We can think of gravity as the invisible force that pulls two masses together. When we speak of mass, we're talking about the amount of matter in a substance. Density is a measure of how much mass is concentrated in a given space. Sir Isaac Newton discovered that as an object's mass increases, the gravitational attraction of that object increases. For example, a container filled with a more dense material like granite rock has more mass and thus more gravitational attraction than that same container filled with water. The Earth's Moon has considerably less mass than the Earth itself. Not only is the Moon smaller than the Earth, but it is only about 60 percent as dense as Earth. Thus, the gravitational attraction on the Moon is much less than it is here on Earth, and a person weighs less on the Moon. This weaker gravity is why we have the famous images of the Apollo astronauts taking "one giant leap for mankind" on the Moon's surface. On planet Earth, we tend to think of the gravitational effect as being the same no matter where we are on the planet. We certainly don't see variations anywhere near as dramatic as those between the Earth and the Moon. But the truth is, the Earth's topography is highly variable with mountains, valleys, plains, and deep ocean trenches. As a consequence of this variable topography, the density of Earth's surface varies. These fluctuations in density cause slight variations in the gravity field.