Dictionary category
Calendars & Timekeeping
Calendar systems, chronology, civil and astronomical time scales, Earth rotation, date conventions, and recurring cycles.
Direct answer
Calendar systems, chronology, civil and astronomical time scales, Earth rotation, date conventions, and recurring cycles. This category gathers sourced definitions, concrete examples, observing or calculation cautions, and connected reference terms in one place.
What this category covers
Calendars & Timekeeping pages begin with the measured or historical definition, then explain why the term matters, how it is used, a concrete example, relevant limits, and the sources behind the entry. Scientific observation, historical practice, cultural star lore, and symbolic interpretation remain explicitly separated.
How to use this dictionary category
Use this category as a reference layer beside the live Almanac guides. Start with the definition, follow the sibling terms that explain the same process or instrument, then open the relevant sky, season, constellation, ephemeris, or historical guide for a wider view.
Calendars & Timekeeping terms
Anno DominiAnno Domini, abbreviated AD, labels years in the traditional Christian era.
Anomalistic MonthAn Anomalistic Month measures perigee-to-perigee return of the Moon.
Anomalistic YearAn Anomalistic Year measures perihelion-to-perihelion return.
Apparent Solar DayAn Apparent Solar Day runs from one true-solar noon to the next.
Astronomical DayAn Astronomical Day is a technical day convention that must be defined with its starting instant.
Astronomical Year NumberingAstronomical Year Numbering adds year zero and negative years for calculation.
Barycentric Coordinate TimeTCB is the coordinate time used in the solar-system barycentric frame.
Barycentric Dynamical TimeTDB is a barycentric ephemeris time scale kept near TT.
Before Common EraBCE is the inclusive label for years numbered before the Common Era.
Besselian EpochA Besselian Epoch is the historical epoch scale used by many pre-J2000 star catalogs.
CalendarA Calendar organizes days into named and numbered cycles for reckoning time.
Calendar MonthA Calendar Month is a conventional named division of the year.
Calendar YearA Calendar Year is one full year as organized by a named calendar system.
Callippic CycleThe Callippic Cycle refines the Metonic pattern across 76 years.
Civil CalendarA Civil Calendar supplies the official dates used in everyday public affairs.
Civil DayA Civil Day is the midnight-to-midnight day used for legal and everyday dates.
Common EraCommon Era uses the same numbered years as AD under the label CE.
Common YearA Common Year is a 365-day Gregorian or Julian year with no leap day.
Coordinated Universal TimeCoordinated Universal Time is the international reference standard for world time.
Delta TDelta T is the offset between Terrestrial Time and UT1.
Dominical LetterA Dominical Letter is a historical device for locating Sundays in a calendar year.
Double DatingDouble Dating records both of two competing historical date conventions.
Draconic MonthA Draconic Month measures the Moon node-to-node orbital cycle.
Draconic YearA Draconic Year is the Sun node-to-node return period used in eclipse cycles.
DUT1DUT1 is the broadcast difference between UT1 and UTC.
Dynamical TimeDynamical Time supplies uniform time variables for celestial mechanics.
Earth Rotation AngleEarth Rotation Angle is the IAU-standard angular measure of Earth daily rotation.
EpactThe Epact is a computus value used to track the calendar Moon age.
EpochAn Epoch is the reference instant for time-dependent astronomical data.
Equation of TimeThe Equation of Time is the seasonal offset between sundial time and mean clock time.
EraAn Era organizes numbered years from a declared reference point.
Geocentric Coordinate TimeTCG is coordinate time for a reference frame centered on Earth.
Golden NumberThe Golden Number identifies a year place in a nineteen-year lunar cycle.
GPS TimeGPS Time is the continuous atomic system time used by GPS satellites.
Greenwich Mean TimeGreenwich Mean Time is the historical mean-solar time reference at Greenwich.
Greenwich Sidereal TimeGreenwich Sidereal Time expresses the equinox hour angle at Greenwich.
Gregorian CalendarThe Gregorian Calendar is today's widely used civil calendar with a 400-year leap-year rule.
IndictionAn Indiction labels a year position within a repeating fifteen-year cycle.
Intercalary DayAn Intercalary Day is an extra day added by a calendar correction rule.
Intercalary MonthAn Intercalary Month is a leap month used to reconcile lunar months with the solar year.
