Al Stahler: Swing close, swing away
The closer you get to the campfire, the warmer you get … works for people, works for planets.
Year after year, Earth circles the sun … but the figure we trace out in space is not a true circle. If our orbit were indeed a circle, we’d always be ninety-three million miles from our star … the distance we learned in school. But ninety-three million miles is only our average distance from the sun.
Playing catch, we don’t simply throw the ball straight to our friend. We also throw the ball upward, to allow for gravity, pulling the ball downward. The path of the ball is thus a mix of motions: Straight and up-and-down.
We wind up and pitch, hard as we can. But, fighting gravity as it moves upward, the ball slows down. Halfway there — at the top its curve — the ball moves at its slowest.
Once past its high point, the ball begins to fall downward. Now gravity pulls it downward, and the ball flies faster and faster, until it hits our friend’s glove.
Four-and-a-half billon year ago, the universe began a game of catch. Earth was the ball flying over the sun. The sun pulled – still pulls – Earth downward. Fortunately, Earth moved so fast, it never fell all the way down to the sun. Rather, Earth fell around the sun. We still fall around the sun, one complete circuit per year.
Because we do not orbit the sun in a perfect circle, we’re sometimes closer to the sun, sometimes farther away.
Playing catch with our friend, the ball moved most slowly when it was highest above the ground – farthest from Earth. Likewise, fighting the sun’s gravity, Earth moves most slowly when it’s farthest from the sun. After reaching our farthest – and slowest – point, we sped up again, as we fall closer and closer to the sun. Earth moves fastest when it’s closest.
A week-and-a-half ago – just after midnight, on the morning of the Fourth – Earth reached its farthest point from the sun. Today, we’re only a smidgeon closer … not ninety-three million miles, but ninety-four-and-a-half million miles from the sun.
Year after year – give or take a day – we are farthest from the sun on the Fourth of July. For the next six months, we’ll fall, faster and faster, toward the sun.
We can easily prove to ourselves that we’re now moving slowly. All we need is a calendar (I suggest a paper calendar) and a pencil to mark up that calendar (which is why I suggest paper).
Hypothesis (an educated guess): Since Earth moves slowly in summer, it should take us longer to go from spring equinox, through summer solstice, to fall equinox … than it takes to go from fall equinox, through winter solstice, to spring equinox … closer to the sun, we move faster in winter.
Let’s count the days. Starting with the day after equinox, March 21, count the days thru April, May, June, July, August … and stop, on the fall equinox, September 21. Jot down the number of days from spring equinox to fall equinox … the summer half of the year, when we’re far from the sun, moving slowly.
Now, beginning on the day after fall equinox, September 22, count the days through October, November, December, January, February … and stop, on spring equinox, March 20. Jot down that number of days, fall equinox to spring equinox … the winter half of the year, when we’re close to the sun, moving fast.
True or false: Summer lasts roughly a week longer than winter.
Farther from the sun, our summers are a touch cooler than they would be if we spent summers closer to the sun. Over tens of thousands of years, our orbit evolves, and we do, indeed, find ourselves closer to the sun in summer, rather than farther. In combination with other orbital changes, such factors send our planet into an ice age … or thaw us out of one. More on that in a future column.
Local astronomers will set up scopes for a star party on Saturday, July 23, at 9 p.m. at the intersection of Highway 49 and the old Downieville Highway. This event is free to the public — bring the kids, and a sweater.
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 may be reached at email@example.com.
It’s the second week in August in Nevada County, which means it is time for the Nevada County Fair. I’ve already been out to the fairgrounds to look around and am impressed by the work…
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