Dusk & Seeing Conditions Venus & Jupiter

[jabeoo1]

Whilst driving back from work in full sunshine (for the first time in months) I made a few mental notes.  I observed on route from the edge of Southdown above Newton St Loe (which is around 4 miles west of central Bath) the distant terrain of the Brecon Beacons.  Normal range of seeing I estimate to be around 20 miles, it was near to 50 miles & I was struck by the transparent view.  

I was fixated to set the telescope up and test the observation against the quality of seeing (fingers were mentally crossed for other factors to be settled to take advantage of this transparency).  

I decided to set up during dusk with Venus still high enough over the next doors fence & Jupiter over head.  'Dusk' according to a popular online encyclopaedia is simplified & split into 3 categories.  Civil Dusk (when the Sun is 6 degrees below the horizon), Nautical Dusk (Sun is 12 degrees below horizon) & Astronomical Dusk (18 Degrees below the horizon).   Based on this scale I think I set up and started observing from late Civil Dusk onwards.  

For the UK I found the seeing conditions to be the best I have witnessed with only one other time in mind (also during dusk). 

The conditions gradually got worse and worse until they reached the normal kind of seeing I am commonly used to.  I would say that at best nearly 30 seconds in every 60 allowed for still pristine seeing of many surface details not normally easily seen, by the end I was getting maybe 2 seconds out of the 60. Venus was clearly a crescent with a defined shadow line on its thick atmosphere.  Jupiter had visibly many extra defined bands and the GRS was a defined separate feature from the normally mushy overlapping belts.    

So what factors make this narrow window of opportunity during dusk the 'best' time to view like this ? Why do the conditions deteriorate so fast & do they re-stabilise again at dawn ?   Its a sad truth but do we ever get an entire tremendous night of seeing here in the UK ?  Or are we tied to these 60 minute opportunities if we are lucky ?

[ronin]

We do get them but people expect them sort of on demand at any tme.

If you want it clear Monday and Bob in Keynsham wants it clear Wednesday one of you is going to complain.

And if Mike in Corston want it clear any time at a weekend he must be expecting a miracle.

Deterioration/change is caused by cooling and the effect that cooling has on the layers in the atmosphere. Some flaming great fusion bomb floating around disappears below the horizon and we get irradiated less. Mind you someone else gets irradiated more.

Recall several nights of tremendous seeing and clarity in a single run.

Also recall the cries of anguish from people here.

First 2 nights were "great", third was a bit less great (sleep loss), fouth was half great, half "Oh no not another", 5 and 6 were "WHY? I cannot take this any more".

The final night, think No 8, was 98% complaint. Next cloudy night was "Thank God it's not clear", second cloudy night was "Oh hell more cloud!".

Do surfers cpomplain about the surf as astronomers do about the sky?

They are the only ones I can think of that might.

[dick_dangerous]

Do surfers cpomplain about the surf as astronomers do about the sky?

They are the only ones I can think of that might.

Skiers complain about the snow - whether there is any an whether it's the right "type." I always thought that the "Other Type of Snow" was called rain. Or maybe hail. I just remember staying in a ski town where no-one seemed to go skiing, even though snow was waist deep.

Personally if it's clear and I'm not knackered or doing something else I usually reckon it's worth a punt. Tonight is looking hopeful and it'll be my first session in ages, so I'll be rolling the Dob down the park pretty soon. And if the seeing's off then... well... nothing ventured etc!

DD