This is something that still perplexes me. Why don’t we feel the earth move? Or worse yet, why aren’t we thrashed about by its incomprehensibly fast movement?
I recall seeing something on tv once where some ancient, but way-ahead-of-his-time, astronomer speculated that momentum is the reason. As an example, he explained a man on a moving horse tossing a ball up and down doesn’t necessarily feel movement, unless the horse’s speed increases or decreases. But that is what I don’t understand. Apparently the earth’s movement has increased and decreased over the years. I recall hearing some place (I forget where–but it was a reliable source) that the earth is moving slower than it was thousands?/millions? of years ago. Furthermore, I don’t know if this is relavent. But a couple of years ago I was seriously thinking of purchasing a sun dial. And to practice calculating sundial time (which is different from regular “mean solar” time), I used an almanac. The wierdest thing is, the solar day is longer and shorter depending on the time of year. To figure out the difference, you use a key in the almanac tables called “sun fast”. Anyways, I could be wrong, but I assume this difference is due to the earth spinning faster as it is closer to the sun.
The gravity of the Earth keeps you in place and you only feel motion when speed changes. Maybe the earth’s movement has increased and decreased over the years but not as dramatically as it would need to be in order for you to feel it.
You don’t feel the Earth move, because you don’t perceive “movement” or velocity with your senses; instead, you feel acceleration, which is the change of velocity with respect to time.
Think about when you are driving in your car. Going in a straight line down the highway, you don’t feel anything. However, if you go around a curve (changing direction) or speed up or slow down (changing speed), you feel the acceleration. When you change the direction or speed with with you are travelling, you are changing your velocity, which means you are accelerating.
The Earth is actually accelerating at it rotates on its axis and revolves around the Sun, but the acceleration involved is so slight that you do not perceive it.
Think of riding as a passenger in a car. If the car is moving along at a constant speed, you can’t really feel the motion. If the car changes speed from 65 MPH to 64.9999997 MPH you won’t detect that either. That’s about the same as asking why we don’t feel the change from the earth slowing down.
You’re right on the momentum bit - people don’t inherently know when they’re moving, they feel acceleration, as you said; or can work out they’re moving by comparing their position to a stationary reference point (eg. if you look at a tree outside your car, you can tell you are moving because your position is changing relative to the tree, which you know is stationary.)
The problem with the motion of the Earth is that you have no stationary point of reference to which to compare the Earth, and the acceleration/decelleration of the Earth is much to slow for you to perceive. The only time you can ‘sense’ the motion of the Earth is during large scale disturbances, such as earthquakes. (For example, the earthquake sparking the tsunami late last year moved the Earth about an inch.)
The length of the solar day is determined by the tilt of the Earth on its axis at a given time of year, not its speed of rotation. In the summer months Earth is tilted on its axis so you are closer to the sun, in winter you’re further away. (A full explanation can be found here: http://www.bbc.co.uk/weather/features/understanding/dayandnight.shtml)
In the northern hemisphere, that is. Obviously it’s the opposite for the southern hemisphere.
This is also subject to a long-term cycle. In another 50,000* years or so, the northern hemisphere will be the one closer in the summer.
*I just read the number but can’t refind it. It’s in the tens of thousands, but may be less than this.
Sundials need adjustment not because the rotation of the earth changes, but because its orbit around the sun is elliptical, not circular.
What we think of as a “day” is not one rotation of the planet; it’s the time from when the sun is in one point in the sky (say, noon) until it next reaches that same point again. One rotation takes 23 hrs, 56 mins; but in that time the earth has moved along in its orbit and it takes (on average) an extra 4 minutes of rotation to point back at the sun again. Google “sidereal day” if you need more explanation.
When the earth is closest to the sun, it travels a bit faster in its orbit, and moves farther along in 23 hrs 56 min , so it takes more than 4 minutes of extra rotation to get back to solar noon; when at its far point, it takes less. People long since decided to just use the average figure of 24 hours; since your sundial doesn’t know this, you need to adjust for the accumulated difference bewteen sun time and averaged time.
If you accept the fact that the earth is rotating you can certainly see the rotation. Go outside during a full moon just before sunset and watch the moonrise. It is surprising how fast it climbs above the horizon.
Actually, you are accelerating since you are not moving in a straight line (due to the curvature of the earth). But the force of gravity is (obviously) much, much stronger and what you “feel” is your own weight-- ie, gravity pulling you towards the earth. If there were absolutely no friction, the earth would slide right under you as it rotated, with gravity still holding you in place on the surface.
If you jump up into the air, you will not land in the same spot partly because the earth has rotated a bit before you land. Of course, you can’t stay up in the air long enough to detect that slight movemen. Even if you were to rise up in a balloon, other forces (eg, from the wind) would overwhelm the effect of the earth moving so you wouldn’t notice it.
If the surface of the earth near you is moving at a certain speed and direction (taken together, that’s velocity) and you are too, you move together. Since there’s no difference, you feel no motion.
If you are used to equating “motion” with “vibration”, as in a car, that is a mistake. If you equate it with “acceleration”, that is, too. A object moving with a constant velocity has no acceleration. And vibrations are introduced by other factors (engines, wind interaction, etc.).
A slight adjustment in the above statements is necessary for a very large rotating object like the earth. Indeed, there is a slight and continuous acceleration and change of velocity, but it is constant and unnoticable to the average bear, and is overcome by the larger force of gravity.
Same reason why you can get up and walk around in a 747 without being thrashed around even though it’s flying 500 mph. The air around you in an airplane is moving along with you at 500 mph also. Earth’s atmosphere rotates along with the planet so you aren’t going to feel any wind against your face as you would if you stuck your head out a sunroof.
Also, the acceleration/deceleration of the earth is so slight and occurs over such a long period of time that you’ll never feel it.
Again, same as being in that plane. If the pilot suddenly decreases speed from 500 to 400 you’ll feel it. But if he does it over the period of 15 minutes you’ll never feel it. Like driving in your car, if you accelerate and brake sharply all the time it’s going to feel eratic. Do it gradually over a distance and it’s a nice smooth ride.
Having re-read the OP, I see that I answered a slightly different question-- one concerning the rotation of the earth about its axis, not the movement of earth thru space. But similarly, it’s incorrect to say the earth is not accelerating. From any reference point you choose that is NOT on earth, the earth IS accelerating. The answer, though, is the same as I originally gave. The gravity of earth is by for the strongest force acting on us and that’s what we feel-- the other forces are too small to notice.
Just remember that you’re standing on a planet that’s revolving
revolving at 900 miles an hour
that’s orbiting at 19 miles a second, so it’s reckoned
a sun that is the source of all our power
We can’t feel the Earth accelerate, but we can detect it. The plane of oscillation for Foucault’s pendulum, as demonstrated in every science museum in the US, turns as time progresses. The forces that cause this are due to the acceleration of the Earth’s surface in it’s daily rotation. Similarily if one drops an object from a great height it will deviate measurably from falling straight down.
