(a) Light from the limb is reddened by solar rotation and appears cooler and therefore less intense.
Explanation:
As the sun is entirely made up of gases and the light that is emitted out from the limbs is less intense as the light is emitted out of the cooler regions of the photo sphere and the light that reaches the observer is dimmed out as it reaches the outer layer.
<u>And thus the limbs look darker than the middle part as it is hotter. And the rays of light take more time to reach the observer than the direct center spot. Which appears to be yellow and red as compared to the orange and dim limbs.</u>
It's because of the tilt and the seasons and the the way on the earth it's located. During the summer it has long days of sun and some days have 24 hours of sun and in winter there might be only 3 hours of sun and total darkness.