點解我們會有日夜/ why we have day and night

A day (symbol: d) is a unit of time equal to 24 hours. For a given planet, there are two types of day defined in astronomy:
1 apparent sidereal day = a single rotation of a planet with respect to the distant stars (for Earth it is 23.934 solar hours or 24 sidereal hours)
1 solar day = a single rotation of a planet with respect to its Sun.
The term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as dies in Latin and dive in Sanskrit.Night or nighttime is the period in which the sun is below the horizon. The opposite of night is day (or "daytime" to distinguish it from "day" as used for a 24-hour period). Time of day varies from season to season, but night typically can begin around 8:00 or 9:00 p.m. and goes to dawn.

Nights are shorter than days on average due to two factors. One, the sun is not a point, but has an apparent size of about 32 minutes of arc. Two, the atmosphere refracts sunlight so that some of it reaches the ground when the sun is below the horizon by about 34 minutes of arc. The combination of these two factors means that light reaches the ground when the centre of the sun is below the horizon by about 50 minutes of arc.Without these effects, day and night would be the same length at the autumnal (autumn/fall) and vernal (spring) equinoxes, the moments when the sun passes over the equator. In reality, around the equinoxes the day is almost 14 minutes longer than the night at the equator, and even more closer to the poles. The summer and winter solstices mark the shortest night and the longest night, respectively.The closer a location is to the North or South Pole, the larger the range of variation in the night's length. Although equinoxes occur with a day and night close to equal length, before and after an equinox the ratio of night to day changes more rapidly in locations near the poles than in locations between the Tropic of Cancer and the Tropic of Capricorn. In the Northern Hemisphere, Denmark has shorter nights in June than India has. In the Southern Hemisphere, Antarctica has longer nights in June than Chile has. The Northern and Southern Hemispheres of the world experience the same patterns of night length at the same latitudes, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other is experiencing short nights (summer).Between the pole and the polar circle, the variation in daylight hours is so extreme that for a portion of the summer, there is no longer an intervening night between consecutive days and in the winter there is a period that there is no intervening day between consecutive nights.Throughout history, night has primarily been a time of resting and sleep for humans, since little work or labor can be done in the dark. On the other hand, exciting clandestine activities such as romance, sex, and crime and police activity flourish.As artificial lighting has improved, night-time activity has increased and become a significant part of the economy in most places. Many establishments, such as nightclubs, bars, convenience stores, fast-food restaurants, gas stations, distribution facilities, and police stations now operate 24 hours a day or stay open as late as 1 or 2 a.m. Even without artificial light, moonlight sometimes makes it possible to travel or work outdoors at night.Night is often associated with danger and evil, because bandits and dangerous animals can be concealed by darkness. The belief in magic often includes the idea that magic and magicians are more powerful at night. Similarly, mythical and folkloric creatures as vampires, and werewolves are thought to be more active at night. Ghosts are believed to wander around almost exclusively during night-time. In almost all cultures, there exist stories and legends warning of the dangers of night-time. In fact, the Saxons called the darkness of night the 'death mist'.

We get day and night because that Earth goes around the Sun. And the speed is depends there distance from the Sun.Earth is the thrid away from the Sun . So,that every time when we are in the morning (day) and the other side of the Earth will be at night because that the Sun get to shine at us and the Sun can not shine at the other slid of the Earth so they are at night.(What the Secienctist think)

The other is that christians belive that the God creat day and night at the first day.