Falling through space affects climate
Earth’s seasons are determined by our jaunty 231/2 degree tilt (seen on any classroom globe). In summer, we tilt toward the sun during the day. This gives us long days, intense sunshine and warm weather. In winter, we tilt away by day, giving us short days, weak sunshine and cold weather.
The tilt of the Earth, toward or away from the sun by day, determines our seasons; but other factors can make summer or winter warmer or cooler. In its orbit around the sun, Earth traces out an ellipse (not to be confused with an oval, an egg-shaped figure that is broader at one end than the other).
A circle is a special sort of ellipse, one with zero ‘eccentricity.’ If our orbit were perfectly circular, we would always be the same distance from the sun: 93 million miles.
But our orbit is not a circle; it’s slightly eccentric – a sort of ‘squashed circle.’ This carries us first closer to and then farther from the sun.
The batter swings and connects with the ball. As it heads for the outfield, the ball moves upward fast. But gravity pulls it back toward the Earth, slowing it down.
Halfway to the outfield, the ball reaches the top of its trajectory, its upward velocity dropping to zero, and it begins to fall – faster and faster – back toward Earth. Much like a ball, Earth is ‘falling’ around the sun.
Last July, we reached the ‘top’ of our trajectory, the point in our orbit farthest from the sun. Since then, we’ve been falling back down – faster and faster. Tomorrow night – at 9 p.m. in the dead of winter – Earth will reach perihelion, the point in our orbit at which Earth and sun are closest. Our distance from the sun will then be, not 93 million, but just under 911/2 million miles.
Since Earth moves faster and faster as it falls toward the sun, we’re moving fastest in the months before and after perihelion – the months from autumn, through winter, and into spring.
And since we slow down as we move away, we’re moving most slowly during the rest of the year, from spring, through summer, and into fall. The calendar reflects our motion through space.
Beginning with the spring equinox (March 20), count the days of spring, then summer, and then fall, until you get to the autumn equinox (Sept. 23). This is the ‘summer half’ of the year, when Earth is farthest from the sun.
Next, count the days beginning with the autumn equinox (Sept. 23), moving through the winter solstice and ending up with to the spring equinox (March 20).
Since we’re moving most slowly during the ‘summer half’ of the year, this part of the journey around the sun takes longer than the ‘winter half’ by roughly a week.
Closer to the sun in early January, Earth receives 7 percent more sunshine than in early July. Winters aren’t as cold as they would be, if the situation were reversed, nor are summers as warm.
Twelve thousand years ago, the situation was reversed; Earth was closest to the sun in summer and farthest in winter. Northern hemisphere summers were warmer then, and winters were colder.
The combined effects of such variables in Earth’s orbit, and a host of other factors, cause our climate to change – to drop in and out of ice ages.
Now, of course, the question is whether – and how much – we ourselves are changing the climate by changing the atmosphere’s chemistry.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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