Oblate Sunset – Hawaii

Reminiscent of a Chinese lantern, the Sun takes on a squashed, oblate appearance shortly before plunging into the Pacific Ocean as seen from the west coast of Big Island, Hawaii on 3rd June 2012.

The uneven appearance of the edge of the Sun is caused by the different layers of air through which its light is passing, each layer having a slightly different refractive index. Refraction increases sharply as we look nearer the horizon which is why the lower half of the Sun looks more squashed than the upper.

As most people know, the Sun appears red near the horizon because the redder wavelengths of light are scattered much less than shorter wavelengths such as green and blue. Since the amount of scattering depends on the fourth power of the frequency, this means that green light is scattered about 2½ times as much as red, and blue light about 4 times as much. This phenomenon is called dispersion and explains a range of effects including rainbows and why a prism splits white light into a spectrum.

Though the horizon is not visible in this image, boosting the brightness reveals it to be just half the Sun’s diameter below its lower limb.

The other effect of refraction as the Sun (along with all celestial bodies) approaches the horizon is to bend light slightly over the horizon, allowing us to see it a few minutes longer each day. The amount of refraction at the horizon is about equal to the diameter of the Sun’s disk, thus if there were no atmosphere the Sun at this moment would appear to be cut in half by the horizon.