Walking across the field to school the other morning:
Younger offspring: How come the earth is round but when you're walking on it, it looks flat?
Dr. Free-Ride: The earth is pretty big. We're so tiny compared to the earth that the curvature isn't something we notice.
Elder offspring: This field would barely be a speck on even a really big globe.
Dr. Free-Ride: Yeah, so it's not a big enough chunk of globe that you appreciate the roundness.
Younger offspring: OK. And where's the top?
Elder offspring: Huh?
Younger offspring: Is the North Pole the top of the earth, or is the South Pole really the top?
Dr. Free-Ride: That's a matter of convention. Most of the maps and globes made in the Northern Hemisphere have traditionally put the North Pole at the top, but that's pretty arbitrary. It would be just as reasonable to put the South Pole at the top.
Elder offspring: Or to make a globe like the one we have at home where you can change what's at the top and what's at the bottom however you like.
Dr. Free-Ride: Yeah, I like that feature.
Younger offspring: But isn't there a top?
Dr. Free-Ride: What do you mean?
Younger offspring: Things have tops and bottoms. If we're right side up here, why aren't the people on the other side off the earth upside down?
Elder offspring: They aren't upside down.
Younger offspring: I know they aren't upside down! But why?
Dr. Free-Ride: That's a good question. If you didn't know the earth was spherical, up and down would be pretty easy.
Elder offspring: Up is away from the ground and toward the sky, down is toward the ground and away from the sky.
Dr. Free-Ride: Gravity sets that one up for us pretty nicely. And, since we're so small compared to the sphere of the earth, each of us feels like we're on a flat piece of land.
Younger offspring: Or sea.
Elder offspring: And the gravity gives us up and down the same way -- away from the earth and toward the earth -- no matter which part of the earth's surface we're on.
Dr. Free-Ride: Uh huh.
Younger offspring: It still seems like there ought to be a top and a bottom to the earth.
Dr. Free-Ride: Globes kind of make it seem that way, don't they. But the earth isn't an object like a globe sitting on a surface. It's a planet hurtling through space.
Younger offspring: How come we don't feel the earth spinning?
Dr. Free-Ride: How come you don't feel it orbiting the sun, either?
Elder offspring: Maybe we've just gotten used to it.
Dr. Free-Ride: This is another good question. If the earth is zipping through space at a few million kilometers per second, you'd think we should feel it.
Younger offspring: Why don't we feel it?
Dr. Free-Ride: When you're in a car, how do you know how fast it's going?
Elder offspring: Look at the speedometer.
Younger offspring: Or ask the driver, if you can't see the speedometer.
Dr. Free-Ride: Is there any other way to tell, when you're riding in the car, if it's going really fast or really slow?
Younger offspring: Maybe by how fast things outside the car seem to go by.
Elder offspring: Or if the windows are open, by how fast the air is coming in.
Dr. Free-Ride: But it's really hard to tell, if you're sitting inside a closed car, how fast it's moving just from how fast it feels like it's moving.
Younger offspring: Airplanes are like that, too.
Elder offspring: If you were on top of a train you could tell, though. If you jumped up and it was moving, you'd land on top of a different train car from the one you jumped up from.
Younger offspring: Or you might land on the tracks, if the train moved all the way past where you jumped from by the time you landed.
Dr. Free-Ride: That's a good thought experiment. How is being on top of the train car different from being inside a train car?
Younger offspring: Inside the train car, you can't fall off the train?
Elder offspring: I bet inside the train car it's like being inside a car or a plane -- you can't really tell how fast you're going just from how it feels.
Younger offspring: But if you jump really high inside a train that's moving, would you land in a different spot because the train moved while you were jumping?
Dr. Free-Ride: That's an experiment I'd actually let you try.
Younger offspring: If I jump really high on the earth, I land in the same place that I jumped from, even though I know the earth is moving.
Dr. Free-Ride: It's hard to really grasp why that should be. It really does feel to us like the earth is stationary. But maybe it feels that way because all this nifty atmosphere around us is moving with the earth.
Elder offspring: So if the earth was a car, we'd be inside the car with the atmosphere.
Younger offspring: And we can't put the windows down to feel how fast we're moving.
Elder offspring: That's a good thing. If you put the windows down, you could feel how fast the earth was moving, but what would you breathe?
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Hi Janet. First time I read your blog. Nice post.
I found myself answering these kind of questions to friends and family. Last weekend I was explaining a friend why is not true that an airplane goes faster if it travels agaisnt the rotation of the earth and viceversa.
Asimov wrote one good book to answer questions like these. I can't remember the title right now.
Saludos (greetings)
Do your kids ever say, "Mom, what the fuck are you talking about?"
Did you tell them that if they jumped up and down inside a moving train they'd land on the same spot they jumped from? I can see merits and disadvantages to hearing if from you vs. trying it themselves. On the one hand, it's much better to actually do the experiments and try things out for themselves, rather than just trusting the authority. On the other hand, when will they next be on a train?
The curvature is 0.6 ft/mile, so your house was probably built on a flat earth.
How old are the sprogs? I'm guessing younger sprog is around 7 or 8? I've been getting questions like these from my dotter (6), and trying to come up with answers that don't provoke an "I don't know what on earth you're talking about...". Which, alas, she got from me.
PhysioProf: Not in those words, no.
Susan: The sprogs are fans of BART, so maybe that's the place to mount an empirical test. From there, we can move on to predictions about cannon balls dropped from the Leaning Tower of Pisa.
Jim: I have no doubt that there was some human-initiated flattening, especially of the field we were crossing.
OmegaMom: At this point the younger one is 7 and the older one is creeping up on 9. Also, I'm starting to think that my ability to avoid (or at least delay) "I don't know what on earth you're talking about..." stems from experience in graduate philosophy seminars -- we got good at constructing plausible theories about what someone might mean, and running through a bunch of them (did you mean X? or maybe Y? ...) before giving up.
Might be fun to do a compare/contrast on this dialogue and one of Galileo's _Dialogues_
Sprogs,
Way back when the ancient Egyptians set the upriver Nile as north. Since upriver on the Nile is our south, that was the Egyptian's north.
First, the Earth is only moving about 30 km/s around the Sun, not "a few million kilometers per second". We can't feel the motion of the Earth around the Sun because the Earth is in free fall. However, we CAN "feel" the rotation of the Earth on its axis by making careful measurements. For instance, you can build a giant pendulum and observe its precession (e.g., the Franklin Institute Foucault Pendulum).
Very interesting, I like how it branches off to answer many questions instead of the one.
Is "a few million kilometers per second" the estimated speed of the Earth traveling around Sol, traveling around the Milky Way, traveling away from the center of the universe?