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Post by Bill W on Jun 1, 2009 8:45:38 GMT
Something to ponder whilst the camera clicks away. Since NLC form at a fairly uniform height above the ground this means that it is possible to estimate the range to the NLC from an "altitude" measurement. Altitude, in this case, being the number of degrees above the true horizon. With a bit of number crunching (we're dealing with two concentric spherical surfaces). For example, if you had a properly leveled small telescope and read off an altitude of 20 degrees then the NLC at that reading would be 235km distant. Of course if the NLC appears right overhead then it is around 83km above your head! The one other point to remember is that the range value is only valid for a great circle through you (the observer), the NLC and the meridian of the pole. Start measuring things well off to the "side" and the trigonometry gets ugly. Enjoy...!
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Post by André on Jun 1, 2009 12:21:43 GMT
Thanks, that is really a nice chart. Nice to see it plotted like this. You say: The one other point to remember is that the range value is only valid for a great circle through you (the observer), the NLC and the meridian of the pole. Start measuring things well off to the "side" and the trigonometry gets ugly. Maybe I get you wrong, but it shouldn't change at all. You can just "define" new coordinates of the earth and it will be fine. The trigonometry might be ugly, but the result will be identical! André
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Post by Bill W on Jun 1, 2009 12:41:00 GMT
Hi,
I think I understand what you mean. But considering any given observer looking due north (or south for that matter) will see the NLC layer at the range I've calculated if it is on that great circle. That's the shortest line between the two surfaces, in section.
Post mod 10, my head now hurts, I thought this would be easy...
If you swing round in azimuth, seeing the NLC at say altitude 10 degrees, azimuth 040 rather than altitude 20 degrees azimuth 000 doesn't mean that the NLC at 10 degrees is 415km (from the graph) as opposed to 235km looking due north. This is because from a single point on the Earth when you swing round in azimuth you are no longer looking along the great circle that was used to determine the range initially.
Both surfaces (the Earth and the NLC sheet) are curving away from you so observing off of the great circle you would need to generate a spherical triangle and correct for the curvature of the Earth and NLC. Is that what you mean by Define new co-ordinates?
The effect of this is that from your point of observation,the NLC which is all at the same physical height is further away than the graph indicates as the azimuth increases. I'll try and find a suitable graphic but if you draw it out I think you'll see what I mean.
Ugly, doesn't really cover it, time for coffee!
Cheers, Bill.
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Post by markt on Jun 1, 2009 22:18:42 GMT
I like this Bill, very, very nice - thanks for sharing
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Post by André on Jun 2, 2009 5:58:37 GMT
Ok Bill. I think I got you now. You are assuming the NLCs are spread out over a circle with the same lattitude and the calculation will tell you the distance to the closest point (which will be in north direction). If you just want to measure different points of the NLC, not assuming anything how they spread, the calculation works for any direction.
Did I get you right?
Cheers, André
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Post by Bill W on Jun 2, 2009 9:06:45 GMT
Hi, That's right!, It's really difficult to put into words what a diagram would show easily.
cheers, Bill.
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Post by Bill W on Jun 3, 2009 12:12:40 GMT
Hi,
Another little exercise which is instructive....
Use the slant range curve to get an estimate of distance of the upper most part of the NLC and using a map or atlas plot this distance. It's surprising how far away the NLC suddenly seem when you put it on a map like this.
If its a bright and expanding display and you do this at intervals you can then get a: the approximate location of where the NLC leading edge is exactly overhead and b: from the changing locations on the ground, the "ground speed" of the NLC sheet as it blows out from the higher latitude regions.
There are geometry errors which limit the accuracy but with a couple of simple measurements you can get a good feel for what the NLC is doing. They're not quite a serene as the pictures make them look!
cheers, Bill.
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