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So we're halfway then--we've got one of the two numbers we need
to know our position on the earth--latitude.
So, this unit is about covered up but not quite. Longitude is much more difficult.
Do you have some intuition for why longitude is so much more difficult. Let's take a look at these lines.
Here we've drawn five lines of the longitude and in the middle we have, of course, the equator.
Now, the equator wasn't chosen randomly.
The earth rotates around an imaginary axis that goes through the center of the earth.
Now the equator define this sort of flat plane through the middle of the earth--
that makes a right angle with the axis of rotation.
The equator isn't some sense real. It's based on actual physical property of the earth.
Now let's see. Which of these lines is the 0 line of longitude.
Is it line 1, 2, 3, 4 or 5, and I've labeled them here.
If you can't tell these to my drawing, you can also select this option.