Why does the Sun rise in the east and set in the west?



The Sun, the Moon, the planets, and the stars all rise in the east and set in the west. And that's because Earth spins -- toward the east.

For a moment, let us ignore Earth's orbit around the Sun (as well as the Sun's and solar system's revolution around the center of the Galaxy, and even the Galaxy's journey through the universe). For the moment, let us just think about one motion - - Earth's spin (or rotation) on its axis.

Earth rotates or spins toward the east, and that's why the Sun, Moon, planets, and stars all rise in the east and make their way westward across the sky. Suppose you are facing east - the planet carries you eastward as it turns, so whatever lies beyond that eastern horizon eventually comes up over the horizon and you see it!

People at Earth's equator are moving at a speed of about 1,600 kilometers an hour -- about a thousand miles an hour -- thanks to Earth's rotation. That speed decreases as you go in either direction toward Earth's poles. In the state of Texas, you'd moving at about 1,400 kilometers an hour due to rotation. If you're in southern Canada, you're moving at only about a thousand kilometers an hour. Now think about what would happen if you stood exactly at the North Pole. You'd still be moving, but you'd be turning in a circle as Earth spins on its axis.

You may wonder why you don't feel this speed: it's because human beings have no 'speed organs' which can sense absolute speed. You can only tell how fast you are going relative to something else, and you can sense changes in velocity as you either speed up or slow down. But we cannot really tell whether or not we are just moving at a constant speed unless something else tips us off!

Think about this:

Suppose you are in a car traveling down the road. How can you tell how fast you are going? The speedometer tells you how fast your wheels are turning, but you could be standing dead still, spinning your wheels trying to get off a patch of ice, so let's remove the speedometer from the car. As you go faster, your car may vibrate more because it's working hard, but these vibrations only tell you the car is working hard, not what velocity you are moving at. So get a good car and some cushions to remove the vibrations you feel. Then get some good earplugs so any misleading sounds won't distract you. And paint your car windows black so that the motion of objects relative to you don't throw you off. Remember that when it comes to the rotation of the Earth, everything around you is moving at the same speed - you, the trees, the houses, your pet dog, everything. OK, now, how fast are you going? You have no way to tell. You don't feel like you're moving. You feel just as you would if you were standing still! Human beings have no ability to tell absolute motion.


The StarChild site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC.

StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A. Whitlock
Responsible NASA Official: Phil Newman

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