Earth’s orbit is currently fairly circular around the Sun. However, this is not always the case. Our planet experiences shifts from the current circular orbit to a much more elliptical one during which the difference between perihelion and aphelion is very marked. The entire cycle—from circular to elliptical and back again—takes about 95,000 years to complete. During the periods when the Earth is in its more elliptical orbit and ventures farther from the Sun, ice ages tend to occur. The theory was first formulated by Serbian geophysicist Milutin Milankovic (also spelled Milankovitch, 1879–1958), who was well known for his research into ice ages. His theories were later verified, in 1976, through the study of sediment cores taken during deep-sea explorations.