The horizon line can be a tricky thing to measure...
This is partly due to pollution, but even in the cleanest air the sky is lighter near the horizon due to the effect of earth's atmosphere on sunlight.
If you look at the sky on a cloudy day, you will find more clouds near the horizon than directly overhead. Why? Learn more on this Moment of Science.
Star light, star bright, the first star I see tonight... What is this legendary star? Is it even a star at all?
During voyage of the Beagle, Charles Darwin was often overcome by seasickness and discovered that “nothing but lying in [his] hammock did any good.”This remedy–lying down with eyes shut–is one of the most effective for treating motion sickness.
Now, at sunset, the lowest mile or so of the atmosphere is filled with things like vehicle exhaust, dust, smoke, and water vapor, and all these pollutants scatter light.
If you look at the horizon at sunset–exactly as the last sliver of sun disappears–you might see a flash of brilliant green blaze across the sky.