Glossary of lunar timekeeping

Thirty terms, each with a definition of fifty words or fewer, the terms it connects to, and the documents that use it.

Lunar timekeeping has two vocabularies. One belongs to metrology and is old: atomic time, civil time, the geoid. The other is new and still unsettled: a lunar reference system, a lunar coordinate time, and a proposed standard that has a name but no realization. This glossary keeps both in one place, so that a sentence from a policy memo and a sentence from a relativity paper can be read side by side.

Every lunar clock value on this site is a model value, computed from the published definitions: see how we calculate lunar time. The articles in the learn hub use these terms in context.

Time scales

Eight entries, and only two of them are produced as running scales today: TAI and UTC. TT is the ideal they approximate; TCB, TCG and TCL are the coordinate times of three reference systems, and TDB is the second barycentric scale a lunar conversion has to reach; LTC is a proposed name for a lunar standard that has not been built.

• Coordinated Lunar TimeLTC — Coordinated Lunar Time (LTC) is a proposed operational time standard for the Moon, named in a 2024 United States policy memorandum. ^[1]Official
• Lunar Coordinate TimeTCL — Lunar Coordinate Time (TCL) is the time coordinate of the Lunar Celestial Reference System, defined by the International Astronomical Union in 2024 by analogy with Geocentric Coordinate Time. ^[2]Official
• Coordinated Universal TimeUTC — Coordinated Universal Time is the world's civil time scale. ^[1]Official
• International Atomic TimeTAI — International Atomic Time is the practical realization of Terrestrial Time, produced as a weighted average of hundreds of atomic clocks worldwide. ^[1]Official
• Terrestrial TimeTT — Terrestrial Time is the theoretical time scale of a clock at rest on Earth's geoid. ^[1]Official
• Geocentric Coordinate TimeTCG — Geocentric Coordinate Time is the time coordinate of the Geocentric Celestial Reference System. ^[2]Official
• Barycentric Coordinate TimeTCB — Barycentric Coordinate Time is the time coordinate of the Barycentric Celestial Reference System, centred on the Solar System barycentre. ^[2]Official
• Barycentric Dynamical TimeTDB — Barycentric Dynamical Time is one of the two barycentric time scales that lunar time work has to convert to. ^[5]

Reference systems and surfaces

A time scale has to be defined from somewhere. These entries name that somewhere: the lunar reference system and its first frame, the two equipotential surfaces that clock rates are referred to, and the regions of space in which the definitions are meant to hold.

• Lunar Celestial Reference SystemLCRS — The Lunar Celestial Reference System is a relativistic system of space-time coordinates centred on the Moon, built with the same techniques as the Geocentric Celestial Reference System. ^[2]Official
• International Lunar Reference FrameILRF — The International Lunar Reference Frame is the first realization of a lunar reference system: axes attached to the Moon and co-rotating with it, with the origin at the lunar centre of mass. ^[6]
• Selenoid — The selenoid is the Moon's equivalent of Earth's geoid: an equipotential surface of the Moon's gravity field used as the reference level for lunar clock rates. ^[3]
• Geoid — The geoid is the equipotential surface of Earth's gravity field that mean sea level follows. ^[3]
• Cislunar space — Cislunar space is the region between Earth and the Moon, including lunar orbits and the lunar surface. ^[1]Official
• Hill sphere — A body's Hill sphere is the region around it where its own gravity wins over the Sun's. ^[7]

Physics

Why two identical clocks disagree at all, and why a coordinate is not the reading of any clock.

• Proper time — Proper time is the time measured by a real clock along its own path through space-time. ^[7]
• Coordinate time — Coordinate time is the time label of a reference system, not a reading of any physical clock. ^[8]Official
• Gravitational time dilation — Gravitational time dilation is the slowing of clocks deeper in a gravitational field, predicted by general relativity. ^[1]Official

Units and conventions

What a second is, what gets inserted into a civil scale, where a scale starts counting, what ties a measurement back to a reference, and the Earth convention that no published lunar proposal carries over.

• SI second — The SI second is the base unit of time, defined by the unperturbed ground-state hyperfine transition frequency of caesium-133. ^[1]Official
• Leap second — A leap second is an extra second inserted into Coordinated Universal Time to keep it aligned with Earth's rotation. ^[1]Official
• Epoch — An epoch is the instant that fixes where a time scale starts counting. ^[2]Official
• Traceability — Traceability is an unbroken chain of comparisons that ties a measurement to a stated reference. ^[1]Official
• Time zone — A time zone is a region whose civil clocks share one offset from Coordinated Universal Time. ^[1]Official

Infrastructure

The services that need one shared lunar time, the clocks and the ensembles that turn them into a scale, and the ephemerides that convert one scale into another.

• LunaNet — LunaNet is a framework of agreed standards, protocols and interface requirements for lunar communication and navigation services, so that missions from different agencies can work together. ^[9]Official
• Positioning, Navigation and TimingPNT — Positioning, navigation and timing is the service triad that satellite systems such as GPS provide. ^[1]Official
• Weighted clock ensemble — A weighted clock ensemble is a set of atomic clocks combined into one time scale, each clock contributing according to its stability. ^[1]Official
• Atomic clock — An atomic clock keeps time by counting oscillations of a fixed transition in atoms; the caesium-133 transition defines the SI second. ^[1]Official
• Ephemeris — An ephemeris is a computed table of the positions and motions of celestial bodies over time. ^[5]
• Lunar Time Ephemeris LTE440LTE440 — LTE440 is a numerical lunar time ephemeris: a software package that calculates Lunar Coordinate Time and its relations to the barycentric scales, built on the JPL DE440 ephemeris. ^[5]

Bodies that decide

A lunar time scale becomes a standard only if a diplomatic body says so. Two entries name the meeting that votes and the committee that prepares the text it votes on.

• General Conference on Weights and MeasuresCGPM — The General Conference on Weights and Measures is the body of the Metre Convention in which member states vote on resolutions about units and time. ^[4]Official
• Consultative Committee for Time and FrequencyCCTF — The Consultative Committee for Time and Frequency is the BIPM committee that prepares the work on time scales. ^[10]Official

All terms A–Z

All thirty entries in one list, sorted by name. Two of them, Terrestrial Time and the selenoid, name the two comparisons behind the two rate figures that circulate; both are worked through in 56 vs 58.7 microseconds^[3]. The dates on which these documents appeared are on the timeline.

• Atomic clock
• Barycentric Coordinate TimeTCB
• Barycentric Dynamical TimeTDB
• Cislunar space
• Consultative Committee for Time and FrequencyCCTF
• Coordinate time
• Coordinated Lunar TimeLTC
• Coordinated Universal TimeUTC
• Ephemeris
• Epoch
• General Conference on Weights and MeasuresCGPM
• Geocentric Coordinate TimeTCG
• Geoid
• Gravitational time dilation
• Hill sphere
• International Atomic TimeTAI
• International Lunar Reference FrameILRF
• Leap second
• LunaNet
• Lunar Celestial Reference SystemLCRS
• Lunar Coordinate TimeTCL
• Lunar Time Ephemeris LTE440LTE440
• Positioning, Navigation and TimingPNT
• Proper time
• Selenoid
• SI second
• Terrestrial TimeTT
• Time zone
• Traceability
• Weighted clock ensemble

Last verified 23 August 2026