Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, 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.
Night is the time after sunset until sunrise.[1] Night can also be defined as a period (time) when a place is in a position that is not facing the sun, and therefore becomes dark. When one hemisphere of the planet is experiencing night time, the other hemisphere will experience daylight. Night is sometimes defined as the time between the sun setting on the western horizon until the sun rises on the eastern horizon the next day. In English (12 hour system) night is 19:00-00:00, 18:00 is late afternoon (magrib), in ISO 8601 time (24 hour system) night can be 19:00-03: 00 (Isha) or 19:00-00:00, Midnight at 00:00, the evening before the morning (Early Day) 01:00-03:00 (tahajjud).
Due to the rotation of the earth that rotates from west to east, day and night will alternate with each other, and form a day consisting of 24 hours.
The length of the night will vary for different seasons, especially for areas near the poles. In the United States, for example, in summer (July and August) the nights will be shorter than in winter (December and January).
Night in the use of daily activities is the time between 19:00 and 00:00.