IntercalationIntercalation adds calendar time to correct accumulated drift.
International Atomic TimeTAI is the international continuous atomic time scale maintained from an ensemble of clocks.
ISO Week DateAn ISO Week Date writes dates as a week-year, numbered week, and weekday.
Julian CalendarThe Julian Calendar uses a regular four-year leap cycle and preceded the Gregorian reform.
Julian CenturyA Julian Century is exactly 36,525 days in astronomical formulas.
Julian DateJulian Date is a continuous astronomical count of days and day fractions.
Julian EpochA Julian Epoch labels an epoch on a fixed 365.25-day year scale.
Leap SecondA Leap Second adjusts UTC by one second relative to Earth rotation.
Leap YearA Leap Year adds calendar time to correct accumulated mismatch with a natural cycle.
Local Apparent Solar TimeLocal Apparent Solar Time is local sundial time based on the true Sun.
Local Mean Solar TimeLocal Mean Solar Time is longitude-based clock time referenced to the mean Sun.
Local Sidereal TimeLocal Sidereal Time tells which right ascension is crossing a site's meridian.
Lunar CalendarA Lunar Calendar bases its months on recurring Moon-phase cycles.
Lunisolar CalendarA Lunisolar Calendar coordinates lunar months with the solar seasonal year.
Mean Solar DayA Mean Solar Day is the averaged solar day underlying the conventional 24 hours.
Metonic CycleThe Metonic Cycle approximately matches 19 years with 235 lunar months.
Modified Julian DateMJD is a shortened midnight-based form of Julian Date.
New Style DateA New Style Date converts a historical date to the newer calendar convention.
Old Style DateAn Old Style Date preserves a pre-reform calendar convention.
Ordinal DateAn Ordinal Date is a year plus its numbered day of year.
Proleptic Gregorian CalendarThe Proleptic Gregorian Calendar applies Gregorian rules to dates before Gregorian adoption.
Proleptic Julian CalendarThe Proleptic Julian Calendar applies Julian rules before their historical adoption.
Reduced Julian DateReduced Julian Date is a historical shortened JD convention whose offset must be stated.
Regnal YearA Regnal Year dates an event within a ruler reign.
Revised Julian CalendarThe Revised Julian Calendar uses a refined leap rule adopted by some Eastern Orthodox churches.
Sidereal DayA Sidereal Day measures Earth rotation relative to the celestial reference frame.
Sidereal MonthA Sidereal Month measures the Moon orbit against a stellar reference.
Sidereal TimeSidereal Time is star-based rotational time used to orient the sky.
Sidereal YearA Sidereal Year measures Earth orbit against the stars.
Solar CalendarA Solar Calendar keeps the calendar year aligned with the seasons.
Solar NoonSolar Noon is the local meridian transit of the Sun.
Synodic MonthA Synodic Month is the approximately 29.53-day lunar phase cycle.
Terrestrial TimeTerrestrial Time is the uniform geocentric time argument used for astronomical ephemerides.
Time ZoneA Time Zone is a region sharing civil clock rules and a UTC relationship.
Tropical MonthA Tropical Month measures lunar return relative to the equinox.
Tropical YearA Tropical Year is the seasonal year measured relative to the equinox.
Universal TimeUniversal Time tracks time through Earth's rotation, especially the UT1 scale.
Unix TimeUnix Time counts computer-system seconds from the 1970 UTC epoch.
UT1UT1 is modern Earth-rotation time.
UTC OffsetA UTC Offset states a local clock difference from UTC.
Week NumberA Week Number labels a week according to a stated calendar convention.
Year ZeroYear Zero exists in astronomical numbering but not in traditional historical era notation.
Continue learning
Place this timekeeping idea beside the current sky, universal timing, and live celestial positions.
Open Worldwide WatchCommon questions
What is in the Calendars & Timekeeping dictionary?
Calendar systems, chronology, civil and astronomical time scales, Earth rotation, date conventions, and recurring cycles.
How should I use this category?
Use it to understand a term, compare neighboring concepts, check sources and cautions, and continue into the relevant live or long-form Almanac guide.
How do these entries connect to the live Almanac?
The dictionary defines the reference language. The Almanac guides add current sky data, larger explanations, and practical observing or historical context.
