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With an equatorial diameter of 12,756 km, Earth is the largest of the Solar System's four rocky planets. Although it is generally portrayed as a sphere, the planet is very slightly oblate, i.e. it has a slight outward bulge at the equator and flattening at the poles. This means that the polar diameter is 43 km less than its equatorial diameter. As is the case with other non‐spherical planets, the reason for this difference is Earth's fairly rapid rotation, which causes it to bulge outward at the equator.

It is interesting to note that Earth's shape, defined by the planet's gravity field, or geoid, also changes a little over time. Satellite data have shown that the shape is influenced by major earthquakes and climatic events that cause changes in the mass of water stored in oceans, continents, and atmosphere.

Earth's bulk density is 5.52 g/cm³, higher than any of the other planets in the Solar System. Since the density of the rocks in Earth's crust is much lower than this, it follows that the material in the interior must compensate by having a much higher density.

Figure 3.18 Satellite data show a significant decrease in the area of summer Arctic sea ice since the late 1970s. These maps show sea ice extent in September 1979 (top) and September 2015 (bottom). The September 2015 ice cover was 1.81 million sq. km lower than the 1981–2010 average. This decades‐long decline, which affects how much solar radiation is reflected back into space, have been attributed to global warming.

(NASA)