On Earth, daytime is roughly the period on any given point of the planets surface during which it experiences natural illumination from indirect or direct sunlight. Other planets that rotate in relation to a primary, such as a local star, also experience daytime of a sort. Approximately half of the Earth is illuminated at any time by the Sun, 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 Earths rotation is not perpendicular to the plane of its orbit around the Sun, 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 period at most points on the planets surface. Because the Sun is a disc as seen from the Earth, rather than a point source of light, sunrise and sunset are not instantaneous. Additionally, the Earths atmosphere further bends and diffuses light from the Sun and lengthens the period of sunrise, 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 ended until the Suns disc is actually well below the Earths horizon. Areas experiencing summer are tilted toward the sun, 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 angles of the sun is what causes the tropics to be warm while low angles at the poles is what causes them to be cold. Although the length of the period is always twelve hours at the Equator, in all seasons. During the winter, the period is shorter than twelve hours, during the summer. When it is north of the Equator, it is summer south of the Equator. At the Equator, the period is always almost twelve hours in length. 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, the fact that the Sun is always so close to the vertical at noon on the Equator explains why equatorial regions are the hottest regions on the planet overall. The tropics occupy a band of the Earths 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 equivalent line of south latitude is called the Tropic of Capricorn, for similar reasons
Daytime sky with white clouds
Earth daylight Northern Solstice
Earth daylight Southern Solstice
Day length as a function of latitude and the day of the year. Latitude 40° N (approximately New York City, Madrid and Beijing) is highlighted as an example.