The waning moon hung pale in the morning sky today as I walked to my meeting. Tomorrow it will still be there, but further east, and so every day until it disappears into the glare of the sun at new moon.
West to east flies the moon, as west to east rolls the Earth beneath my feet, counting out the days of my life. So too for the Earth itself as it marches through the seasons of the year, revealing constellations further east in my evening sky each month. And so the other planets as they crawl slowly across those constellations.
Last weekend there was a tremendous snowstorm up at the cabin. Almost three feet of snow fell over a couple of days, and the wind blew it into deep drifts. Since we felled the beetle-killed trees there is nothing to slow the snow scoured off of the lake, so it flows unimpeded up and over the cabin until it falls out of still air in the huge eddy east of the cabin.
West to east flows the snow, scouring the back door so we can still get in, but burying the front door and the roof under the winter whiteness. On a windy winter day at treeline in the Medicine Bow it’s hard to stand and almost impossible to talk. West to east, the crystal-laden air bites my nose and crams words unspoken back down my throat.
Where does it come from, this wild west wind of winter? What makes it blow?
It comes from the spin of the Earth itself. It’s the same spin that twirls the planet like a top, that hurls the Moon around each month, sweeping sunspots across the solar disk, flinging the Earth around the Sun each year, driving great Jupiter from west to east across the sky from year to year. Jupiter’s dancing Galilean moons and the swirling rings of Saturn are but eddies in this great vortex of celestial spin.
The wind blows and blows for billions of years, eroding the very bones of the Earth, yet it never tires, never runs down. How can this be?
In the tropical Trade Wind belt, the planet spins faster than the air, so surface friction adds planetary vorticity to the atmosphere. As it flows toward the dark winter pole, the spinning atmosphere moves closer to the Earth’s axis of rotation like the fists of an Olympic skater setting up a triple Lutz. The warm air flows poleward, the cold air equatorward, and spin is exchanged between the solid rock and the overlying gas. Yet the overall spin, the angular momentum of the planet, is perfectly conserved.
When the Chinook winds blow down the Rocky Mountain front to eat the snow in our cities, the day lengthens by a millisecond or two. The solid Earth loans its spin to the air as it crashes in great waves over the Continental Divide. As the roaring wind is dissipated by friction, it’s given back to the rocks below.
There are no brakes on the primordial vorticity!
The only semblance of spindown is the friction of the tides. The Moon pulls the tide up and away from the nearside of the Earth, and pulls the Earth away from the air and ocean on the far side. As the Earth spins under this two-sided tidal bulge it slows just a little over geologic time.
The year is the same length as it was in the Devonian Period 400 million years ago, but the Earth spun 410 times instead of 365 in those years. Where did the extra angular momentum go, as the planet’s rotation slowed from 1/410 of a year to 1/365?
It went to the Moon! The Earth’s loss of spin to the tides has been precisely matched to the acceleration of the Moon’s orbital speed. As the Earth spun down, the Moon climbed to a higher orbit. Our longer days are perfectly matched to longer months.
On clear nights this time of year, the bright stars of winter outline the arc of the Orion arm of the Galaxy. The cosmic spin of the Galaxy has carried our little solar system around perhaps 25 times since the Sun shone for the first time.
Peering out into the winter night, it’s obvious that the spin of the Galaxy and the spin of the solar system are not aligned.
The primordial vorticity was imparted to the solar system when the great cloud of gas and dust collapsed under its own weight out of our natal star cluster 5 billion years ago. An eddy within an eddy of the spinning Galaxy, it spun faster and faster as it fell into itself under gravity. The rings of Saturn, the moons of Jupiter, and the west wind of winter are eddies that remain from that colossal fall.
Yet here we stand, with the wind of the world and the slow march of the lunchtime Moon to remind us of our origins.
