Concepts

Coordinated Universal Time (UTC) is the international time standard from which every time zone is defined as a fixed offset. UTC ticks at the steady rate of atomic clocks, with occasional one-second adjustments — leap seconds — that keep it close to the time told by the Earth's rotation.

Daylight saving time (DST) is the practice of advancing a jurisdiction's civil clock by one hour during the summer half of the year, then returning it to standard time for the winter half. DST changes only the numbers on local clocks; UTC, the Earth's rotation, and the length of the day are unaffected. The sun rises and sets at the same instants it always would, and the local clock simply reads those instants an hour later.

Unix time is a count of the seconds that have elapsed since 00:00:00 UTC on 1 January 1970 — a moment known as the Unix epoch. The same count is also called POSIX time, epoch time, or a Unix timestamp; the encoding is fixed by the POSIX standard and is the way nearly every computer represents an instant in time internally.

Solar noon at a place is the moment the Sun crosses the local meridian — the imaginary line in the sky running due north–south through the observer's zenith. The Sun reaches its highest altitude of the day at that instant and casts its shortest shadow; the same event is also called the solar transit or local apparent noon.

The equation of time is the gap between apparent solar time — what the Sun in the sky shows, on a sundial — and mean solar time, the steady time scale a clock keeps. It is positive when the Sun is ahead of the clock and negative when behind, and reaches up to about 16 minutes either way through the year, peaking near early November and bottoming out near mid-February.

Apparent solar time is the time told directly by the real Sun — the time a sundial shows — while mean solar time runs on a fictitious mean Sun that moves at a perfectly uniform rate through the year. Civil clocks keep mean solar time because the length of the real solar day varies from one date to the next; the gap between the two on any given day is the equation of time, which reaches up to about 16 minutes either way.

Sidereal time is time reckoned by the stars rather than the Sun — formally, the hour angle of the spring equinox, which completes one full cycle each time the Earth turns once relative to the distant stars. That turn, the sidereal day, lasts about 23 hours 56 minutes 4 seconds, roughly four minutes shorter than the 24-hour solar day, because the Earth's orbital motion forces it to rotate nearly one degree further each day to bring the Sun back to the meridian. Astronomers use sidereal time to track which celestial objects are crossing the meridian; everyday clocks keep solar time instead.

A time zone is a region that observes a uniform civil time, defined as a fixed offset from Coordinated Universal Time (UTC). Standard offsets in use today range from UTC−12 in uninhabited US Pacific possessions to UTC+14 in the eastern Pacific, with several half-hour and quarter-hour offsets in between; the IANA Time Zone Database is the operational source of truth for which offset applies where, and how that has changed over time.

Twilight is the period before sunrise and after sunset when the Sun is below the horizon but the sky is still partially lit by sunlight scattered through the atmosphere. It is divided into three phases by how far the Sun has fallen below the horizon: civil twilight ends at 6° below, nautical at 12° below, and astronomical at 18° below — the threshold at which scattered sunlight drops below natural starlight and night begins for observational astronomy.

An equinox is one of two moments each year — in March and September — when the Sun, in its apparent yearly path across the sky, crosses the celestial equator and stands directly above the Earth's equator. Day and night are approximately twelve hours long everywhere on the planet on each equinox, but never exactly so: atmospheric refraction and the finite size of the Sun's disc bias the day longer than the night by about seven minutes near the equator, growing to ten minutes or more at mid-latitudes.

A leap year in the Gregorian calendar is a year of 366 days — one more than the usual 365 — formed by inserting an extra day, 29 February, to keep the calendar aligned with the tropical year of about 365.2422 days. The rule is precise: a year is a leap year if it is divisible by four, except that years divisible by 100 are not leap years unless they are also divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be.

ISO 8601 is the international standard for representing dates, times, durations, and time intervals as strings — for example 2026-05-09 for a date or 2026-05-09T14:30:00Z for an instant in UTC. The current edition is ISO 8601-1:2019, with an extension in ISO 8601-2:2019, and most everyday uses on the internet follow a stricter profile of it called RFC 3339.

Greenwich Mean Time (GMT) is mean solar time at the Royal Observatory in Greenwich, England — the time the Sun shows there, averaged over the year to remove its small seasonal swings. GMT was the international civil time standard from 1884 until 1972, when it was superseded by Coordinated Universal Time (UTC); the two are equivalent for everyday purposes and the name "GMT" survives as the time-zone label at the prime meridian.

Standard time is the uniform civil time a region keeps by law throughout the year, defined as a fixed offset from Coordinated Universal Time (UTC) — for example UTC−5 for the eastern United States or UTC+5:30 for all of India. It is the baseline that clocks return to outside the daylight saving period, and it replaced the older system in which every town kept its own local solar time — a patchwork the railways and the telegraph made unworkable during the nineteenth century.

