Alan Stahler: Fall equinox approaches; Nevada County viewing of Saturn’s rings Sept. 24 | TheUnion.com
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Alan Stahler: Fall equinox approaches; Nevada County viewing of Saturn’s rings Sept. 24

Rivers carve V-shaped valleys (like the valley of the Yuba). As a glacier flows down a river valley, it flattens the floor and steepens the walls, turning the valley into a U-shape, as here, in Glacier Bay National Park.
Submitted photo |

Keep close track of where Earth is in its orbit, and you’ve got a calendar. Is Orion well above the horizon before dawn? Autumn’s approaching.

But the calendar’s slipping. There will come a time when we celebrate the Fourth of July in mid-winter.

Take a trip to the North Pole, look straight up, and there’s Polaris, the north star. The North Pole always points up to the north star. But the north star is not always Polaris.



The tilt of the classroom globe reminds us that Earth orbits the sun at a slant, leaning always toward Polaris. In summer, Polaris lies in the same direction as the sun; during the day, as we lean toward Polaris, we also lean toward the sun. The summer sun climbs high in the sky; it rises early, sets late. High overhead, the sun warms the ground for hours. Warm ground, in turn, warms the air.

Half a year later, on the opposite side of our orbit, Polaris and the sun are in opposite directions in space — as we lean toward Polaris, we lean away from the daytime sun. The sun stays low in the sky; it rises late, sets early. Slanting sunlight, for only a few hours a day, can barely warm the ground; the cold ground can barely warm the wintertime air.




On one day of the year, we lean more toward the sun than on any other. Six months later, we lean away from the sun more than on any other day. These two points in our orbit are the solstices.

Halfway between the solstices are the equinoxes, one in spring, one in fall, when we lean neither toward nor away from the sun. Today marks the fall equinox.

Spin your arm like a propeller and blood flows outward, toward your fingertips. As the Earth spins, rock flows outward at the equator, making our planet bulge at the beltline. That bulge gives Jupiter and Saturn, sun and moon, something to pull on. Jupiter might try to pull the Earth upright, while the moon tries to pull us over. Then, they might trade off, each pulling the other way. Or both could pull the same way.

Pulled this way and that, our planet wobbles; the wobble makes the north pole point to different parts of space. For the better part of the coming century, the north pole will inch closer to Polaris. And then it will start moving away. Over thousands of years, the north pole will point to one star, then another, then another.

The wobble changes the position of the equinoxes in our orbit, and, therefore, the timing of the seasons — sometimes it’s summer in July, sometimes it’s winter.

Nowadays, Earth is closest to the sun in January, farthest from the sun in July. Winters are less cold, summers less hot, than they could be. This, too, changes as we’re tugged on. Thousands of years from now, summers will be super-hot, winters, super-cold.

And our jaunty tilt — that changes too. Sometimes we’re a bit more upright; other times, a bit more slouched over — again, on a cycle of thousands of years.

As the cycles interact with one another, Earth’s climate grows warmer and colder, dropping into, pulling us out of, ice ages. And our great geophysical experiment — injecting greenhouse gases, soot and sulfur into the atmosphere, destroying ozone and other gases — this, too, tweaks Earth’s response to sunlight, and to the cycles.

Local astronomers will set up scopes Saturday night at the junction of SR 49 and the old Downieville Highway, and invite the public to join us. We’ll view Saturn’s rings, galaxies, star clusters, and we’ll track Earth’s wobble, by tracing all the north stars, past, present and future.

Al Stahler teaches nature classes for students of all ages, kid to adult. His science programs can be heard on KVMR-FM, and he may be reached at stahler@kvmr.org.


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