dayandnight

Day and night refers to the recurring daily pattern of light and darkness on Earth's surface as it rotates around its axis relative to the Sun. A solar day is about 24 hours.

Earth's axis tilt of about 23.5 degrees causes different hemispheres to receive varying solar elevation throughout the year, lengthening days in summer and shortening them in winter; at the equator day and night are nearly equal year-round.

Sunrise and sunset mark when the Sun’s upper limb crosses the horizon, and the intervals of civil, nautical, and astronomical twilight occur before sunrise and after sunset as light changes gradually to dark and back.

The day–night cycle is a fundamental environmental cue that helps regulate circadian rhythms in many organisms, including humans. It influences daily activity patterns, agriculture, and energy use, and it plays a prominent role in cultural and religious practices and in the structure of calendars.

Observations of day and night vary with latitude and season, producing long days in high latitudes during summer and long nights during winter. On planets that are tidally locked, one side may experience permanent day while the opposite side remains in permanent night, illustrating the variety of day–night patterns possible in the universe.