Earth is a spinning ball, we are insects on its surface, yet we don’t feel ourselves rushing along – indeed, we aren’t even aware of the direction of our travel (eastward) unless we stop to think about it.
Why? Why aren’t we feeling an almighty wind? People around 1600 AD used this to cast doubt on the new Copernican cosmos, and Galileo answered them with a thought experiment: if you’re inside a smoothly traveling coach with its window closed, you don’t feel motion – the air and the furnishings around you stay still.
My ally Daniel Cummings has a set of simple activity-experiments for making you sense the speed of Earth’s rotation, by means of the speed of sound. He calls the speed of the ground you are standing on “the speed of day” – a beautiful idea. It is the same as the speed at which the Sun passes over your meridian.
The speed of day at a spot on the equator is about 1,000 mph, for the north pole it is zero, and in between, for latitude 42.97° – which is about Boston – it equals the speed of sound: 767 mph.
Watch a friend clap on the far side of a large field: the sight arrives instantaneously (well, in 1/3,000,000 of a second) and the sound takes about a third of a second to arrive. Its speed through the air to you was the same as your Speed of Day.
That’s amazing. Never thot much about our speed on the spinning Earth. At a Cleveland Indians baseball game I ventured up to the top row of the nosebleed section in right field. I saw the batter get a hit but I didn’t hear the crack of the bat until about half a second later. That gives me an idea of how fast we’re rotating.
Fireworks might be another fun way of imagining the speed of the turning of the Earth.
Speaking of track and field guns, timekeepers start their stopwatches when they see the flare or smoke from the gun (rather than listening for the sound).
Thanks for the shout out, Guy!
Would it make a difference if the friend clapped east or west of the observer?
Nope. I thought of using some more words on that, but didn’t. The sound is traveling through the air, which is standing still in relation to you. It could be traveling northwestward, or in any direction – relative to the ground. The speed of the whole scene is in its own eastward direction.
The Coriolis force may make a microscopic difference, but…
Won’t someone think of the barometric pressure!!!
If there were a gale blowing and your friend clapped upwind from you and then downwind from you, the difference between the sounds’ arrival times might not be discernible to the senses, but it would be measurable. Now that I think about it, in a gale your friend would need to make a louder noise than just clapping. Perhaps a track and field starting gun?