 Obliquity refers to the tilt of the Earth's axis. Over the last million years, axial obliquity has varied between 22.1 and 24.5 degrees perpendicular to Earth's orbital plane. Obliquity is the reason why Earth has seasons. The greater the Earth's axial tilt, the more extreme our seasons become, as each hemisphere receives more solar radiation during its summer. When the hemisphere is tilted towards the sun, it receives less radiation during the winter when it is tilted away. It takes around 41,000 years for the axial tilt to cycle from most to least tilted and back again. Earth's axis is currently tilted at 23.4 degrees, or about halfway between its extremes, and this angle is very slowly decreasing. Smaller tilt angles make Earth's seasons milder, resulting in warmer winters and cooler summers. Over time, snow and ice at high latitudes build up into large ice sheets. During glacial periods when ice cover increases, it reflects the sun's energy back into space, meaning Earth cools further. Larger tilt angles favour periods of deglaciation and the melting and retreat of glaciers and ice sheets.