At latitudes where the Sun is directly above the Earth's equator (on the equinoxes of approximately March 20 and September 23), sunlight shines perpendicular to the Earth's axis of rotation, and all latitudes have a 12-hour day and 12-hour night. At other times of the year, solar radiation does not come straight down but instead is scattered by clouds and aerosols in the atmosphere. The phrase "equatorial region" is used to describe any region on or near the equator, including both the tropics and the higher latitude continents. The term refers to the fact that the axis of Earth is tilted at an angle with respect to the orbit of Mars, so that the Sun appears to rise about 6 degrees north of east at the North Pole and to set about 6 degrees south of west at the South Pole. This is why people living at high northern and southern latitudes experience seasons: winter at the poles and summer at the equator. But the tilt of the Earth's axis is not constant; it changes over time due to gravitational forces acting on the planet's core. As a result, the length of each season varies from year to year and from place to place on Earth. In addition to the change in length of the days and nights, the location of the Earth's center of mass also affects how much energy is received at different locations on its surface.