to methods of navigation that take into account a presumed statistical error.

For example, you know you're likely within (don't you love that word likely? If you're a worrywart, it sure doesn't help the sleep process!) 30 miles, that's plus or minus 30 miles! That means there is a 60 mile range north/south and east/west that you could be in. You're approaching a coast and want to make sure you go pretty directly to a port.

Do you navigate to the position of the harbor? NO!!!!! That would be stupid.:arrgh!:

Why is it stupid. Well, your position is not a point but a probability distribution 60 miles long. When you achieve landfall there are as many possible positions north of the harbor as there are south. You ARE Shoedinger's Cat, and you've just been let out of the box. Quick, which direction do you turn to get to the harbor? Choose the choice of your choice, but either way there is a 50% chance that you have actually turned away from your destination! How cool is that?:rotfl2:

Now let's do it taking into account the error range of +-30 miles. If instead of aiming at the port, we aim for a point 55 miles to the north, we know two things. If we end up on the south end of the error probability range, we'll be in sight of the port and can easily turn whichever direction we need to. When we get there we'll brag about how accurate our navigation is.

If we come to landfall and can't see the port, we KNOW we're north of it. We turn south decisively, like we know what we're doing ('cause we do!) and sail directly into port, where we brag about how accurate our navigation is. In practice it is normal to aim 100 miles from your destination to ensure that you turn in the correct direction. If you wanted to end up at Daytona Beach you would aim for Jacksonville! Of course I'm ignoring that when you get within 50 miles of Daytona you could pick up a radio station and vector in on that. I'm assuming you are using only celestial navigation.

The gradations on the sextant are irrelevant in determining how accurately you can navigate. On a boat getting to within half a degree is darned good shooting. Like I said, atmospheric scintillation can make Sirius dance around like it hasn't been to the toilet for 24 hours. I've seen it move more than half a degree and there is no way you can fix its exact position. The accuracy of the instrument far exceeds the accuracy of the man.

These videos are not extreme examples. Note that Sirius also flashes all the colors of the rainbow while it does its "I gotta go" dance.
https://www.youtube.com/watch?v=jwn6KIAZH5c&feature=mfu_in_order&list=UL
https://www.youtube.com/watch?NR=1&v=4f35gIWdCz0
https://www.youtube.com/watch?v=umvgCr32ssk

[Pilot_76]

In practice, at sea, in a small craft or a submarine, you're doing incredibly well to get within 20 miles of your true position. Larger errors than that are very common, leading to methods of navigation that take into account a presumed statistical error.

For example, you know you're likely within (don't you love that word likely? If you're a worrywart, it sure doesn't help the sleep process!) 30 miles, that's plus or minus 30 miles! That means there is a 60 mile range north/south and east/west that you could be in. You're approaching a coast and want to make sure you go pretty directly to a port.

Do you navigate to the position of the harbor? NO!!!!! That would be stupid.:arrgh!:

Why is it stupid. Well, your position is not a point but a probability distribution 60 miles long. When you achieve landfall there are as many possible positions north of the harbor as there are south. You ARE Shoedinger's Cat, and you've just been let out of the box. Quick, which direction do you turn to get to the harbor? Choose the choice of your choice, but either way there is a 50% chance that you have actually turned away from your destination! How cool is that?:rotfl2:

Now let's do it taking into account the error range of +-30 miles. If instead of aiming at the port, we aim for a point 55 miles to the north, we know two things. If we end up on the south end of the error probability range, we'll be in sight of the port and can easily turn whichever direction we need to. When we get there we'll brag about how accurate our navigation is.

If we come to landfall and can't see the port, we KNOW we're north of it. We turn south decisively, like we know what we're doing ('cause we do!) and sail directly into port, where we brag about how accurate our navigation is. In practice it is normal to aim 100 miles from your destination to ensure that you turn in the correct direction. If you wanted to end up at Daytona Beach you would aim for Jacksonville! Of course I'm ignoring that when you get within 50 miles of Daytona you could pick up a radio station and vector in on that. I'm assuming you are using only celestial navigation.

