lunartime.org

Lunar clock (model)

No lunar time scale has been realized, so this page shows a model: Earth scales as your browser reports them, and lunar scales as the published rate figures predict them.

Key facts

  • A clock resting on the Moon's selenoid runs 56.02 microseconds per day faster than a clock resting on Earth's geoid. [1]
  • The OSTP celestial time memorandum cites 58.7 microseconds per Earth day for a lunar coordinate time scale measured against Terrestrial Time. [2]Official
  • The epoch of Lunar Coordinate Time (TCL) is 1977 January 1, 0h 0m 32.184s TCB at the centre of the Moon. [3]Official
  • TT minus UTC is 69.184 s today: TAI minus UTC is 37 s and TT minus TAI is 32.184 s by definition. [4]
  1. EARTH · UTC13:43:01.596Coordinated Universal Time, read from the browser clock. Milliseconds are where a browser clock stops being honest, so this line has no microsecond field.
  2. EARTH · TT13:44:10.780Terrestrial Time: UTC plus 69.184 s, the sum of TAI − UTC = 37 s and TT − TAI = 32.184 s. Not a model, an exact offset.
  3. MOON · TCL (model)13:44:11.844model offset +1.064382 sLunar Coordinate Time as a model: TT plus the accumulated 58.7 µs/day by which TCL runs ahead of TT.
  4. MOON · surface clock (model)13:43:02.611model offset +1.015787 sA clock on the lunar surface as a model: UTC plus the accumulated 56.02 µs/day by which a selenoid clock runs ahead of a geoid clock.

+1.015787 s since 1977-01-01

Selected pair: lunar surface clock vs Earth geoid clock, 56.02 µs/day. The model runs ahead of UTC by 1.015787 s, accumulated since 1977-01-01.

Epoch t₀
Rate for the counter

The microsecond field shows a modelled offset, not a measurement. Browser clocks are accurate to milliseconds at best.

What each line means

Each line names the pair of scales it compares, because a rate figure without its pair is not a fact about the Moon — it is a number without a referent.

The four lines of the clock and the pair of scales behind each one.
LineWhat it showsCompared withSource
EARTH · UTCCoordinated Universal Time from the browser clock, to milliseconds.Nothing: this is the reference every other line is measured against.[5]Official
EARTH · TTTerrestrial Time: UTC plus 69.184 s, an exact offset rather than a model.UTC, through TAI: TAI minus UTC is 37 s and TT minus TAI is 32.184 s.[4]
MOON · TCL (model)TT plus the drift accumulated at 58.7 µs/day since the epoch.TT: this rate figure compares TCL with Terrestrial Time, not with UTC.[2]Official
MOON · surface clock (model)UTC plus the drift accumulated at 56.02 µs/day since the epoch.A clock on Earth's geoid: this rate figure compares selenoid proper time with geoid proper time.[1]

The rate switch above chooses which of those two pairs the counter and the spoken summary use. The clock lines themselves never change rate: each one keeps the figure that belongs to its own pair of scales.

What this clock does not show

  • No realized scale. Nothing here is read from a clock on the Moon. A coordinate time scale is not read by any real clock, and no lunar reference time scale has been realized yet, so every lunar line is a model. [6]Official
  • No leap seconds inside the model. The model treats UTC as uniform, so the 27 leap seconds inserted since 1977 are not applied; over 49 years that omission is worth about 0.0175 microseconds of drift. [4]
  • No periodic terms. The published rate for a selenoid clock against a geoid clock carries a periodic term of about 0.108 microseconds per day, which this model drops and keeps only the secular part. [1]
  • No lunar time zones. No time zone system for the Moon has been defined; the question of how a lunar time scale relates to UTC is explicitly left to international agreement. [6]Official

Methodology: how we calculate this

The same model values are available as data: the JSON API returns the offset for either pair of scales right now, and the drift accumulated between any two dates.

Short answers to the questions this clock raises — what time it is on the Moon, whether the standard exists yet, why two rate figures are both correct — are in the lunar time FAQ.

Keep reading

Sources

  1. A Relativistic Framework to Estimate Clock Rates on the Moon — Neil Ashby and Bijunath R. Patla, NIST — The Astronomical Journal 168:112, . Peer-reviewed. Verified .
  2. Policy on Celestial Time Standardization — White House Office of Science and Technology Policy, . Official document. Verified .
  3. Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL) — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .
  4. Our calculation: the lunartime.org clock model — Lunartime Editorial, . Our calculation. Verified .
  5. What Time Is It on the Moon? — National Institute of Standards and Technology, . Official document. Verified .
  6. Resolution on the establishment of a coordinated lunar time standard by international agreement — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .