May 19, 2022

A day is any of several different units of time. The word refers either to the period of light when the Sun is above the local horizon or to the whole day which covers both the dark and the light period. Day (ancient day, from the Indo-European root * din-, with the basic meaning: radiance, radiation) is the time during which the Earth once revolves around its axis (Earth's rotation). In astronomy, that time is determined by the period that passes between two consecutive passages of a celestial body through the same meridian or meridian. If the body is the Sun, it is called a sunny or solar day (synodic day). But as sunny days of unequal duration are observed, due to the ellipticity of the Earth's path around the Sun, the average sunny day is considered an astronomical day. The difference between a medium sunny day and a real sunny day is expressed by the equation of time. The deviation of a real sunny day from the middle has two maxima (+3.8 minutes on May 14 and +16.4 minutes on November 3) and two minima (–13.4 minutes on February 12 and –6.4 minutes on July 26) , and coincide four times a year (April 16, June 14, September 1 and December 25). Real local time (tp) is obtained by the equation of time: tp Ts + es + λgde is: Ts - mean Greenwich Mean Time (GMT, today UTC), es - deviation for a given date (from tables), and λ - longitude. The time interval between two consecutive passages of a star through the meridian is called a star or sidereal day (sidereal day); it lasts 23 hours 56 minutes. A lunar or lunar day lasts 24 hours and 50 minutes.


Different definitions for the day are based on the apparent movement of the Sun across the sky (solar day). The reason for this apparent movement is the rotation of the Earth around its axis, as well as the revolution of the Earth in orbit around the Sun. Day, unlike night, is often defined as the period when sunlight reaches the earth, assuming no logical assumptions. Two effects make the day on average longer than the night. The sun is not a point, but it has an apparent size of about 32 arc minutes. In addition, the atmosphere refracts sunlight in such a way that some of that light reaches the earth even when the sun is still below the horizon for about 34 arc minutes. So the first light reaches the earth when the center of the sun is still below the horizon in about 50 arc minutes. The difference in time depends on the angle at which the sun rises