We stick to Earth’s surface without giving it a thought. Yet can anyone honestly explain gravity, the most far-reaching of nature’s four forces? Newton said it was a force. Einstein insisted it wasn’t a distinct entity but simply how motion unfolds when space-time has been distorted. To be honest, nothing satisfactorily explains it, or why it doesn’t blend with nature’s other forces, no matter how hot or strange the circumstance.
Any kid who’s belly-flopped into a swimming hole knows that the higher the dive, the more it hurts. That’s because falling for a single second makes you hit the surface going 22 miles an hour. Each additional second you’re airborne makes you land another 22 miles an hour faster. It’s pretty simple.
If you want to stay in the air for exactly one second, you’d have to jump from a height of 16 feet, or one-and-a-half stories. If you land on sand or a wet lawn, it might not even hurt. To stay aloft for two seconds, however, means leaping from a five-story rooftop, a very bad idea. You’d now accelerate to 44 miles an hour, and such an impact is usually not survivable. So humans, unlike squirrels, have a very narrow range of safe falling possibilities. One second of plummeting is usually okay. Two seconds means death.
No ancient culture had the concept of a downward pulling force. It took until modern times for this gravity business to become dominant for space travel and such. Meanwhile the parallel topic of air resistance also developed and was central to aircraft engineering and sky diving. It was always a natural design feature in the animal kingdom that explains why cats and squirrels can never accelerate to a lethal speed no matter how far they fall.
It was Johann Kepler, almost exactly 400 years ago, who declared that each planet speeds up as it approaches the Sun in its elliptical orbit, but travels more and more slowly as it heads away. So Earth continually slows down and speeds up. Turns out, this month, January, is when Earth is moving fastest in our orbit.
It probably doesn’t matter that you’re now zipping along at nearly 19 miles a second instead of your more customary 18 ½. The only practical consequence is that we have less time to fully complete one rotation on our axis when we’ve moved far enough in our orbit for the Sun to next be positioned due south on the meridian. Which throws sundials off and makes our clocks up to 16 minutes wrong if we use it to mark noon as defined by the Sun standing highest. Since such activities are nowadays often absent from our to-do list, our fast January speed pretty much happens with very few noticing or caring.
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