Celestial Navigation with RFB / TMO mods?

[DaveR]

thx Nathaniel.

home port is midway.

thought of another way. once on station, measure sunrise/set time and calculate offset from that. will try that tonight.

[DaveR]

tried several times; all with similar results. currently, being at truk; from midway, I came up with a 2:08 hour difference. date is 20Nov43. sun up is listed 6:27 and measured time was 7:08 or 0:41 difference. sun down is listed at 17:00 and measured time was 18:54 or 1:54 difference. so far, really confusing; , why the differences?

so then checked when the moon rose and it peeked out at 02:30. the listing for moon rise/set is:
Nov19 23:35 12:23
Nov20 ' - ' 12:59
Nov21 00:28 13:33

For Nov20, if the ' - ' means zero, at least closer to the 2:08.

So what does the ' - ' mean? that measured moon rise was at 02:30 on 'Nov21'; but the chart list 00:28 for the 21st.

so far, all the moon phases' have been correct and all though I measured a lot of weird times: still find this all useful for planning.

[Sean C]

The ' - ' means that there is no moonrise on that date.

[Rockin Robbins]

I fear that you can calculate till the cows come home and only (possibly) come up with sunrise times for reality. But the game will stubbornly refuse to cooperate because it is not a solar system simulator, ESPECIALLY where the moon is concerned., I have an SH4 screenshot with a crescent moon less than 10 degrees from a setting (or rising) sun. The phase of the moon is directly related to its rise and set times. Since the screenshot is impossible, the moonrise tme must not be just a little bit, but radically wrong.

And all those calculations just go to waste. SH4 did a TERRIBLE job with solar system astronomy. And just a bad job with deep space astronomy.

[Sean C]

Originally Posted by Rockin Robbins

The phase of the moon is directly related to its rise and set times.

No, it isn't. The phase of the Moon is a function of its apparent angular separation from the Sun. The percent of illumination can be calculated as the haversine¹ of the separation angle in degrees.

The apparent angular separation can be calculated using the following formula:

θ = acos(sin(Dec1)⋅sin(Dec2)+cos(Dec1)⋅cos(Dec2)⋅cos(G HA2-GHA1)) (Not sure why there's a space inserted into "GHA2". I can't remove it.)

If the Moon was only 10 degrees from the Sun, it should be a crescent. A very thin sliver at 0.7% illumination, but a crescent nonetheless.

¹Haversine = (1-cos)/2

[Rockin Robbins]

Originally Posted by Nathaniel B.

No, it isn't. The phase of the Moon is a function of its apparent angular separation from the Sun. The percent of illumination can be calculated as the haversine¹ of the separation angle in degrees.

You're just repeating my contention that moon phase is dependent on time. For instance, for a last quarter, the moon has to rise 90º before the Sun, and so MUST rise at midnight local time. The last quarter moon can't rise at any other time. And it always sets at noon. (I'm driving myself crazy here trying to visualize this stuff.)

At sunset, the first quarter moon is 90º from the sun, so is at culmination. The first quarter moon, then, MUST set at midnight local time and rise at noon local time. It cannot rise at any other time. And of course, your math verifies that but is meaningless for most humans. Even you, comfortable with the math, haven't made the connection to reality that is vital to understanding whether it is true or false and vital to actually understanding what the numbers mean. Numbers have no meaning in and of themselves. They must be connected to reality.

Just as a check, what time MUST the full moon rise? There is only one time it can rise.

Originally Posted by Nathaniel B.

The apparent angular separation can be calculated using the following formula:

θ = acos(sin(Dec1)⋅sin(Dec2)+cos(Dec1)⋅cos(Dec2)⋅cos(G HA2-GHA1)) (Not sure why there's a space inserted into "GHA2". I can't remove it.)

If the Moon was only 10 degrees from the Sun, it should be a crescent. A very thin sliver at 0.7% illumination, but a crescent nonetheless.

