varies because of the mass gravitational attractions among Earth, the Moon, the Sun, and other planets. These changing gravitational attractions cause variations in the Earth's angle, position, and path; they even slightly alter the positions of the seasons. Slight variations in the Earth's orbit can lead to changes in the distribution and amount of sunlight reaching the Earth's surface.
There are three aspects of the Earth's orbit that change periodically and have an influence on climate—eccentricity, tilt, and precession. Not only do they vary throughout time but they all vary in unique cycles, or periodicity, and the interaction of the cycles also affects the climate. These orbital variations are called the Milankovitch cycles after Milutin Milankovitch, a Serbian astrophysicist.
The Milankovitch cycles are also associated with glaciation and the Earth's past glacial and interglacial cycles. In fact, glacial episodes are related to the cyclical changes in the Earth's orbit around the Sun. Cyclic variations in the Earth's eccentricity, axial tilt, and precession is what creates differences over time in the seasons and the amount and intensity of sunlight reaching various parts of the Earth's surface. During some parts of the cycle, the Earth receives increased amounts of solar radiation; at other times it receives decreased amounts. Clima-tologists believe this plays a significant part in influencing the advance and retreat of glaciers because it impacts the seasonality and location of incoming solar energy.
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