July not nearly as cold as expected
Ninety-three million miles – the textbook distance from sun to Earth – is only an average. Earth’s orbit is not circular. As we move through the year, we draw close to the sun – as close as ninety-one-and-a-half million miles – and then draw away.
Next week, Earth will be far from the sun, as far as it gets all year: ninety-four-and-a-half million miles.
Farther from the solar fusion furnace, we’ll receive seven per cent less solar energy next week than we do in early January.
July, it would seem, should be the coldest month of the year.
Another event, which occurred last week, ensures that July does not mark the middle of winter: the summer solstice.
Earth circles the sun at a slant. No matter where we are in our orbit, we always lean toward Polaris, the north star.
It’s this lean that makes July hot, January cold. To see why, you can build a model of the solar system, using your body to represent Earth (Don’t think about this – DO IT!).
Do this in a room with a table – a table you can walk completely around.
The table is the sun. Starting anywhere, walk completely around the sun/table. It takes you – the Earth – a year to do this.
Standing in one place, face the sun/table; you’re also facing the wall on the far side of the sun/table. Pick a spot high on the wall, where the wall meets the ceiling. Let that spot be the north star. Lean toward the north star/wall spot.
As you lean toward the north star, you’re also leaning toward the sun/table. To look at the sun/table, you’ve got to lift your eyes. The sun is high in the sky. It must be summer.
Now walk half-way around the sun/table, until you’re standing between the sun/table and the north star on the wall.
Face the sun/table, but lean – backwards – toward the north star. You’re now leaning away from the sun.
To look at the sun/table, you’ve got to lower your eyes. The sun is low in the sky. It’s winter.
Enough exercise – time for a break … and a quick experiment.
Shine a flashlight straight down onto a newspaper so it illuminates a small circle of newsprint. Every word within the circle of light is well-lit.
With the summer sun high overhead, its light falls (almost) straight down on us: We’re well-lit, the sunlight intense.
Now tilt the flashlight to shine on the newspaper at an angle; the circle of light is stretched out. It illuminates a larger part of the page – more words are lit up … but, with the light spread out, each individual word is more dimly lit. The sun’s light is weak.
Now for some observation. Stars overhead twinkle because the atmosphere is always in motion, bending the star’s light this way and that before it can reach us.
Notice that stars near the horizon twinkle like mad; their light, skimming close to the surface, travels though more air to reach us. (When a really bright star shines near the horizon, it’s light gets so bent it twinkles in all the colors of the rainbow).
Air robs light of its energy. The more air the light must travel through, the more energy is robbed. The low sun of winter is attenuated as it plows through the air, making winter sunlight weaker still.
Back to the model. Stand with your back to the table/sun: It’s midnight. You can’t see the sun, because most of your head’s in the way.
Turn slowly to your left, until the table/sun just enters the edge of your vision – it just begins to rise.
Like your head, the Earth blocks our view of the sun at night, until we turn enough for the sun to rise over the horizon. When we lean away from the sun, in winter, there’s a lot more of the sun in the way, so the Earth has to turn more before the sun rises. The sun rises late.
In summer, with our hemisphere leaning toward the sun, the Earth needs to turn very little before the sun rises. It rises early, and then it sets late. Soaking in the direct, intense sunlight for hours, our hemisphere grows warm.
One caveat: The timing and intensity of all these events – the date of the summer solstice, the directness of sunlight, the length of the day, and so on – all depend on how much we tilt, the shape of our orbit (how circular), and a number of other parameters. As sun and moon, Venus and Jupiter tug on us with their gravity, these parameters change, and so does seasonality – the intensity and timing of the seasons.
Alan Stahler’s program, “Soundings,” can be heard on alternate Tuesdays, after the noon news, on KVMR, 89.5 FM. This Tuesday, he’ll be looking at how wind and water make beaches, how the Earth makes ores, and how New Orleans is struggling to re-build. He offers classes in science, math and writing, and may be contacted through the Union, or by e-mail at email@example.com
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