The ISO week date is a calendar scheme defined in ISO 8601 that identifies a date by its ISO week-numbering year, week number, and weekday — written as 2026-W19-5 for Friday of the 19th ISO week of 2026. ISO weeks start on Monday, end on Sunday, and week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. The week-numbering year occasionally differs from the calendar year for a few days at the boundaries, and a year contains 52 or 53 ISO weeks depending on which day of the week 1 January falls on.

The 24-hour clock writes time-of-day with the hour running from 00 to 23, so a single number — 14:30, for example — names a particular moment without an AM or PM suffix. It is the international civil convention in most of the world, the format used by ISO 8601 and every major internet timestamp standard, and the format used in aviation, the military, and hospitals, where the AM/PM ambiguity of the 12-hour clock is a recognised source of error. "Military time" is the same 24-hour convention written without the colon — 1430 rather than 14:30 — with the day starting at 0000 and ending at 2359.

Sunrise and sunset are the moments when the upper edge of the Sun's disc appears to touch the horizon — sunrise as it climbs into view, sunset as it slips out of view. By the standard convention used in the United States by the U.S. Naval Observatory and the National Oceanic and Atmospheric Administration, both events are computed for the moment the centre of the Sun is 50 arcminutes (0.8333°) below the horizon: 16 arcminutes for the apparent radius of the Sun, plus 34 arcminutes for the average bend imparted by the atmosphere.

Day length is the duration between sunrise and sunset at a given location on a given date — the total time any portion of the Sun is above the horizon. It varies from about 12 hours year-round at the equator to as much as 24 hours (or as little as zero) inside the polar circles, swinging through the year as the Sun's declination changes with Earth's axial tilt.

Golden hour is the period after sunrise and before sunset when the Sun is above the horizon but no higher than about 6° in altitude, casting warm, soft, low-angle light prized in photography and cinematography. Duration depends on latitude and season — typically 30 to 60 minutes at mid-latitudes, as little as 20 minutes near the equator, and stretching to many hours or persisting all day in polar summer.

Solar position is where the Sun sits in the sky at a given place and instant, fixed by two angles: altitude — how high the Sun stands above the horizon, from 0° at the horizon to 90° straight overhead — and azimuth, its compass bearing measured clockwise from true north. Together the two angles make up the horizontal coordinate system, and both change continuously with the observer's latitude, the date, and the time of day.

Blue hour is the short period of twilight before sunrise and after sunset when the Sun is several degrees below the horizon and the sky takes on a deep, saturated blue tint. The site's solar tools place blue hour between 4° and 6° of solar depression — inside civil twilight — though photographic references widen the band as far as 8° below the horizon. The blue colour comes from a combination of long-path Rayleigh scattering and ozone's Chappuis-band absorption of yellow and red light at altitudes still lit by direct sunlight.

The day of the week is the position of a date within a recurring seven-day cycle — Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday — that runs continuously and independently of months and years. The cycle has been kept unbroken since at least the early Roman Empire, was made the international civil standard by ISO 8601 (which numbers the days Monday = 1 through Sunday = 7), and is the same cycle whether you count from Monday in business and computing or from Sunday in everyday calendars across most of the Americas, East Asia, and the Middle East.

The seven-day week is a continuous cycle of seven named days — Monday through Sunday — that orders civil and religious life almost everywhere, yet alone among the everyday units of the calendar it follows nothing in the sky. The day tracks one rotation of the Earth, the month roughly one round of the Moon's phases, and the year one circuit of the Sun, but the week answers to no astronomical period: it is a purely human rhythm that took shape in the Roman Empire from the fusion of the Jewish Sabbath week with the planetary week of Hellenistic astrology, has run unbroken ever since, and never realigns with the calendar because seven divides neither the 365 days of a common year nor the 366 of a leap year.

The International Date Line is the imaginary line on Earth's surface where the calendar date changes — places immediately west of the line are one calendar day ahead of places immediately east. It runs roughly along 180° longitude through the Pacific Ocean, but is not defined by any treaty or international body: each segment is set by the time-zone choices of the bordering nations, and the line jogs east or west to keep island groups on a single calendar day.

The prime meridian is the line of 0° longitude — the north–south reference from which every other longitude on Earth is measured east or west, and historically the line from which civil time was reckoned. By an international agreement reached in 1884 it runs through the Airy Transit Circle at the Royal Observatory in Greenwich, London; the satellite-based reference line used by modern navigation lies about 102 metres east of the historic instrument.