¹Haversine = (1-cos)/2

The problem with trotting out formulas is that they are not intuitively validating. You can use any form of equation you wish and it looks equally valid. Not only that but results of a formula are not intuitively verifiable either. Your calculation could show that the sun rose at midnight, but being a cloud of meaningless numbers, an error picked up in less than a second graphically only yields to calculation by calculation checking of the math. Often the conclusions of pure mathematics are in stark contrast to the phenomenon they describe, and our mental picture of what they describe is entirely bogus.

I apologize, my memory was wrong. The moon, in the SH 4 screenshot I was describing, was rendered a completely wrong size with the crescent moon actually in contact with the disk of the sun. Phase angle would have been less than .001% illumination at those positions.


So here you go, courtesy of Kim Rohnoff from Dutch Harbor and I have the date somewhere but it doesn't matter. This is just a mangling of solar system astronomy so bad it really doesn't matter when and where the shot was taken..... In reality, the sun and moon are essentially the same size in the sky, 1/2 degree, and the moon in that position would not only be invisible, but we would have a partial solar eclipse, which, needless to say did not happen on that day. I believe in this screenie the moon is rendered more accurately to scale than the sun, which is pretty close to double its real size.

SH4 is not an astronomy simulator and any attempt to use it as such is doomed to failure.

[Sean C]

Originally Posted by Rockin Robbins

(I'm driving myself crazy here trying to visualize this stuff.) [...]

And of course, your math verifies that but is meaningless for most humans. Even you, comfortable with the math, haven't made the connection to reality that is vital to understanding whether it is true or false and vital to actually understanding what the numbers mean. Numbers have no meaning in and of themselves. They must be connected to reality.

First of all, I'd like to say that I'm no mathematician. Math was actually my worst subject in school and I absolutely hated it. However, if you're like me and find you have a passion for celestial navigation and devote years to studying it, you will learn to do the spherical trig because that's the heart of celnav.* Whether or not the equations mean anything to you personally is actually irrelevant. The equations I quote are literally the science behind the planning and practice of celnav.

Having said that...I totally understand what you're saying about visualization, because that's also how I understand and integrate new knowledge. So, with that in mind, I have prepared a few pictures to help illustrate why the Moon's phase is only tangentially related to rise or set times and even then only under certain circumstances. (Apologies for only being able to link to the pictures. Apparently, I haven't yet earned the privilege of attaching my own pics in-line in a post.)

Let's take a look at the case you cited as a "check":

Originally Posted by Rockin Robbins

Just as a check, what time MUST the full moon rise? There is only one time it can rise.

Many times, it is useful to look at an extreme example of a situation to understand the mechanics behind it. So, let's assume we are almost at the North Pole...our latitude is 89°59'59"N. Only one arc second away from the pole itself. Our longitude is 0°00'00"E. We are on the Greenwich meridian and therefore our local time is exactly equal to UTC. The date is September 6th, 2017 and the time is 07:30 UTC. The Moon is full (as full as it will get at this time at 99.9% illumination). Now, what is the Moon's altitude? If you click on Fig. 1, you will see that it is at 8°54'21" below the horizon (represented by the green circle).

Next, we wait six hours until 13:03 UTC. Now what is the Moon's altitude? Fig. 2 shows that it is still 7°53'18" below the horizon. Another six hours later at 19:03 UTC and Fig. 3 shows that the Moon is still 6°50'38" below the horizon. So what time will it rise? Well, the bottom left corner of Fig. 3 gives away the fact that the Moon will not rise at all on this date, at this location. In fact it won't rise until two days later, on September 8th, at which point it won't set again until September 20th...another twelve days away.

Suppose our latitude was 45°00'00"N (still on the Greenwich meridian, back on September 6th). Now what time will the moon rise? Fig. 4 shows that it already rose at 18:22 UTC the previous day. And if we were almost at the South Pole...what then? Fig. 5 shows that the Moon never rose on this day. At this latitude and time it is "circumpolar", meaning it stays above the horizon all day.

So, "what time MUST the full moon rise?" The answer is: it depends. But, on what? ... If you answered "the observer's latitude", congratulations! You are correct...partially. Unlike the Sun, the Moon is revolving around the Earth as the Earth itself rotates. The speed of the Moon's revolution is great enough that, the farther away from Greenwich you are, the more the time of Moon rise/set changes from what is listed in the Nautical Almanac.

