<P> The hemisphere of the Earth experiencing daytime at any given instant changes continuously as the planet rotates on its own axis . The axis of the Earth's rotation is not perpendicular to the plane of its orbit around the Sun (which is parallel with the direction of sunlight), and so the length of the daytime period varies from one point on the planet to another . Additionally, since the axis of rotation is relatively fixed in comparison to the stars, it moves with respect to the Sun as the planet orbits the star . This creates seasonal variations in the length of the daytime period at most points on the planet's surface . </P> <P> The period of daytime from the standpoint of a surface observer is roughly defined as the period between sunrise, when the Earth's rotation towards the east first causes the Sun's disc to appear above the horizon, to sunset, when the continuing rotation of the Earth causes the Sun's disc to disappear below the horizon to the west . Because the Sun is a luminous disc as seen from the Earth, rather than a point source of light, sunrise and sunset are not instantaneous and the exact definition of both can vary with context . Additionally, the Earth's atmosphere further bends and diffuses light from the Sun and lengthens the period of sunrise and sunset . For a certain period after sunset and before sunrise, indirect light from the Sun lightens the sky on Earth; this period is often referred to as twilight . Certain groups, such as Earthly astronomers, do not consider daytime to be truly ended until the Sun's disc is actually well below the Earth's horizon, because of this indirect illumination . </P> <P> Given that Earth's own axis of rotation is tilted about 23.5 ° to the line perpendicular to its orbital plane, called the ecliptic, the length of daytime varies with the seasons on the planet's surface, depending on the observer's latitude . Areas tilted toward the Sun are experiencing summer . Their tilt toward the Sun leads to more than half of the day seeing daylight and warmer temperatures, due to the higher directness of solar rays, the longer period of daytime itself, and less absorption of sunlight in the atmosphere . While increased daylight can have some effect on the higher temperatures in the summer, most of temperature rise results from the directness of the Sun, not the increased daylight . The high angles (around the zenith) of the Sun causes the tropics to be warm, while low angles (barely above the horizon) causes the polar regions to be cold . The slight effect of daylight hours on average seasonal temperature can be seen with the poles and tropical regions . The poles are still cold during their respective summers, despite seeing 24 hours of daylight for six months, while the Equator remains warm throughout the year, with only 12 hours of daylight per day . </P> <P> Although the daytime length at the Equator remains 12 hours in all seasons, the duration at all other latitudes varies with the seasons . During the winter, daytime lasts shorter than 12 hours; during the summer, it lasts longer than 12 hours . Northern winter and southern summer concur, while northern summer and southern winter concur . </P>

In the northern hemisphere the day is always shorter at higher latitudes than at lower latitudes