Day length

On Earth, daytime is roughly the period of the day during which any given point in the world experiences natural illumination from especially direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. During daytime, an observer sees indirect sunlight while in the shade, which includes cloud cover. 'Day' is sometimes used instead of 'daytime', in this case 'day' will mean 'the period of light between dawn and nightfall; the interval from sunrise to sunset', which is synonymous with daytime. However, in this context, in order to be clear 'daytime' should be used distinguish it from 'day' which typically refers to a 24-hour period. On Earth, daytime is roughly the period of the day during which any given point in the world experiences natural illumination from especially direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. During daytime, an observer sees indirect sunlight while in the shade, which includes cloud cover. 'Day' is sometimes used instead of 'daytime', in this case 'day' will mean 'the period of light between dawn and nightfall; the interval from sunrise to sunset', which is synonymous with daytime. However, in this context, in order to be clear 'daytime' should be used distinguish it from 'day' which typically refers to a 24-hour period. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime of some sort, but this article primarily discusses daytime on Earth. Approximately half of the Earth is illuminated at any time by the Sun. The area subjected to direct illumination is almost exactly half the planet; but because of atmospheric and other effects that extend the reach of indirect illumination, the area of the planet covered by either direct or indirect illumination amounts to slightly more than half the surface. 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. 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. 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.