Altitude Measurements

[Bill W]

Hi,

I don't want to be a boring pedant but being a professional anorak can I use last nights fine display to try and put a bit of perspective on things.

The "height" of ANYTHING in the sky IS deceiving. As humans we normally deal with a very limited range of elevations. I have heard people argue blind that this or that was "overhead" (just because they had to tilt their head a bit) but when a proper theodolite is used and it turns out to be only 40 degrees, it must be the scale that is wrong! (yeah right!, a student demo some time ago.)

To be seen in the zenith, that is at 90 degrees up, the solar depression angle (the angle the sun is below the horizon) is only a few degrees therefore it is physically impossible to see NLC at or near the zenith except during particular times of the day.

Unless you actually measure the altitude angle with some sort of horizon leveled device please take care with height estimation.

If one doesn't have a theodolite to hand then conduct the following test. Lie on the ground and look straight up. I'd be prepared to bet that the first thing you do is look "down" towards ones toes. Right away this means that you're not looking at something in the zenith.

Let's not get carried away.

Cheers,

Bill.

[Bill W]

Hi,

Yes, I have Mikes book too. The graphic given in the book demonstrates the theory and shows the point. That to illuminate the layer at 83km up at the zenith the sun must only be 6 degrees on so below the horizon.

NLC are generally optically thin (you can see through them) and are visible by forward scattering of sunlight. If there's no sunlight on them then they can't be seen. Maybe they're getting super dense.

I have seen NLC genuinely in the zenith on only a handful of times and these have always been 0130UT onwards when the solar depression angle is appropriate and never at 0000UT

You can easily predict when it'll happen by using any planetarium programme.

It's just a note of caution.

cheers,
Bill.

[Bill W]

A PS.
You're pictures have just materialised. I think they actually prove my point and what's in the book!
You're second picture has needed stretching so you are using the instrument properties to make the observations of very weakly scattered light from the NLC sheet.
As your pictures were taken at 0245UT this is entirely appropriate.
It's the claims from the less experienced observers that they are very high around midnight that needs caution. That was the point I was trying to make.

[Bill W]

As an example here's a picture from 12/13 July 2006 at 0230UT
This is what a near all sky NLC looks like, the southern horizon is on the right. The bright object is the moon. This picture too has been stretched to bring out the detail as visually it was quite faint.



This season is looking like a good one so here's hoping we get some more of these!

Cheers,
Bill.

[dgrutherford]

Hi,

I hope you were not getting at my report about the overhead display. I was in the Met Office for 42 years as Observer, Forecaster and Manager so I am well aware how to observe and what overhead is.

Dave Rutherford

[Bill W]

Oh dear,
This is obviously being mis-read.

I am not trying to get at anyone. However there ARE reports of the NLC being very high, year on year when the sun can't be illuminating the NLC layer. It is a note of caution to the less experienced. In light of my own experience!

If I wanted to critisize or pass a comment specifically I can assure you I would do it directly, but there are less experienced observers who need to learn and gain experience.

If there is no realistic and honest guidance then it just becomes a photography hobby, which is fine if that's all one wants to do, or worse a joke.

By informing people of what is factually and scientifically correct makes for better reporting.
Then any genuinely unusual events can be discussed in that light.

Bill.

[markt]

Interesting thread guys!

I was certainly not aware of the geometry involved of NLCs and the zenith etc, but i'm a sucker for this kind of stuff and find it quite fascinating.

[scotweather]

I am no expert in the field of NLC's nor do I have a scientific background however, I do have a genuine interest in seeing and photographing NLC at any opportunity I have. Last night with my report on elevation I would not have mentioned at all as I cannot accurately judge the elevation but what I will say is - from where I was standing in Central Scotland I was looking above my head at the outer pale edges of the NLC. Yes the brighter and more fanciful NLC was sitting at around 45 degrees but there was distinct whispy glows of NLC at a much higher elevation to the main event. I was out early before it all started and the first indication of NLC appeared at approx 45 degrees but as the sun went down all this whispy stuff appeared extending from the NLC at 45 degrees.

Have a look at the picture to see what I saw.