<P> Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter . However, these effects are not uniform everywhere on the Earth's surface . Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator . </P> <P> The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend . Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the onset of an ice age for two reasons: There is less overall summer insolation, and also less insolation at higher latitudes, which melts less of the previous winter's snow and ice . </P> <P> Axial precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of 25,771.5 years . This motion means that eventually Polaris will no longer be the north pole star . It is caused by the tidal forces exerted by the Sun and the Moon on the solid Earth; both contribute roughly equally to this effect . </P> <P> Currently, perihelion occurs during the southern hemisphere's summer . This means that solar radiation due to (1) axial tilt aiming the southern hemisphere toward the Sun and (2) the Earth's proximity to the Sun, both reach maximum during the summer and both reach minimum during the winter . Their effects on heating are additive, which means that seasonal variation in irradiation of the southern hemisphere is more extreme . In the northern hemisphere, these two factors reach maximum at opposite times of the year: The north is tilted toward the Sun when the Earth is furthest from the Sun . The two forces work in opposite directions, resulting in less extreme variation . </P>

The feature of earth's movement that takes about 26000 years is