That is why there is a "Table for Interpolating Sunrise, Moonrise, Etc." included in the N.A. This table allows for the adjusting of sunrise/set times for latitude and Moon rise/set times for both latitude and longitude.Because these are two of the primary factors which govern what time the phenomena occur for a given observer.

As I said: I have spent years intensely studying celestial navigation. I'm not just speaking from some cursory understanding of the subject. I own several sextants and have taken hundreds of sights, including "lunars". I have even generated my own "Time Sight Logarithm Tables" which can be used to navigate the way sailors did back in the 19th century. You may have noticed that my forum nickname is actually the name of the author [or editor, more precisely] of one of the most famous American books on navigation at sea: Nathaniel Bowditch...author of The American Practical Navigator, otherwise known simply as Bowditch.

Anyway, I hope all of that was clear enough. And I hope to be able to help DaveR with his problem...although from my initial calculations, it seems that SH might actually have some inaccuracies. I'll need to investigate further and get the answers to my questions to really know. We'll see.

BTW, for some reason I can't see the picture you posted. All I get is a rectangle containing a circle with a horizontal line through it. (?)

*Actually, learning and/or understanding the math is not required to practice celnav. One can simply use pre-calculated tables and fill in pre-printed forms and do just fine. To be perfectly honest, I still don't completely understand it, myself.

[Sean C]

DaveR,

I'd be happy to help you figure this out, but first I need to know a few details:

1. What, exactly, are you trying to accomplish? Are you trying to figure out the time of Sun/Moon rise/set at Midway or at your ship's actual position or are you trying to find the position of your ship?
2. Are you able to measure the altitude of the Sun/Moon in game? If so, is the measurement at the lower limb, center or upper limb? What precision can be achieved? (Ideally, you want to be able to measure the altitude to within one tenth of an arc-minute. E.g. 37°12.8'.)
3. Where are you getting your almanac data? The times you listed for moon rise/set match the Nautical Almanac times for those dates at 30°N. But, the sunrise/set times are a little off.
4. Is your ship's clock set to Midway time, as I assume it is from reading aanker's post?
5. Is your ship actually at Midway or some other location? (Still 140°30'W?) What is your DR latitude? Are you underway or stationary?

[aanker]

Originally Posted by DaveR

tried several times; all with similar results. currently, being at truk; from midway, I came up with a 2:08 hour difference. date is 20Nov43. sun up is listed 6:27 and measured time was 7:08 or 0:41 difference. sun down is listed at 17:00 and measured time was 18:54 or 1:54 difference. so far, really confusing; , why the differences?

so then checked when the moon rose and it peeked out at 02:30. the listing for moon rise/set is:
Nov19 23:35 12:23
Nov20 ' - ' 12:59
Nov21 00:28 13:33

For Nov20, if the ' - ' means zero, at least closer to the 2:08.

So what does the ' - ' mean? that measured moon rise was at 02:30 on 'Nov21'; but the chart list 00:28 for the 21st.

so far, all the moon phases' have been correct and all though I measured a lot of weird times: still find this all useful for planning.

Nuts! - lol ...... Yeah, a lot of weird times. I used these charts for rough information, a "close enough for Government work" type thing.

It is frustrating not to have precise times though.... plus your in-game times that you posted seem to be closer to a Pearl home port which is interesting. I forget if the Stock game has Midway as a 'Home Base' - maybe that's it?

The time differences do seem to be closer to a Pearl home port even though they still are not precise times, they are close (for a game ; ) Maybe these charts are only 'good' if using/calculating from Pearl as 'Base Time' for PacFlt? Sail out of Midway but calculate from Pearl where the Admirals are - ha!

I wonder how close SoWesPac is, (one way to find out) although those missions are roughly in the same time zone, or close to it.

The phases are 'good' but the times aren't exact, however they're still close enough to plan an attack or Special Mission.

That's all I got.... it's a game - and the Dev's did get it close. I'm going with the use Pearl to calculate from theory.

Happy Hunting!