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Earth’s orbit around the sun is not precisely circular, but elliptical, with the sun positioned slightly to one side of the center point. Currently, this brings Earth about 3% closer to the sun in early January (perihelion) than in early July (aphelion), with about 7% more solar energy reaching Earth at perihelion. The eccentricity, or “off-centeredness,” of the orbit varies over time in a complicated way. The net result is two main cycles, one that averages about 100,000 years and another that runs about 400,000 years. When the eccentricity is low, there’s little change through the year in the Earth–sun distance. When eccentricity is high, the sunlight reaching Earth can be more than 20% stronger at perihelion than at aphelion. This either intensifies or counteracts seasonality, depending on the hemisphere and the time of year that perihelion and aphelion occur. The last eight ice ages have come and gone roughly in sync with the 100,000-year eccentricity cycle, but it’s not yet clear how strongly the two are related (see p. 296).