Our daily experience leads us to believe in Newton's laws. When you drop a ball, it falls straight down. When you throw a ball, it travels in a uniform (compass) direction — and falls down. We appear to be in a constant gravitational field, but apart from that there are no forces acting on the ball. This isn't the full story, of course, as we are ignoring things like air resistance and the spin of the ball. Nevertheless, it seems to give a pretty good description of what we observe, and so we base our intuitive understanding of physics on it.

But it's wrong.

Yes, gravity is more complicated than this simple picture. The gravitational field of the Earth isn't really constant. And there are other nearby objects, notably the Moon, whose gravity acts on us. As discussed in the final chapter, this causes tides.

A bigger problem is that the Earth is round. Due East is not a straight line, defined in this case as the shortest distance between 2 points, as anyone who flies from San Francisco to New York is aware. In fact, if you travel in a straight line that (initially) points due East from my home in Oregon, you will eventually pass to the south of the southern tip of Africa! 1)

So East is not East.

But the real problem is that the Earth is rotating. Try playing catch on a merry-go-round! Balls certainly don't seem to travel in a straight line! Newton's laws don't work here, and strictly speaking they don't work on (that is, in the reference frame of) the Earth's surface. The motion of a Foucault pendulum can be thought of as a Coriolis effect, caused by an external pseudoforce. And a plumb bob doesn't actually point towards the center of the Earth!

So down is not down.