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.
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Approximately half of the Earth is illuminated at any given time by sunlight. 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 exactly 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 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. 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.
Although the length of the daytime period is always twelve hours at the Equator, in all seasons, at all other latitudes the length varies with the season. During the winter, the daytime period is shorter than twelve hours; during the summer, it is longer than 12 hours. When it is winter north of the Equator, it is summer south of the Equator, and vice versa.
At the Equator, the daytime period is always almost twelve hours in length, no matter what the season. The sun always rises nearly perpendicular to the horizon. From the March Equinox to the September Equinox, it rises a bit north of east, and sets a bit north of west. From the September Equinox to the March Equinox, it rises a bit south of east and sets a bit south of west. The path of the Sun lies entirely in the northern half of the sky for the period from the March Equinox to the September Equinox and is entirely in the southern half of the sky from the September Equinox to the March Equinox, with the Sun passing directly overhead at noon on the equinoxes.
The fact that the Sun is always so close to the vertical at noon on the Equator (never being more than 23.5 degrees north or south) explains why equatorial regions are the hottest regions on the planet overall. Additionally, sunrise and sunset always occur very quickly at the Equator, because the Sun's path across the sky is so nearly vertical with respect to the horizon; at the equinox, the Sun requires only two minutes to traverse the horizon at sunrise and sunset.
The tropics occupy a band of the Earth's surface between 23.5° north latitude and 23.5° south latitude. Within this band, the Sun will pass almost directly overhead on at least one day per year. The line of 23.5° north latitude is called the Tropic of Cancer, because when it was named, the Sun passed overhead at this location at the time of year when it was within the zodiac sign of Cancer. The equivalent line of south latitude is called the Tropic of Capricorn, for similar reasons. If they had been named today using the constellation in which the sun is currently in at the time it is directly overhead the tropic line, they would have been called, respectively, the Tropics of Leo and Aquarius. The sun enters and leaves each sign of the zodiac slightly later each year at the rate of about 1 day every 72 years. For more information, see precession of the equinoxes.
On the Tropical Circles, the Sun is directly overhead only once per year, on the corresponding solstice. At latitudes closer to the Equator and on the Equator itself, it will be overhead twice per year (on the equinoxes in the case of the Equator). Outside the tropics, the Sun never passes directly overhead.
Near the poles, which coincide with the rotational axis of the Earth as it passes through the surface, the seasonal variations in the length of daytime are extreme. In fact, within 23.5° latitude of the poles, there will be at least some days each year during which the sun never goes below the horizon. In that there will be days when the sun never rises above the horizon. This number will be fewer, but close to the number of days in the summer where the sun doesn't set (for example the sunrise is usually a few days before the spring equinox and extends a few days past the fall equinox, even without accounting for twilight). This phenomenon of more daylight than night is not unique to the poles. In fact, at any given time slightly more than half of the earth is in daylight. The 24 hours of summer daylight is known as the midnight sun that is famous in some northern and southern countries. To the north, the Arctic Circle marks this 23.5° boundary. To the south, the Antarctic Circle marks the boundary. These boundaries correspond to 66.5° north or south latitude, respectively. Because the Sun's disc itself is about half a degree in diameter and is very bright, truly dark days during which the sun never seems to rise are only seen beyond 72° north or south latitude.
At and near the poles, the sun never rises very far above the horizon, even in summer, which is one of reasons why these regions of the world are consistently cold in all seasons (others include the effect of albedo, the relative increased reflection of solar radiation of snow and ice). Even at the summer solstice, when the sun reaches its highest point above the horizon at noon, it is still only 23.5° above the horizon at the poles. Additionally, as one approaches the poles the apparent path of the Sun through the sky each day diverges increasingly from the vertical. As summer approaches, the Sun rises and sets become more northernly in the north and more southernly in the south. At the poles, the path of the Sun is indeed a circle, which is roughly equidistant above the horizon for the entire duration of the daytime period on any given day. The circle gradually sinks below the horizon as winter approaches, and gradually rises above it as summer approaches. At the poles, apparent sunrise and sunset may last for several days.
In the middle latitudes, far from both the Equator and the poles, variations in the length of daytime are moderate. In the higher middle latitudes where Montreal, Paris and Ushuaia are located, the difference in the length of the day from summer to winter can be very noticeable: the sky may still be lit at 10 PM in summer, but may be dark at 5 PM in winter. In the lower middle latitudes where Southern California, Egypt and South Africa are located, the seasonal difference may be quite small and only slightly noticeable to the locals.
Also in the middle latitudes, the seasonal climate variations produced by changes in the length of daytime are the most marked, with very distinct periods of cold and heat, and other secondary seasonal changes such as snow and ice in winter that disappear in summer and so on. At high latitudes, it is cold most of the time, with constant snow and ice, so the seasons are less obvious; and in the tropics, it is hot most of the time, with no snow or ice at all, so again the seasons are less obvious.
The exact instant of solar noon, when the sun reaches its highest elevation in the sky, varies with the seasons everywhere except at the poles and on the Equator. This variation is called the equation of time, and the magnitude of the variation is usually in the range of minutes over the course of a year, depending on the observer's latitude.
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