One evening in the late 1800s, Joseph Holden measured the water in a pail and set the pail on top of a post. By morning, if the earth was round and spinning on its axis, some of the water would slosh out and the pail would shift on the post or fall off entirely.
In the morning he found the pail exactly where he’d placed it, and after measuring the water, found that none was missing. Conclusion? The earth is flat.
The other day, I was sitting in my car in a parking lot. I had started the engine, but not yet shifted into gear, needing to check a message on my phone first. I caught a glimpse of movement from the corner of my eye and quickly stepped on the brake. Somehow my car was rolling forward.
It took a second to realize that my car hadn’t moved. The car next to me was in reverse and pulling out. I had lost my frame of reference and thought the other car was stopped and mine was moving. For an instant, it seemed real — real enough to make me step on the brake.
Frame of reference is a tricky thing. It fooled Joe Holden, and for a moment, fooled me.
The Earth is spinning on its axis at about 1,000 miles per hour. How come we don’t feel the ground spinning? How come the motion doesn’t make us lose our footing and fall down? And how come no water sloshed out of Joe Holden’s pail?
Frame of reference.
When we are moving at a constant rate, that rate becomes our frame of reference and we don’t feel the motion. Riding in the passenger seat of a car going 70 miles an hour, we don’t feel like we’re moving at all, particularly if our eyes are closed.
If we open our eyes suddenly, our frame of reference may be temporarily confused. It may look like the trees, houses, and signs are moving backwards and we are standing still, but it only takes a second for our brains to adjust.
We don’t feel a constant speed, but do feel when the car speeds up or slows down. If we jam on the accelerator, we are pressed back into our seats. If the car comes to a sudden stop, we lunge forward and the seatbelt bites into our chest.
The Earth is rotating at about 1,000 miles per hour, but we are going the same speed, so to our frame of reference, it doesn’t look or feel like the Earth is spinning. We stand or travel about on what seems like a non-moving surface, the way a fly can walk around on the dashboard of a speeding car.
Joe Holden probably thought that frame of reference was a bunch of scientific humbug. If he were alive today, I’d insist he fasten his seatbelt. Believing water should slosh out of a pail is one thing. Exiting through a windshield is another.
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