Length of day

On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or (especially) direct sunlight.

Other planets that rotate in relation to a luminous primary, such as a local star, also experience daytime of a 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 the Earth's own axis of rotation is inclined by about 23.5 degrees from the perpendicular (as compared to its orbital plane), the length of the daytime period varies with seasons on the planet's surface, depending on the observer's latitude. Areas experiencing summer are tilted toward the sun. Their tilt toward the sun leads to over half of the day being in daylight and warmer temperatures due to the increased directness of the sun's rays, the longer day period itself and less absorption of sunlight in the atmosphere. While increased daylight can have some effect on the increased temperature in the summer, most of the increase in temperature is due to the directness of the sun, not the increased daylight. The high (near 90 degrees) angles of the sun is what causes the tropics to be warm while low (barely above the horizon) angles at the poles is what causes them to be cold. Hours of daylight having little effect on temperature can be seen with the poles still being cold in their respective summers despite 24 hours of daylight, while the equator is warm with only 12 hours of daylight.

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Wikipedia