The gradations on the sextant are irrelevant in determining how accurately you can navigate. On a boat getting to within half a degree is darned good shooting. Like I said, atmospheric scintillation can make Sirius dance around like it hasn't been to the toilet for 24 hours. I've seen it move more than half a degree and there is no way you can fix its exact position. The accuracy of the instrument far exceeds the accuracy of the man.

These videos are not extreme examples. Note that Sirius also flashes all the colors of the rainbow while it does its "I gotta go" dance.
https://www.youtube.com/watch?v=jwn6KIAZH5c&feature=mfu_in_order&list=UL
https://www.youtube.com/watch?NR=1&v=4f35gIWdCz0
https://www.youtube.com/watch?v=umvgCr32ssk

I understand that Stellarium does simulate this scintillation?

[Rockin Robbins]

No program simulates scintillation that I am aware of. It would be a non-useful function. If there were still such a thing as a telescope simulator out there, then scintillation would be necessary.

[Pilot_76]

Just another doubt I have for you guys using Stellarium with SH4/SH3.

When taking sights during sunrise/set exactly when do you mark the time? When the first glimpse of the upper limb of the sun shows up or when its lower limb is not touching the horizon anymore? How accurate is the sun movement and position located against Stellarium's?

[Rockin Robbins]

Astronomically, sunset is defined as the moment of last contact between the sun's disk and the horizon. That actually puts my sunset time above 60 seconds too early, because I was solving for the midpoint of the disk as sunset. Hey that's what I get for not consulting any reference materials.......

Sunrise is the first contact of solar disk and the horizon.

Stellarium is accurate to the second on astronomical positions and timings. I prefer Cartes du Ciel for astronomical purposes, but Stellarium is just as good for navigation.

In practice, I time from first contact on sunset and last contact at sunrise. Why? Because those are easiest to time accurately. Then I apply corrections for the diameter of the sun and refraction to get my real time of sunset or sunrise.

Food for thought: How many times have you been out on a dark night (there ARE none in any of our urban areas), looked up and wondered at the countless number you see? It's a cliche from the time before telescopes, "as countless as the stars in the sky." Well it might surprise you that all the stars you can see with the naked eye are identified, their positions, brightness, variability, color precisely tabulated. And they are actually counted.

From a city, only the very brightest can punch their light through the excess lighting and the thickly obscuring atmospheric pollution. I'm not even going to hazard a guess there, but it could be less than a couple hundred. But lets get out of the city into a suburban area with no Wal-Mart within 10 miles. Here there are about 2,000 stars that are visible to the naked eye. You might find that number surprisingly low. Well the reality is half that because half are completely obscured by the Earth below your feet!

But what about the pristeen darkness many miles out to sea or in the middle of the Australian outback? How many stars can you see then? The total number of visible stars from such a location is only about 6,000, and you can only see half of them at any one time because the other half are below the horizon.

So countless? Not hardly! With nothing but your eyes you can only see a tiny number of stars, fewer by far than the population of any of our well-known cities. No matter where on Earth you are, if you could count every one you see, you wouldn't get to 3,500. Isn't that amazing?

[Daniel Prates]

So countless? Not hardly! With nothing but your eyes you can only see a tiny number of stars, fewer by far than the population of any of our well-known cities. No matter where on Earth you are, if you could count every one you see, you wouldn't get to 3,500. Isn't that amazing?

The lesser artificial light to offuscate your vision, the more you can count (not to mention other factors, such as weather, pollution, air temperature etc. The atmosphere acts as a giant lens). What you are saying in terms of numbers is the absolute maximum amout of stars, in perfect sighting conditions?

[ETsd4]

At 05:13 7. sept '39 the uboat started from harbor Swinemuende with course 017° T and speed 05 kts.
At 06:42, at twilight, the commander asked for a position-fix because the executed heading and speed was incorrect.
The navigator's result:
http://i714.photobucket.com/albums/ww143/snDf1/Log_003_A_Testa.jpg
3-star position fix
(from SH5)

[Sailor Steve]

What you are saying in terms of numbers is the absolute maximum amout of stars, in perfect sighting conditions?
That's what he's saying. And I'm amazed, because I never knew that.

[TorpX]

At 05:13 7. sept '39 the uboat started from harbor Swinemuende with course 017° T and speed 05 kts.
At 06:42, at twilight, the commander asked for a position-fix because the executed heading and speed was incorrect.
The navigator's result:

3-star position fix
(from SH5)

I'm curious. I know how the three LOP with the stars are obtained, but how were the three light grey lines obtained?

