The Half-Astrophysicist Blog

The Fat Sun

Just getting caught up after my Florida trip.  The last night I was there, I went out for sunset as I do many nights.  Each night brings something a little different. Tuesday night’s sunset showed off how Earth’s atmosphere refracts sunlight and can distort its shape.

As sunlight passes through Earth’s atmosphere, its path is bent thought a process called refraction. You have probably seen refraction before. It’s the same process that causes a pencil to appear bent when it is partially submerged in a glass of water and the same process that your eyes (and glasses if needed) use to focus light. In the Sun’s case, refraction causes the Sun to appear a little higher in the sky than it would if Earth had no atmosphere.

But the Sun is not a point source.  Near sunset, light from the upper limb of the Sun passes through less atmosphere than light from the lower limb of the Sun. That means light from the lower limb of the Sun is shifted upward more than light from the top limb of the Sun. The upshot of this is that the Sun will appear to be squeezed vertically and appear much fatter the lower it gets in the sky.

This effect was pronounced on Tuesday. I took a bunch of my photos and put them into a short video.  The pictures are not equally spaced in time (I tend to take pics more frequently as the Sun gets lower and a rapid burst near the end of sunset). You also may notice when I change camera settings due to the changing lighting conditions as the Sun sets.  And you can look see some birds flying across some pics.  Really watch the last frames and you can see the last tip of the setting Sun gets very stretched horizontally.

I started my burst too soon and the buffer in the camera filled up which killed any chance of a green flash pic. However, I didn’t see a green flash visually so I am not too disappointed with that. I am happy any time I capture some good atmospheric optics at Sunset!



October 28, 2011 - Posted by | Astrophotography, Atmospheric Physics, Sunset

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