Which way is down? Falling balls and planets

When you jump, you come back down. When you drop a pencil, it falls. But what does “down” really mean? Down is not a place in the sky or on a map. It is the direction of the pull of gravity, and Earth’s gravity pulls toward the centre of the Earth. A plumb line shows this. Hang a small heavy object on a string, and the string settles pointing straight toward the middle of the Earth. Builders use plumb lines and levels to make walls straight. Because the Earth is round, “down” points in different directions for people in different places, but for everyone it points toward the centre. A child in Aswan and a child on the other side of the planet both feel pulled toward the Earth’s centre. Nobody falls off. To trust a measurement, scientists repeat it and calculate an average.