Wednesday, January 20, 2010

The Oblate Sun

This shot of the setting sun taken from Seamans Neck Park in Seaford, NY illustrates an interesting atmospheric optical property. Earth's atmosphere causes refraction of the sunlight (and moonlight, and starlight) that passes through it. Near the horizon, refraction will raise an object on the horizon (like a star or the Sun) to an apparent position about 0.5° above the horizon. When the 'bottom' of the Sun is raised more than the 'top', the rising or setting sun can look quite flat! This refraction hastens the apparent rising of the Sun and delays the apparent setting of the Sun, effectively giving us a few more minutes of daylight than we'd have if there was no atmosphere at all.  You can read a little more about the effect this refraction has on the length of day here.

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