Plotting the Intercept

[cagney]

Originally Posted by sneuman

Quick question about doing the actual plot:

I'm confused about how to plot "away" vs. "toward". Is this a function of declination, so if the sun's declination is in the southern hemisphere, "away" is plotted away from south along the Z azimuth?

Let me try to explain it.

"Away" and "toward" always refers to the direction of the body you took the height of. The geographical position is the position on earth where the body is in zenith. The latitude and longitude of this point is tabulated in the almanac. The declination is the latitude of the geographical position and Greenwich Hour Angle is, basically, the longitude of the geographical position.(without going into a few technicalities). So the direction of the body is the same thing as the direction of the geographical position.

I hope this helped!

[sneuman]

Originally Posted by cagney

Let me try to explain it.

"Away" and "toward" always refers to the direction of the body you took the height of. The geographical position is the position on earth where the body is in zenith. The latitude and longitude of this point is tabulated in the almanac. The declination is the latitude of the geographical position and Greenwich Hour Angle is, basically, the longitude of the geographical position.(without going into a few technicalities). So the direction of the body is the same thing as the direction of the geographical position.

I hope this helped!

yes, it did. Declination (which N-S hemisphere the body is in) is key, which is what I thought.

[s/v Jedi]

GP: imagine a string stretched from the center of the earth, through its surface, and into the center of the celestial body. The point at which in passes through the earth's surface is its geographical position or GP.

cheers,
Nick.

[lolarose]

Originally Posted by s/v Jedi

GP: imagine a string stretched from the center of the earth, through its surface, and into the center of the celestial body. The point at which in passes through the earth's surface is its geographical position or GP.

cheers,
Nick.

thanks nick,

and how you find (determine) GP. using sextant, or some other way

[s/v Jedi]

So, GP comes from the sextant reading or from the almanac? Well, I'm actually sure I got it right nayy officer or not

cheers,
Nick.

[sneuman]

GEOGRAPHICAL POSITION OF A BODY. The geographical position of a celestial body is a point on earth's surface which is exactly under the given heavenly body at any one instant. An observer at the geographical position would find the corresponding body exactly at his zenith.

ALTITUDE. Altitude is the least angular elevation of a body above the horizon at any instant.

They are not at all the same thing. GP refers to the position of the body; altitude of that body relative to the observer (as taken uncorrected from the sextant) is the variable Hc.

[HappySeagull]

Originally Posted by sneuman

Quick question about doing the actual plot:

I'm confused about how to plot "away" vs. "toward". Is this a function of declination, so if the sun's declination is in the southern hemisphere, "away" is plotted away from south along the Z azimuth?

...so is this answered yet?

What most people use is a method that compares a calculated angle for an Assumed position against their real observed sights. The Assumed Position angle is really just a point on a very large circle on the earth (with it's center GP)and everywhere on that circle would observe the same angle to that body.The Az helps a lot because it is your bearing from GP, so this marks the place on that very large circle.It's your Assumed position,right?.... . Now, it follows that If your real life observed angle is steeper, your are toward the GP on line Az-if it is shallower, you are away from GP on line Az and of course,also away from AP on that same line... you'll note that you don't just make a dot and say aha! You draw a line across the az...this represents the idea that you are really on a large circle ...that your az won't do because it's a calculated one. A magnetic compass taking a bearing as you took sights wouldn't work well either.It's darned difficult to do in a boat...But that said, one fix and a guess of an az is better than nothing!

So you take another sight on another body(it could even have a different AP and all)...and get another "towards or away" line to cross the az...and yet another gives you a little hat....If you're lucky, you can get all your sights at once. If not, you will have to DR these lines to the time you can get another.



"Declination" as you describe it is a confusion created by the tilted earth and in the moon's case, additionally a wild orbit! The sun and moon wander above and below the equator,but this has no bearing on the assumed and observed sextant angles.Without any lines of long and lat, the angles would still be the same. You are REALLY merely comparing an angle that you do see with one you would see if you are where you think you are.

[sy_gilana]

Agree with Nick. You have to visualize it. I used the example of an obelisk, or any tower, when explaining this. Imagine you calculate the angle of how high the tip of the tower should be, now you shoot it, and its a bigger angle, you have to be standing closer than you thought, so its away. The acronyms help remember, but if you can visualize, you wont even need them its obvious...

I just tried to see if the best online resource is still there, but it seems to be "host not found" it was www.celestial-navigation-course.com

Keep checking it.

PS also used string and globe to explain COP and GP.

[HappySeagull]

just to add...all heavenly bodies have "Declination"Declination is not the same as a sextant angle HcHo whatever.It's about describing their GP versus your AP on this planet with it's arbitrarily chosen system involving an "Equator" and the like..Lat and Long.North south is just a part of the calculation.... nothing to do with direction of intercept

[sneuman]

thanks Seagull and other civil interlocutors. I was thinking about it on the bus and had an epiphany that matches what Seagull says. I realized the azimuth (Zn) is a bearing toward the GP, which would be the GHA and declination expressed in latitude and longitude, though there's no need to bother with actually plotting the position as the azimuth takes care of that for you. I think I had been doing it correctly as an intuitive matter, but I was just fuzzy on the reasoning.

Thanks (almost) all.

[sneuman]

Originally Posted by sneuman

thanks Seagull and other civil interlocutors. I was thinking about it on the bus and had an epiphany that matches what Seagull says. I realized the azimuth (Zn) is a bearing toward the GP, which would be the GHA and declination expressed in latitude and longitude, though there's no need to bother with actually plotting the position as the azimuth takes care of that for you. I think I had been doing it correctly as an intuitive matter, but I was just fuzzy on the reasoning.

Thanks (almost) all.

no need to bother plotting the gp, i should say

[SaucySailoress]

Gentlemen,

I have deleted just a few posts that were off-topic, so as not to detract from the message of this thread. Thank you to those of you have stayed on track, and have posted valuable info here for us, it's appreciated!

Whilst I'm here, this is what the rules say about quoting from other sources:

Under Fair Use provisions you can legally post a small abstract of an article - or perhaps the opening paragraph. In either case please post a link to source.

[sneuman]

Thanks, Moderator. I do need to correct myself from earlier. The variable I was referring to in sight reduction should have been Ho sted Hc. I was working some problems last night and discovered my misstatement.