distance NLC's mathematical puzzle

Forum für Leuchtende Nachtwolken (NLC)

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janlameer
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distance NLC's mathematical puzzle

Beitrag von janlameer » 15. Jul 2006, 20:32

Hi all
Last night was spectacular, and I very much enjoy all the nice pics that I've seen in the forum.
I have a question that at first sight seems easy, but is a little more complex if you dive into it.

Q: what is the distance of NLC's per degree over the horizon if you assume that they form at an average altitude of 83 kilometers above sealevel ?

I once made a table of distance per degree for the various heights of the colors of auroral curtains, through a grafical way of drawing the curvature of the Earth, with a scale of 2 centimeters = 100 kilometers.
With NLC's it is a bit different because they can be visible very near to the horizon where there is a lot of diffraction.

Like a NLC that is at altitude 83 kilometers and a distance of 500 kilometers from the observer would be at a height X degrees above the horizon but because of the atmospheric defraction, it would appear at height Y = X plus d (d=diffraction).

I'm mostly interested in the NLC's between 0,5 and 3 degrees above the horizon, who has an idea ???

JanL

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Matthias Juchert
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Wohnort: Wien (48° 15') + Potsdam-Mittelmark (52°23' n.B.)
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NLC distance

Beitrag von Matthias Juchert » 16. Jul 2006, 16:24

Hi Jan,

probably, these link will be helpful for you:

http://home.arcor.de/alexander.wuensche ... lccalc.htm

A distance calculator for NLC - but it looks like the calculation doesn't
include the refraction.

But you can try to use corrected values as input. For example, if
the hight of the NLC is 5 degrees, the refraction value is 10'15"
and so on. But as you already wrote - refraction is only interesting
for very low NLC (<3deg above horizon)

Clear skies
Matthias
---
www.Serifone.de

Joann S.
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Registriert: 4. Aug 2004, 10:08
Wohnort: Bremen

Beitrag von Joann S. » 17. Jul 2006, 11:29

Hi Jan,

the "refraction probem" is common in astronomical navigation. For astro navigation it is recommended not to use angles below 3° because of the refraction. For hights over 3°, normal temperature and normal preassure there are some formulas which approximate the refraction (e.g: http://www.seesack.net/navi/navi_4.htm). Try googling for some links to astronomical navigation webpages, refraction models of the atmosphere, ... I sure you will find a lot.
In general I would not try to calculate anything with any angles close to horizont! There are not only refraction influences caused by (homogenous) temperature and preasure profiles of the atmosphere, but also a lot of influences due to turbulence and refraction caused by inhomogenous temperature layers in the lower athmosphere. Just remember the nice, but distorted pictures of the setting sun...

Many greeting
Joann

Alexander Haußmann
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Beitrag von Alexander Haußmann » 17. Jul 2006, 14:58

Hi all,

some time ago I did some calculations about NLC distances involving atmospheric refraction. A very detailed explanation about the refraction problem is given here:

http://mintaka.sdsu.edu/GF/explain/atmo ... nding.html

From this site I took the refraction data. The NLC calculations were suggested by Richard Löwenherz to have a simple method for distiguishing NLC from cirrus, therefore the plotted lines represent object heights of 10+/-3 km and 82+/-3 km. Similar to the NLC calculator by Alexander Wünsche, one could manually look up the distance in such a graph, and draw the corresponding position into a map (and, if the clouds are real NLC, the reconstructed positions obtained by different observers should match if the 82 km line is used. If the positions coincide by using the 10 km graph, the observation can be classified as cirrus).

Bild

Finally, its worth to think about what is really meant by "distance". It might be the distance between the observer and the real NLC along a straight line in 3D-space, or otherwise the arc on the earth sphere (if the approximation of the earth as a sphere is made) between the observer and the point just below the NLC on the sphere. The difference between those distances is not much for NLC at the horizon.

Best regards,
Alex

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