Limb darkening

 

My enthusiasm for the natural world wanes when it comes to things such as earthquakes, floods, tornadoes, wildfires, and mosquitoes. Nevertheless, while our present smoke-filled valleys are distinctly unpleasant, they offer some interesting features. Consider a view of the Sun.

Colour:
Molecules in the atmosphere preferentially scatter bluish light and this produces the blue of skylight. The smoke particles do likewise and, as they add to the scattering, direct sunlight gets distinctly orangish.

Brightness:
As the smoke removes light from the direct beam, the Sun becomes dimmer making it easier to view and even see…

Sunspots:
A sunspot can be seen at seven 0’clock. 

Solar prominences:
It may be that some of the fuzziness around the edge results from solar prominences.

Limb darkening:
The picture shows a distinct variation in brightness and colour across the solar disc. This is a consequence of a real variation from the Sun, itself, and is known as limb darkening. Sunlight is emitted by (what is known as) the photosphere, but it must then pass up through the Sun’s atmosphere which absorbs some of that light. More sunlight is absorbed on the longer path through the solar atmosphere near the Sun’s limb than on the shorter path near the centre making the sunlight dimmer towards the edge. (The Moon, which lacks an atmosphere, does not show limb darkening.)

Elevation darkening:
Finally, the limb darkening is asymmetric, with more darkness near the bottom of the image than the top. This is a result of the variation in the path with elevation through the smoke in our own atmosphere. 

A view of the Sun through our smoke-filled sky reveals interesting features.

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7 Responses to Limb darkening

  1. Carlo says:

    Outstanding! A whole lesson in a one-page blog!

    • Alistair says:

      Carlo, I suspect that the use of physics to make sense of the natural world is an acquired taste that not all naturalists share.

  2. Lorna says:

    You may well be right Alistair but I too find it most interesting.

  3. Denise Brownlie says:

    “Count me in”, Alistair. I’m with Carlo and Lorna in responding positively to posts such as this one. Even when I can understand only a fraction of the physics, it’s exciting to be with the life-long learners who seek out sites such as yours.
    Please keep teaching us, Alistair!

  4. Trevor Goward says:

    Hi Alistair

    The thought that the sun has an ‘atmosphere’ has never occurred to me until now. Of course it must do, as it’s forever giving off gases, right? I must look into this. Thanks much for this fascinating post!

    • Alistair says:

      Trevor, and the fascinating thing for me is that evidence of the solar atmosphere can be seen by a casual observation from Earth — no big telescopes or satellites required.

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