Earth's Structure and Plate Tectonics

Earth is the third planet from the Sun, orbiting at an average distance of about 150 million kilometers, a span astronomers use as the baseline unit known as the astronomical unit. This position places Earth within the Sun's so-called habitable zone, the narrow range of orbital distances in which liquid water can persist on a planetary surface, a condition central to the emergence and maintenance of life. Earth's rotation on its axis, completed roughly every 24 hours, produces the daily cycle of day and night, while its revolution around the Sun, completed in about 365.25 days, defines the year. Critically, Earth's axis is tilted approximately 23.5 degrees relative to the plane of its orbit; this tilt, rather than variations in distance from the Sun, is what produces the seasons, since it changes the angle at which sunlight strikes different latitudes throughout the year. The tilt and the planet's spherical shape together explain the latitudinal zonation of climate, from the intense, near-vertical sunlight of the tropics to the oblique, weaker illumination of the polar regions, a pattern that underlies nearly every subsequent topic in physical geography, from atmospheric circulation to the distribution of biomes.