Sunday, 17 June 2012

THE EARLIEST SUNRISE

Sunday 17th June 2012

Holbeach

52°43'N 000°00'W

Sun
Rise 04:35 BST
Set 21:27 BST
Moon
Rise 03:04 BST
Set 19:36 BST
Phase Morning Crescent
I wrote about this in connection with the shortest day, and now  I find myself here. Not the longest day, but the EARLIEST SUNRISE. From now until late December I get extra time in bed every day.
To celebrate, I thought I'd put together some basic information on sunrises (mostly nabbed from Wikipedia)
 
The earliest sunrise does not occur on the shortest day
This is all to do with atmospheric refraction. Basically, light bends over the horizon. The average amount of refraction is 34 arcminutes! Bet you didn't know that.
It's also because Earth has an eccentric orbit and an axial tilt. (Wobbly circles)
 
What is the definition of sunrise?
Sunrise time is the moment at which the upper limb of the sun appears tangent with the horizon.
I think that means the moment the first bit of the sun appears.(So, not when the centre of the sun or the whole sun has crossed the horizon)
 
The Sun does not actually go round the Earth.
If you thought this was the case then this is a stunning piece of information.
A geezer called Nicolaus Copernicus came up with this idea way back in the 16th century.Of course, it is the rotation of the Earth that causes the sun to rise, cross the sky and set again.
This may sound simple enough, but it's amazing to stand in the open fenland landscape, watch the sun rise and actually concentrate on the fact that the sun is staying still. Just for a few minutes you can actually get a sense of the earth spinning (though perhaps not as much as after I've tried some of my home-made cider!)
 
Why does sunrise occur at different times every day?
Again, this is all to do with the tilt and rotation of the earth and it's elliptical orbit around the sun. (Wobbly circles again) I guess if everything were perfect circles (or spheres) on the right angle, then we would get the same 12 hours of sunlight every day all year.
 
Why does the sun change colour?
Rayleigh scattering of course!
Pure sunlight is white in color, containing a spectrum of colors from violet to red. When sunlight interacts with atmospheric particles the light is scattered. Violet, blue and green are scattered more than orange and red. The sun appears yellow because the violets, blues and greens have been scattered into the sky. This is also why the sky appears increasingly blue further away from the sun.
This effect is exaggerated during sunrise and sunset, as the light is travelling further through the atmosphere. Hence oranges and reds.
 
Then there's Mie scattering.
This is caused by larger particles in the atmosphere, such as volcanic ash and cloud droplets. So the already reddened sunlight is scattered further to light up the horizon orange and red.
 
So, Mie scattering is responsible for the light scattered by clouds, and also for the daytime halo of white light around the Sun. Without this, the sky along the horizon has only a dull-reddish appearance, while the rest of the sky remains mostly blue and sometimes green.
 
Why are sunsets more dramatic that sunrises?
Simply because the evening air typically contains more large particles, such as clouds and smog, than morning air. (Having said that, too much cloud is obviously not great for spectacular skies, as witnessed in about half of my sunrise photos.)
 
 
So, I urge you, once in a while make the effort to see the sun rise. If you really concentrate on the scale of what is happening it'll teach you a lot about yourself.

 

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