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Heavenly orbs affect Earth

Earth’s climate would be easier to understand if we were alone in the solar system – no other planets, no moon, just us and the sun.

But we’re not alone: The moon and Jupiter, the sun and other planets, pull us this way and that, alter our orbit, and thus change our climate.

Earth orbits the sun in a near-perfect circle … sometimes. Oft times, the circle gets squished.



Squeeze a hula-hoop and it gets narrower in one direction, longer in the other. So does our orbit, as Jupiter and the other planets pull us about.

When our orbit is nearly circular, Earth remains always the same distance from the sun … from the “campfire.” But when our orbit gets squished, we draw closer to the sun at some times of year – some parts of our orbit – farther in others.




Summers spent close to the sun are hot; winters far from the sun are cold.

Summers far from the sun are mild, as are winters that are close.

Earth’s orbit goes from circular to squished, and back to circular, in a cycle of roughly 100,000 years.

One needn’t climb very high in the Sierra to find evidence that vast rivers of ice repeatedly covered the mountains: Evidence of previous Ice Ages. From beginning to end – cooling, then warming, then cooling again – the Ice Age cycle lasts about 100,000 years.

A glance at the school-room globe reminds us that Earth orbits the sun at a jaunty 23 1/2 degree tilt. This tilt causes the seasons: Leaning toward the sun brings summer; leaning away, winter.

Again, the gravitation of other worlds – especially, of the moon – tweaks the tilt, down to a less-jaunty 22 degrees, (we lean less toward the sun in summer, less away in winter), then up to a more-jaunty 24 1/2 degrees (we lean more toward the sun in summer, lean away more in the winter).

The greater we lean, toward the sun or away, the greater our seasonality – the warmer our summers, the colder our winters. The cycle, from 22 degrees to 24 1/2 and back, takes roughly 40,000 years.

Ice Ages today recur on a 100,000-year cycle, but they haven’t always. Until some 900,000 years ago, they recurred on a 40,000-year cycle.

Judged solely by brightness, there’s nothing impressive about Polaris, the north star. What makes Polaris special (to us) is that it hovers over the north pole.

Pulled by sun, moon and planets, Earth wobbles, like a toy top on a table. The wobble causes the north pole to point, tonight, toward Polaris; but, 12,000 years from now, the north pole will point toward Vega, a future (and, 12,000 years ago, a past) north star.

The wobble, too, affects the advance and retreat of the ice.

These orbital cycles – dubbed “Milankovitch cycles,” for the mathematician who, almost a century ago calculated how they would affect sunlight striking the Earth – are the pacemakers of the Ice Ages, triggering the ice to wax and wane.

But while orbital cycles keep the beat, it’s their interaction with myriad other factors – the subtly changing brightness of the sun; the jig-saw puzzle pattern of continents, created by plate tectonics; ocean currents; patterns of vegetation; greenhouse gases – that ultimately determine Earth’s climate.

North stars of the past, present and future shine in the night sky at this time of year. We’ll point them out – and see just how much our planet wobbles – at the Nevada County Astronomers’ Sky Watch on Saturday night at the North Star House in Grass Valley. It begins at 8 p.m., and it’s free. Bring the kids.

Al Stahler teaches science classes to students of all ages, and talks about science on KVMR-FM. He can be reached at stahler@kvmr.org


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