Saturday, August 27, 2016

Sunrise, Sunset


source: Active Rain
The sun rises in the East and sets in the West. Or at least that's what everybody says.

And this is true. But it's not the whole truth.

The sun does appear to move in predictable daily patterns, but the whole truth is quite a bit more interesting.

We say the sun "rises" because it appears gradually over the horizon in the morning.

We see the morning sun in the eastern sky, but it doesn't rise at the same point on the horizon every day. The spot where the sun rises depends on where you live, and the time of year. And that point is not necessarily due East of you, although it might be, once or twice a year, depending on where you live.

The morning sun appears to climb higher and higher in the sky, away from the eastern horizon. Its path appears to take it on an arc directly over your head, or to the North or South of a point directly over your head, depending on where you live and the time of year.

In the Northern hemisphere, the sun climbs to higher and more southerly positions in the sky until mid-day, when it reaches its highest and most southerly position.

If the Southern hemisphere, the sun climbs to higher and more northerly positions in the sky until mid-day, when it reaches its highest and most northerly position.

The closer you are to the equator, the less the sun appears to move to the North or South, and the closer the sun appears to move directly over your head. Again, this depends on the time of year, and where you live.

In the afternoon, the sun begins to descend. It moves through lower and more westerly positions, until the evening, when it appears to "set," gradually disappearing under the western horizon.

The sun doesn't set at the same point on the horizon every evening. The spot where the sun sets depends on where you live, and the time of year. And that point is not necessarily due West of you, although it might be, once or twice a year, depending on where you live.

The sun is so bright that we can hardly ever look at it directly, so most of the sunlight that we see reaches our eyes indirectly. And we hardly ever notice it.

But we do notice indirect sunlight when the sun itself is obscured by clouds, or below the horizon. Indirect sunlight is very striking before sunrise or after sunset, for example.

The size and color of the sun also appear to change in predictable daily patterns.

During most of the day, the sun appears to be yellow, or almost white, and it is relatively small compared to its size at sunrise and sunset, when it appears larger and a bit darker, throwing fiery shades of orange and red across the sky.

Among all cosmological phenomena, the daily progression of the sun in the sky is by far the most important, not only to humans but to all life on earth; not only now but as far back in our history as you care to go.

It is also by far the most easily observed. We can all see it every day. We don't even need a telescope.


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