A business day is a day on which most banks, government offices, and financial markets are open for ordinary business — typically Monday through Friday in most of the world, with national public holidays excluded. The exact set of weekdays counted as business days is fixed by national convention rather than by any international standard, and the variation is real: most Arab countries take Friday and Saturday as the weekend, Israel works Sunday through Thursday with an optional half-day on Friday, and Brunei is the only country whose working week splits across the weekend with Friday and Sunday off.

International Atomic Time (TAI) is a continuous reference timescale formed by averaging more than 300 atomic clocks at laboratories around the world, all ticking the SI second — the international unit of time, fixed by a resonance of the caesium-133 atom. TAI never pauses for leap seconds, which makes it the steady backbone of modern timekeeping: Coordinated Universal Time (UTC), the basis of every civil clock, is simply TAI minus a whole number of leap seconds — 37 seconds as of 2026.

A leap second is an extra second occasionally inserted into Coordinated Universal Time (UTC) to keep it within 0.9 seconds of UT1, the time scale defined by the Earth's actual rotation. Twenty-seven leap seconds have been added between 1972 and 2017, all at the end of either 30 June or 31 December, and announced in advance by the International Earth Rotation and Reference Systems Service (IERS). The international body that governs measurement units voted in November 2022 to retire the leap second in or before 2035.

A solstice is one of the two instants each year when the Sun reaches its greatest angular distance north or south of the celestial equator — equivalently, when the Sun's apparent geocentric longitude is exactly 90° (the June solstice) or 270° (the December solstice). The June solstice falls on 20–22 June and brings the longest day to the Northern Hemisphere and the shortest to the Southern; the December solstice falls on 20–23 December and reverses the two.

The Gregorian calendar is the international civil calendar, proclaimed by Pope Gregory XIII in the papal bull Inter gravissimas of 24 February 1582 and now used in every country for secular dating. It replaces the older Julian calendar by dropping three centurial leap years out of every four — only years divisible by 400 stay leap — and by deleting ten days from October 1582 to bring the spring equinox back to about 21 March. Its mean year is 365.2425 days, about 27 seconds longer than the tropical year and accurate to within a day in roughly 3,300 years.

The Julian day number is a continuous count of whole days that astronomers use to label dates without the complications of months and years; day zero began at noon Universal Time on 1 January 4713 BC. Adding the fraction of a day elapsed since the previous noon turns the count into the Julian date — a single running number, now past 2.46 million, that pinpoints any instant. Its convenience is that the interval between two events is simply the difference of their Julian dates in days, free of the irregular month lengths and leap years that make ordinary calendar arithmetic awkward.

A tropical year is the time the Sun takes to return to the same point in the cycle of the seasons — about 365.2422 days, or 365 days, 5 hours, 48 minutes, and 46 seconds. Also called the solar year, it is the year the civil calendar is built to track, and its surplus of almost a quarter-day over 365 whole days is the reason leap years exist. It runs about 20 minutes shorter than the sidereal year, the time the Earth takes to complete one full orbit relative to the fixed stars.

The midnight sun is the period each year, inside the Arctic and Antarctic Circles, when the Sun never sets — daylight continues unbroken through what would ordinarily be night. The mirror phenomenon, when the Sun never rises and the day stays dark, is the polar night. Both are direct consequences of Earth's 23.44° axial tilt: above 66°33′ latitude in either hemisphere, on at least one day each year the Sun's apparent path no longer crosses the horizon.

A lunar phase is the shape of the Moon's sunlit portion as seen from Earth — new moon, first quarter, full moon, last quarter, and the crescent and gibbous stages between them — set by the changing angle between the Sun, the Earth, and the Moon. One full cycle from new moon to new moon is the synodic month, which averages 29.53 days (29 days, 12 hours, 44 minutes). That is about 2.2 days longer than the 27.32-day sidereal month the Moon takes to circle the Earth once relative to the stars, because the Earth is itself moving around the Sun and the Moon must travel a little farther to line back up.

The analemma is the slender figure-eight the Sun traces in the sky when it is photographed from one place at the same clock time on many dates through the year. Its east–west width is the equation of time and its north–south height is how far north or south of the equator the Sun stands — its declination — so the figure stands about 47° tall, twice the tilt of Earth's axis, with a small upper loop for the northern summer and a larger lower loop for the northern winter.

Perihelion is the point in Earth's yearly orbit where it comes closest to the Sun, reached in early January; aphelion is the opposite point, farthest from the Sun, reached in early July. The two differ by only about 3 percent — Earth's orbit is very nearly circular, with an eccentricity of about 0.0167 — and the dates are a standing rebuttal to a common misconception: the seasons come from the 23.4° tilt of Earth's axis, not from its distance from the Sun, which is why the Northern Hemisphere has its winter at the very time Earth is nearest the Sun.