[ETsd4]

The aim was to find the incenter of the triangle. It is the point of intersection of the triangle's three angle bisectors.

[Rockin Robbins]

The lesser artificial light to offuscate your vision, the more you can count (not to mention other factors, such as weather, pollution, air temperature etc. The atmosphere acts as a giant lens). What you are saying in terms of numbers is the absolute maximum amout of stars, in perfect sighting conditions?
That is correct. Maximum number possible to see in the absolute best of conditions on the darkest spot on earth.

[Rockin Robbins]

The aim was to find the incenter of the triangle. It is the point of intersection of the triangle's three angle bisectors.
However the assumption that you would be at the center of that triangle is almost certainly wrong. What you hope is that you are somewhere in that triangle. The smaller the triangle is, the more consistent your observations are. But you could be anywhere within that triangle, IF you haven't made any mistakes, in which case you could be outside the triangle.

Celestial navigation isn't for sissies..... And it isn't for perfectionists because they go stark raving mad.

[ETsd4]

However the assumption that you would be at the center of that triangle is almost certainly wrong. What you hope is that you are somewhere in that triangle.

yeah, tell me more. Report your "findings" to the public and write a new chapter in celestial navigation calculation. The incenter-point of the triangle should be your FIRST assumption where your actual position could be.

[Rockin Robbins]

Assuming good observations, not a really great assumption at sea, and accurate calculations, a good assumption if your calculators are all working and you don't have to do the figuring by hand, your position is a probability bell curve with the high point in the center of the triangle. If you want to assume perfection and take that center point that's fine by me. However, in the real world, no such assumption is made except by fools.

Real navigators are aware at all times what error is intrinsic in the process and generally laugh at precise points of position. We, who live in a GPS world, expect accuracy measured in terms of six feet, two meters or so. Celestial navigation deals with accuracies measured in error ranges of dozens of miles.

The three lines on your celestial nav charts are position lines. Each line says "I'm somewhere on this line." They should join at a single point. However, due to the intrinsic inaccuracies of the measuring process, they never do. You end up with a triangle, each line saying "you're somewhere on this line." Surely you're not going to argue that they are all accurate, for then you would have to be three places at once!

The correct answer to "where am I" is "probably somewhere in this triangle." The midpoint is just as bad a position as somewhere on one of the lines marking the border of the triangle. Any real celestial navigator knows this. This is not a new chapter, but one two hundred years old.

[TorpX]

The aim was to find the incenter of the triangle. It is the point of intersection of the triangle's three angle bisectors.

OK. I couldn't tell from the picture there.

I believe there are methods for refining the initial LOP's, but when I was using Cel Nav, I didn't get that far into it. RR probably knows all of this stuff.



Celestial navigation isn't for sissies..... And it isn't for perfectionists because they go stark raving mad.

You can say that again.

After doing this for a week or so, I felt like I'd been subjected to a 12 hour depth charging. :dead:

[Rockin Robbins]

OK. I couldn't tell from the picture there.

I believe there are methods for refining the initial LOP's, but when I was using Cel Nav, I didn't get that far into it. RR probably knows all of this stuff.




You can say that again.

After doing this for a week or so, I felt like I'd been subjected to a 12 hour depth charging. :dead:


Doing it for real on a real small craft multiplies the uncertainties tremendously!

[Pilot_76]

I am going to start a new thread about using Stellarium 11 with SH4. I hope you guys can help me out as correcting or optimizing my procedure...

:)

[TorpX]

I am going to start a new thread about using Stellarium 11 with SH4. I hope you guys can help me out as correcting or optimizing my procedure...

:)

I will certainly be interested in how it goes for you. For me, it was just too much extra work. I also didn't like not having a sub marker on the map (needed one for tactical plots).

[Rockin Robbins]

Agreed. You just give up too much of the game to do celestial navigation. Just about everything goes out the window for targeting if you don't know your position relative to the target. If the game had a tactical plot different from the navigation plot it would be a different story and I'd be on board.

[Pilot_76]

Just posted:

http://www.subsim.com/radioroom/showthread.php?t=190017