Introduction
The major physical properties of the earth can be measured with a high degree of precision. They include:
- size - 6,378,099 meters equatorial radius
- shape - oblate spheroid, almost spherical
- mean density - 5.517 gm/cm3
- gravity - 9.8017 gm/sec2
- moment of inertia - 0.331 mr2
- magnetic field - 8.09 x 1025 emu dipole moment
- geothermal flux - 1 x 1028 erg/yr
The interior of the Earth is not accessible beyond about 10 kilometers, so the internal structure is inferred from indirect evidence. The difference between rock densities at the surface and the total mass of the earth is strikingly different. The average density of the earth is 5.5 gm/cm3, but the surface rocks have densities of only 2.7 to 3.0 gm/cm3, so densities must be higher in the interior and the composition must change. This is confirmed by the moment of inertia since for a uniform earth sphere the moment should be 0.4 mr2, the difference from the measured value and the density difference between crustal rocks and total mass all indicate that densities increase inward and that the mass is concentrated toward the center of the earth.