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Two Moonset times in the ...
Forum: General Topics Here
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Old no-plotting method ...
Forum: The Sight Reduction process
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Hello
Forum: The Sight Reduction process
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Electronic Charts: New Bo...
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assumed longitude
Forum: General Topics Here
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What Is Your Favorite Wat...
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Camp Runamuck- TV show
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Help With an Exam Questio...
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USCG Crazy Exam question:...
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How to get LHA and Ap lon...
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Electronic Charts: New Book |
Posted by: PeterB - 10-15-2023, 01:37 PM - Forum: Equipment- Wanted or for Sale
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![](https://thenauticalalmanac.com/Forum/images/default_avatar.png) |
David Burch of Starpath School of Navigation has just released a new book "ENC Essentials" dealing with the transition from paper charts to electronic navigation charts.
It is available from Amazon as a Kindle book or paperback.
I have no association with Amazon, Starpath, or David Burch other than having been an occasional customer of each.
How do you like the ENC's vs. paper charts?
My experience with ENC's is minimal but so far I much prefer the paper version. Much easier to see the "big picture" and then focus in on the details you need. With ENC's some of the elements appear and disappear with the zoom level. So zoomed out you might see the entrance to a destination harbor in relation to your departure point but not some of the rocks or buoys that are there until you zoom in on them.
Also the zoom level doesn't always change the size of the rendered elements meaning that zooming in doesn't change the size of printed information. It seems as if you need a huge electronic screen to render some of this stuff in sizes that are readable at a glance..
I sometimes use raster e-charts which are scans of paper charts on an iPad mini or even my iPhone and with those you can increase print size by zooming in, or see the big picture by zooming out.
I guess it is age related ;)
Peter
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assumed longitude |
Posted by: Rumata - 10-06-2023, 05:38 PM - Forum: General Topics Here
- Replies (2)
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![](https://thenauticalalmanac.com/Forum/images/default_avatar.png) |
Gentlemen,
Quite recently I've identified little ( maybe ;>) problem, I ignored for a long time. Let me make it short and concise:
Working on LOP for a star. To find LHA to use in Pub.229, vol. 3 input, I need to add GHA Aries to SHA of the star and subtract DR longitude. Well, as you know, to get the whole number for LHA I need to use the assumed longitude.
And this is where I found a little problem.:
Depending on the order of adding and subtracting, the magnitude of assumed longitude is changing. In minutes, of course. But still is changing.
The example:
GHA Aries- 212-05.7
SHA of the star 146-09.1
Dr Longitude- 41-50w.
Now, to get LHA I add 212--05.7 + 146-09.1= 358-14.8
LHA of the star= 358-14.8 - 41-14.8= 317.
Assumed longitude is 41-14.8.
But, if I do it slightly different,
GHA Aries- DR Assumed longitude= 212-05.7 - 41-05.7= 171
171 + SHA of the star = 171+ 146-09.1= 317-09.1.=LHA. I can round it to 317.
But in this case assumed longitude is 41-05.7. Not 41-14.8 like in the above.
And my question is: What assumed longitude to take? Because the difference is NOT zero.
Please, enlighten me.
Thank you.
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Help With an Exam Question |
Posted by: PeterB - 05-05-2023, 02:34 PM - Forum: Humor & Laughter
- Replies (1)
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![](https://thenauticalalmanac.com/Forum/images/default_avatar.png) |
Today is May 5th 2023 so this seems like a good time to post the following
A long while back on an obscure forum that I can't seem to find again I found this response to a poster seeking help on a rating exam question:
If the question mentions Arcturus the answer will be A
If the question mentions Denebola the answer will be D
And if the question mentions May 5th the answer will be C -- as in "Cinco de Mayo"
PeterB
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why Astron uses a "decide for yourself" AP longitude |
Posted by: PeterB - 03-30-2023, 08:51 PM - Forum: General Topics Here
- Replies (2)
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![](https://thenauticalalmanac.com/Forum/images/default_avatar.png) |
Carlos Pindle had mentioned he did not understand why the Astron app allows you to use a "decide for yourself" latitude and longitude for an assumed position.
I will try to shed some light on that subject if I can.
When we derive a calculated altitude (Hc) for an observed body what we are actually doing is finding a proxy value for the distance along a great circle from that assumed position (AP) to the geographic position of the body. The reason I say it is a proxy distance is because the actual distance from or AP to the GP of the body is found by
(90° - Hc) = zenith distance.
The reason for using the altitude above the horizon is simply because it would be very difficult to base our observations from a selected spot on the celestial sphere that was directly over our heads (our zenith.) But since we know that the total distance from our zenith to the true horizontal is always 90° it is easy enough to figure out the zenith distance.
We take a measurement of how high a body appears to us above the horizon, which is a little below true horizontal because the Earth is sphere and the surface falls away from us in all directions. We correct this for that dip of the horizon and for atmospheric refraction, and for some bodies parallax and semi diameter. We now have Ho. If we subtract Ho from 90° the result will be how far we are from the GP of the body expressed in degrees. Converting this to all arc-minutes converts that number into nautical miles.
Now suppose we wanted to know how far it is by great circle from New York to London? It turns out we use the same math for that calculation. We input an AP near to New York that either IS the EXACT lat and lon of New York for calculator methods, or we derive an AP nearby to New York as our starting position (usually called "the departure"
but I prefer "starting point") and we put in the location of London (usually called The Arrival, but I prefer "the end point") using its latitude in place of declination and its longitude as a Greenwich Hour Angle. Then we do a perfectly run-of-the mill sight reduction. The Zn is the initial great circle course (it changes throughout the track) and (90° - Hc) = the great circle distance. Because it is a great circle every arc minute is one nautical mile, so converting Hc into all arc-minutes gives the great circle distance in nautical miles.
Now if you are still with me consider the following:
Suppose you knew of a reef that you wanted to give plenty of berth? Or maybe a restricted zone you did not want to enter? If you can pick ANY AP you want you can pick a point on the reef's edge or on the demarcation line of the restricted zone and using that IN PLACE OF a "normal" AP you can get an HC from that spot to compare to the Ho you got of a body from your actual location-- and that result tells you how close you are to the reef.
The one thing you have to keep in mind is that the lines of position (LOP's) plot perpendicular to the azimuth so you need a body that you see in the general direction of the reef; or its reciprocal.
By extension if you want to figure out how far you progressed along your course you could pick an AP some where out on your desired track and use a body ahead or astern and by this means check your speed over the bottom.
It is perhaps for these purposes that the Astron app lets you pick your own AP. It provides increased versatility all out of the same math.
Peter
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Don't lose heart..... |
Posted by: CelNav57 - 03-30-2023, 01:00 AM - Forum: General Topics Here
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![](https://thenauticalalmanac.com/Forum/images/default_avatar.png) |
Celestial navigation isn't particularly easy- especially when you're tired and cold and forgot even the simplest of things about it. Sextant...whats a sextant? Oh, I remember...he's a janitor in a church. (sexton)
Here are some words that'll help cheer you in your voyage-
The Great One himself- Nathaniel Bowditch- that great American navigator and mathematician who said the following words;
“Whenever I meet with the words 'Thus it plainly appears,' I am sure that hours and perhaps days of hard study will alone enable me to discover how it plainly appears.”
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