Search found 389 matches

by JohnM
Sat Oct 05, 2019 8:20 pm
Forum: General chat
Topic: Do you have Time
Replies: 13
Views: 1947

Re: Do you have Time

I once asked a girl 'Do you have the time?' The answer was 'Not today'. That reminds me of 'The Time Lady' who used to take the time from the clock at the gates of The Royal Observatory at Greenwich to the various clock makers working in the centre of London. I presume on occasions she could well h...
by JohnM
Sat Oct 05, 2019 4:41 pm
Forum: General chat
Topic: Do you have Time
Replies: 13
Views: 1947

Re: Do you have Time - the arrow of time

The arrow of time ie why time goes from the past to the future is on of the 12 or so unsolved problems in physics. The basic physics equations that involve time do not have a direction in them and are reversible but time does not seem to reverse. It could be related to The Second Law of Thermodynami...
by JohnM
Fri Oct 04, 2019 8:46 am
Forum: Observing
Topic: Transit of Mercury 11th November 2019
Replies: 3
Views: 870

Transit of Mercury 11th November 2019

The next transit of Mercury will be on 11th November 2019 and the first half will be visible from the UK weather permitting though you will need a good southern and particually SW horizon due to the low altitude of the Sun. Robin will tell us more at the October London meeting but the timings are av...
by JohnM
Wed Jul 17, 2019 7:41 pm
Forum: Observing
Topic: Partial Lunar Eclipse
Replies: 8
Views: 1570

Re: Partial Lunar Eclipse

I saw it from my upstairs window from Carshalton. I got some images with a 300mm lens on a Canon 70D but they are not very good due to camera vibration and the contrast between the light and dark part of the moon exceeding the 14 bit CCD dynamic range. I could have probably cut down on the vibration...
by JohnM
Mon Jul 15, 2019 9:54 pm
Forum: Visual Satellite Observing
Topic: Too many Earth-Satellites..
Replies: 20
Views: 4462

Re: Too many Earth-Satellites..

Mike, As I understand it Entendu is just a restatement of the law of conservation of energy. Rougly it is Energy at teh focal plane / Energy at the input of the telescope thus seems to be a masure of losses in the telescope. From this it seems that Solar Telescopes are deliberatly designed with a hi...
by JohnM
Sun Jul 14, 2019 7:44 am
Forum: Visual Satellite Observing
Topic: Too many Earth-Satellites..
Replies: 20
Views: 4462

Re: Too many Earth-Satellites..

Mike, Wikepedia defines it as 'In photography, bokeh (/ˈboʊkeɪ/ BOH-kay; Japanese: [boke]) is the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens. Bokeh has been defined as "the way the lens renders out-of-focus points of light"' I wonder if he meant É...
by JohnM
Fri Jul 12, 2019 3:38 pm
Forum: Observing
Topic: NLC 2019-07-11/12 from South of London
Replies: 1
Views: 878

NLC 2019-07-11/12 from South of London

Did anyone see the NLC last night (20190711) around 23:00 BST ? I saw it from south of London very near the horizon through a gap in the clouds. It was best in binoculars as I have a street light in my line of sight. I took a few images but they are not much good as I had to hand hold them and the s...
by JohnM
Tue Jul 09, 2019 6:16 am
Forum: Visual Satellite Observing
Topic: Too many Earth-Satellites..
Replies: 20
Views: 4462

Re: Too many Earth-Satellites..

Hello David, Don't forget the two ? Hewitt Satelite tracking cameras. See http://www.royalobservatorygreenwich.org/articles.php?article=1097 for a bit of history and the Hurstmonceux Science Centre at https://www.the-observatory.org/dome-c . These are quoted as Aperture 860mm, 680mm focal length, f/...
by JohnM
Sun Jul 07, 2019 8:31 pm
Forum: Visual Satellite Observing
Topic: Too many Earth-Satellites..
Replies: 20
Views: 4462

Re: Too many Earth-Satellites..

I saw the article on the BAS - DSTL satellite imaging in Astronomy Now and also saw a presentation on this at The National Astronomy Meeting last week. I am not sure what the objecxtive was as the systems used were not at the cutting edge. There are others available with a lot higher Étendue which t...
by JohnM
Fri Jun 28, 2019 7:49 am
Forum: Observing
Topic: Noctilucent Cloud Display 21st June 2019 at 11:00pm
Replies: 13
Views: 2800

Re: Noctilucent Cloud Display 21st June 2019 at 11:00pm

Hello David, I use a old Canon 10D facing north controlled by a Raspberry Pi to give an exposure every 30 secs from around 22:00 BST to 23:00 ish. This download the images from the camera when complete leaving me to check them in the morning. Details at http://www.johnmurrell.org.uk/nlc.html if you ...
by JohnM
Thu Jun 27, 2019 9:43 am
Forum: Observing
Topic: Noctilucent Cloud Display 21st June 2019 at 11:00pm
Replies: 13
Views: 2800

Re: Noctilucent Cloud Display 21st June 2019 at 11:00pm

David, I have had a look at the Meteosat images for the evening of the 21st & morning of the 22nd June and there is no obvious reason for the limited height of the display from where I am located. There was low cloud over East Anglia but the high cloud was over Norway - I would have thought that was...
by JohnM
Tue Jun 25, 2019 4:49 pm
Forum: Observing
Topic: Noctilucent Cloud Display 21st June 2019 at 11:00pm
Replies: 13
Views: 2800

Re: Noctilucent Cloud Display 21st June 2019 at 11:00pm

From south of London there was a very bright display but it was very low on the horizon. I did not notice it at the time but my north observatory camera recorded it. On the morning of the 22nd there was a strong display low down ( below Zeta Per). I thought this was going to develop into a strong di...
by JohnM
Sun Apr 28, 2019 8:37 am
Forum: General chat
Topic: Dark adaption of human eyes
Replies: 5
Views: 1824

Re: Dark adaption of human eyes

There is a page about dark adaptation of the eye here https://webvision.med.utah.edu/book/part-viii-psychophysics-of-vision/light-and-dark-adaptation/ . It iassumes some medical knowledge in places however there are some interesting curves (figure 2) about adaptation particually those that show how ...
by JohnM
Sat Apr 27, 2019 5:52 pm
Forum: General chat
Topic: Finally a Black Hole
Replies: 10
Views: 2365

Re: Finally a Black Hole

Brian, From what I understand of the theory the black hole needs to spin and you need to enter with sufficient velocity to miss the singularity. If you manage that I beleive you will end up in another universe which would have a spinning black hole. If you repeat the process you end up in another un...
by JohnM
Sat Apr 27, 2019 8:27 am
Forum: General chat
Topic: Dark adaption of human eyes
Replies: 5
Views: 1824

Dark adaption of human eyes

I was looking at an online page about human vision and that stated something I never really though of before. If you shine a bright light in only one eye of a person in a dark room the pupil of the other eye that is shielded from the light also contracts. That got me thinking - dark adaption involve...