November 05, 2017: WINTER AHOY!
Last night nearly all people in the USA set their clocks back one hour in order to save us an hour of daylight for useful work. (I must confess, though, I’ve never been able to figure out whether this putative time saving occurs in the morning or the evening!) This annual ritual marks our transit into those gloomy days of winter when children, half asleep, must negotiate knee-deep snow to board school buses in the dark—and reverse the ordeal nine hours later, again in the dark. (Our children must wonder what kinds of sadists we are to make them go to school in winter. So do I!)
But why does the sun forsake us during winter in the northern latitudes for the warmer climes of Australia and South Africa? We know that all energy on earth comes from the sun, but why does the amount of energy vary with the seasons? Is it because we are farther from the sun in winter? No! The distance to the sun varies by only 3% from summer to winter: 141 million kM at its closest (“perihelion”) and 150 million kM at farthest (‘aphelion”). It varies because planetary orbits are not exact circles but ellipses.
The real cause of the seasons is that the earth’s equatorial plane is tilted away from the plane in which the planets revolve around the sun. But that tilt does not bring the summer or winter side of the earth nearer to the sun. (The summer latitudes are only about 0.005% nearer to the sun than the winter latitudes, or by a few thousand kilometers, which is nothing compared to the overall earth-to-sun distance of 141-150 MILLION kilometers.) So, if distances are not the causes of the seasons, what is? It is entirely a matter of angles. The tilt makes the sun’s rays hit the winter side of the earth at shallower angles than the summer side. A low angle of incidence means sun’s rays travel through thicker slices of the earth’s atmosphere where they get absorbed. It’s for the same reason the sun’s rays are dimmer and cooler at sunset.
But why is the earth’s axis of rotation tilted? Planetologists think that, eons ago, an impact from a rogue planetoid knocked the earth askew. The entire heavens are one immense demolition derby; we now know that 65 million years ago an asteroid collided with the earth and caused the fifth and latest mass extinction of life on earth, including the disappearance of the dinosaurs. From space, collision craters may be seen on earth; the moon is covered with them, and so is Mars. (Note that planetary tilt is so large in the case of Saturn it is visually obvious in the diagram below, especially when you consider its rings.)
The degree of tilt (aka “obliquity”) is 23.5 degrees now, but it varies: between 21 and 24 degrees in time cycles of about 40,000 years. That variation is a hint that the rocking/precession of the earth arising from that epic collision with a rogue projectile still continues. When the obliquity increases, the temperature differences between summers and winters become greater. (Next time it increases I will petition the schools to shut down in winter and let my nietos stay in bed!)