Day
Overview
A day is a unit of time defined by the period Earth takes to complete one rotation around its axis, and it is also the fundamental cycle in which human life and activity repeat. In astronomy, it refers to the solar day, the time between two successive passages of the Sun across a meridian; in everyday usage, it means the 24 hours from midnight to the next midnight. A day is subdivided into hours, minutes, and seconds, and functions as the most basic measure constituting rhythms across society, including calendars, labor, rest, and rituals.
Key details
Astronomical definition
Earth rotates once on its axis in about 23 hours 56 minutes 4.09 seconds; this is called a sidereal day (恒星日). However, because Earth orbits the Sun while rotating, the solar day (太陽日), which is based on the Sun, is about 4 minutes longer than the sidereal day. The mean solar day is defined as 24 hours, and this is the basis of what we commonly call a 'day.' Meanwhile, Earth's rotation rate is decreasing very slowly due to factors such as tidal friction, and the length of a day is known to increase by about 1–2 milliseconds per century.
Day as a unit of time
In the International System of Units (SI), the second is defined by the vibration of cesium atoms, and a day is equivalent to 86,400 seconds. However, tiny differences accumulate between Universal Time (UT1), based on Earth's rotation, and International Atomic Time (TAI), and leap seconds have been introduced to correct for this. The day is also a core unit of calendar systems; in the solar calendar, one year is calculated as about 365.2422 days, and the error is adjusted with leap years.
Biorhythms and the day
Most living organisms, including humans, have a circadian rhythm with a period of about 24 hours. The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the center of the biological clock, and sleep–wake cycles, body temperature, and hormone secretion are regulated by the cycle of light and darkness. Research has shown that when the daily rhythm is disrupted, the risk of sleep disorders, metabolic diseases, emotional instability, and other conditions increases.
Culture and the day
Ancient peoples observed the movement of the Sun and divided a day into morning, daytime, evening, and night, and Egyptian and Mesopotamian civilizations divided day and night each into 12 parts, laying the origin of the 24-hour system. In Korea, the twelve Earthly Branches were used to divide a day into 12 double-hours (sijin, 시진), and during the Joseon dynasty, a time system based on midnight was used. Even today, each culture uses the day as a unit for anniversaries, holidays, and rituals, giving order to life.
Linguistic expressions and idioms
In Korean, '하루' (haru, day) is widely used in expressions such as '하루가 다르게' (by the day), '하루아침에' (overnight), and '하루살이' (mayfly) to indicate the speed of change or transience. Also, '하루' can refer to a specific day or be used adverbially to mean 'every day.' In other languages, too, the word for day is distinguished from or used interchangeably with 'daytime,' showing subtle differences in meaning across languages.
Recent trends
In the 2020s, the way the discrepancy between atomic time and astronomical time is managed is being reorganized. At the 27th General Conference on Weights and Measures (CGPM) in 2022, a plan to stop inserting leap seconds after 2035 was effectively confirmed, and the possibility of introducing a one-minute 'leap minute' is also being discussed. Meanwhile, as personal circadian rhythm measurement through smartwatches and wearable devices becomes common, the 'sleep tech' and 'biohacking' markets, which seek to manage daily rhythms through data, are growing rapidly. With the spread of remote work and flexible work arrangements, the traditional notion of fixing the daily work unit at 8 hours is also being shaken, and experiments with a four-day workweek are being conducted in several countries, making social discussion about the meaning and allocation of the day more active.
Related topics
- [[Time]]
- [[Earth's rotation]]
- [[Circadian rhythm]]
- [[Leap second]]
- [[24-hour clock]]