We have been talking about some of the effects that the human beings had on the earth. One you may not be aware of is that we actually begin to change the length of the day. In the other way to say, one day is the amount of time the earth needs to spin completely around an axis, the imaginary line around the center of the earth, from the north to the south. And of course there are a lot of physical causes that can affect the speed of the earth’s rotation, but there was only one that is direct result of the human activity. Since 1950, human beings have built about ten thousand artificial reservoirs all over the world. These reservoirs have redistributed tremendous amount of the earth water. When they are used to be in the area near the equator, the imaginary line surround the middle of the earth, it’s now the reservoirs in the areas of different latitudes. The latitude matters because, well, thinking the earth and axis, the equator contains the areas on the earth that are the farthest away from axis. So water has been redistributed from the equator regions, then wherever the water is, to it’s close to the earth axis. It’s like when ice skaters perform spins when those skaters put their arms enclose to their bodies, they spin faster. So the earth is spinning faster because the reservoirs have redistributed the water close to its axis. And because the earth was spinning faster, since 1950 the length of day has decreased by about 8 millionth of second. I know that doesn’t sound like much but significant in that this is the first time that human beings ever had measurable affect on the earth’s motion.