Published: 9 June, 2005

Gravity keeps you upright, even if you're at the South Pole

How is it that no matter where a person stands on earth they are always upright? Why aren't people in Antarctica upside down?

This is a great question! You're right, people in Antarctica ARE pretty nearly upside down, compared to people in Ithaca. To be more accurate about this, look at a globe of the Earth. As you know, the surface of the Earth is a sphere. If you find Ithaca, and then imagine a line that passes DOWN from there through the center of the spherical Earth (this line is a "diameter"), it will come out of the Earth on the opposite side. (This place is in the ocean near Australia, by the way - check it out). You could even put two little standing people on the globe, one at each place, and you can see that they are exactly upside down compared to one another. To compare to Antarctica, put a person near the North Pole. The diameter line now comes out in Antarctica. So people at these two places would say, correctly, that the other person is upside down!

But, as you said in your question, every single person on Earth thinks that he or she is upright! And the reason we all agree is that, no matter where we live on the Earth, our sensation of "up-ness" is determined by the direction of the FORCE OF GRAVITY at our place. The gravity force is the DOWNWARD pull on our bodies that we always feel. Most of the time, like when we're just standing or sitting there, the downward pull of the Earth is balanced by the upward force produced by the surface we're standing or sitting on. (Physicists like to call this a "normal" force.)

If you're adventurous, maybe you like to go bungee-jumping, or even parachuting out of an airplane. When you fall like that, the ONLY force on you is the downward force of gravity. And part of the thrill you feel is that since gravity is being allowed to act all by itself, you also lose your sense of which way "up" is! This can happen on a roller-coaster too, especially on the loopy parts.

But let's get back on Earth. Gravity points down, which is the opposite of up. So far, so good. If the earth were a flat surface, like a huge table, then everybody would agree that the "down" direction was the same direction. Earth DOES look like it's a huge table (if you ignore hills and valleys, of course) when we look around -- and human beings thought the earth was flat for a really long time!

But the Earth is a sphere, really, and so the meaning of "down" or "up" (the direction of gravity) is the direction of that "diameter" line we talked about earlier -- the line that goes straight "down" toward the center of the Earth. So the down direction is different at different places on the earth. People are "tilted" with respect to each other (but they all think they are upright!). And the further apart these places are, the more different is the downward direction. For example, people on the Equator are "sideways" compared to people in Antarctica. The biggest difference in direction is for people who are at "opposite ends of the earth," like Ithaca compared to somewhere near Australia. Those two people are truly exactly "upside down" compared to one another, even though they are both "upright."