evidence: Earth's orbital changes have influenced climate, life forms for at least 215 million years [...] Scientists have for decades posited that Earth's orbit around the sun goes from nearly circular to about 5 percent elliptical, and back again every 405,000 years. The shift is believed to result from a complex interplay with the gravitational influences of Venus and Jupiter, along with other bodies in the Solar System as they all whirl around the Sun like a set of gyrating hula-hoops, sometimes closer to one another, sometimes further. Astrophysicists believe the mathematical calculation of the cycle is reliable back to around 50 million years, but after that, the problem gets too complex, because too many shifting motions are at play. [...] The planetary motions that spur climate swings are known as Milankovitch cycles, named for the Serbian mathematician who worked them out in the 1920s. Boiled down to simplest terms, they consist of a 100,000-year cycle in the eccentricity of Earth's orbit, similar to the big 405,000-year swing; a 41,000-year cycle in the tilt of Earth's axis relative to its orbit around the Sun; and a 21,000-year cycle caused by a wobble of the planet's axis. Together, these shifts change the proportions of solar energy reaching the Northern Hemisphere, where most of the planet's land is located, during different parts of the year. This in turn influences climate.