The radius of our planet has been known since the time of the ancient Greeks to be about 6400 km (see More Precisely 1-3). More precise measurements are now routinely made by spacecraft, yielding a value of 6378 km.

We saw in Chapter 2 how we can measure the mass of Earth, or any other astronomical body, by observing its gravitational influence on some other nearby object and applying Newton's law of gravity. (Sec. 2.7) By studying the dynamic behavior of objects near our planet—be they baseballs, rockets, or the Moon—we can compute Earth's mass to be approximately, 6.0 10²⁴ kg—6000 billion billion metric tons. A more accurately measured value is 5.974 10²⁴kg. The Earth Data box lists in detail some of Earth's physical properties; however, we will use rounded-off numbers throughout the body of the text.

Dividing Earth's mass by its volume yields an average density of around 5500 kg/m³. (Sec. 6.2) This simple measurement allows us to make a very important deduction about the interior of our planet. The water that makes up much of Earth's surface has a density of 1000 kg/m³, and the rock beneath us on the continents, as well as on the seafloor, has a density in the range 2000—4000 kg/m³. We can immediately conclude that, because the surface layers have densities much less than the average, much denser material must lie deeper, under the surface. Hence we should expect that much of Earth's interior is made up of very dense matter, far more compact than the densest continental rocks on the surface.

Based on measurements made in many different ways—using aircraft in the atmosphere, satellites in orbit, gauges on the land, submarines in the ocean, and drilling gear below the rocky crust—scientists have built up the following overall picture of our planet. As indicated in Figure 7.1, our planet may be divided into six main regions. In Earth's interior, a thick mantle surrounds a smaller, two-part core. At the surface we have a relatively thin crust, comprising the solid continents and the seafloor, and the hydrosphere, which contains the liquid oceans and accounts for some 70 percent of our planet's total surface area. An atmosphere of air lies just above the surface. At much greater altitudes, a zone of charged particles trapped by our planet's magnetic field forms Earth's magnetosphere. Virtually all our planet's mass is contained within the surface and interior. The gaseous atmosphere and the magnetosphere contribute hardly anything—less than 0.1 percent—to the total.

Figure 7.1 The main regions of planet Earth. At Earth's center lies our planet's inner core, about 2600 km in diameter. Surrounding the inner core is an outer core, some 7000 km across. Most of the rest of Earth's 13,000-km diameter is taken up by the mantle, which is topped by a thin crust only a few tens of kilometers thick. The liquid portions of Earth's surface make up the hydrosphere. Above the hydrosphere and solid crust lies the atmosphere, most of it within 50 km of the surface. Earth's outermost region is the magnetosphere, extending thousands of kilometers out into space.