Alan Stahler: Seasons change |

Alan Stahler: Seasons change


Summer is over. Winter is coming. The seasons are changing.

The globe of the Earth, on teacher’s desk, sits at a jaunty angle – the same angle at which our planet is cocked, as we circle the sun. That slant tilts us toward the sun in summer, and makes the summer sun strong; it tilts us away from the sun in winter, and makes the sun in winter wimpy.

When a chef makes pizza, the pie does not start out flat. A pizza begins as a round ball of dough. The chef spins the dough over her head, to flatten it out.

Planet Earth spins fast – all the way ‘round, in just a day. Earth is not made of pizza dough, but – spun fast enough – even rock will stretch and flow, making Earth bulge at the beltline, at the equator.

The first-quarter moon hangs high in the sky tonight (Thursday). The bulge at Earth’s beltline gives lunar gravity something to pull on. Pulling on that bulge, over thousands of years, the moon changes Earth’s tilt, which changes how weak or strong sunlight can get. Changing our tilt changes our seasons, making summers hotter or cooler, winters colder or warmer.

Climate scientists have a name for the familiar seasons – summer, fall, winter, spring – that result from Earth’s tilt: Tilt-seasons.

Tilt-seasons are not the only seasons we enjoy here on Earth.

We’ve all learned that the distance from Earth to sun is 93 million miles. But 93 million is really just an average. For half our orbit, we’re closer than average; for the other half, we’re farther away.

Sitting ‘round the campfire, if we want to get warmer, we move closer to the fire. Just so, when Earth is in that part of its orbit closer to the sun, Earth warms up; farther from the sun, Earth cools down. This divides our orbit – divides the year – into another set of seasons: Distance-seasons.

For some months now, Earth has been moving closer to the sun. We’ll reach our closest point to the sun next January. So even as our tilt-seasons are cooling, our distance-seasons are warming. Distance does not affect Earth’s temperature nearly as much as tilt, but it does make our winters a tad warmer than they would be otherwise … and makes our summers a tad cooler.

Much as the moon tweaks our tilt-seasons by yanking on our beltline bulge, our distance-seasons are also tweaked … though not by the moon.

The first quarter moon will shine high tonight, as soon as the sun goes down. As the sky grows darker, close to the moon will appear a bright “star.” But it’s not a star … it’s planet Jupiter.

Jupiter – largest planet in the solar system – is so massive, its gravity pulls Earth – slightly – away from the sun. Tweaking our distance from the sun, Jupiter tweaks our distance-seasons.

Distance-seasons are totally unrelated to tilt-seasons. But that doesn’t stop distance-seasons and tilt-seasons from working together, to alter Earth’s climate.

The rocks around Bowman lake, off SR-20 – like rocks in many parts of the Sierra – record times when distance-seasons conspired with tilt-seasons to throw our planet into an ice age … and then, ten thousand years ago, to pull us out again.

My thanks to friend John, former 7Hills science teacher, who suggested writing more about extraterrestrial influences on Earth.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at

Glacial striations, carved into bedrock by stones at the base of a flowing glacier, are souvenirs of the last ice age.
Photo courtesy USGS

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