Lunar time and the lunar calendar are not the same
Key facts
- The Moon travels around Earth once every 27.322 days, in an elliptical orbit. [1]Official
- The same cycle of sunlight at a point on the surface repeats about once a month, every 29.5 days. [1]Official
- A clock on the Moon gains about 56 microseconds a day on a clock on Earth. That is a model figure, not a reading. [2]Official
- The International Astronomical Union defined a coordinate time for the lunar reference system in 2024. A coordinate time is not a calendar. [3]Official
Lunar time and a lunar calendar answer different questions. One is about how fast a clock ticks. The other is about dates and sunlight.
The two are confused often, because both carry the word lunar and both are counted in days. This page separates them, and says which of the two this site covers.
What this site covers, and what it does not
lunartime.org is about time scales. It covers how a clock on the Moon runs against a clock on Earth, and which scale a lunar timestamp would be quoted in [4].
This site does not cover lunar calendars, moon phase dates, festival dates or astrology. No page here publishes a phase calendar, and this page is the only one that names the subject at all.
If that is what you came for, this is the wrong site, and saying so is more useful than a page that pretends otherwise.
Three numbers, and what each is a fact about
The first two rows below are the raw material of a lunar calendar. The third is the subject of this site. Nothing converts one into the other.
| Quantity | Value | What it is a fact about | Source |
|---|---|---|---|
| Orbit around Earth | 27.322 days | the Moon’s motion | [1]Official |
| Repeat of the sunlight cycle | about 29.5 days | light on the surface | [1]Official |
| Rate of a lunar clock against an Earth clock | about 56 µs per day, as a model | how fast a clock ticks | [2]Official |
The Moon keeps the same side turned towards us, and its surface gravity is about one sixth of Earth’s [5]Official. Those are facts about the body, and they change none of the three rows above.
Why a calendar and a time scale are different objects
A calendar divides time into named units and fixes where each unit starts. It answers what date it is.
A time scale defines the rate of a clock and the instant that rate is counted from. It answers how much time has passed, and to what precision.
The two do not overlap. A calendar with a wrong rate is still a calendar; a time scale with no calendar attached is still a time scale.
What the lunar time work actually decides
NIST puts the rate of a lunar clock against an Earth clock at about 56 microseconds per day [2]Official. That figure is a model result, and the site’s lunar clock shows values computed from it [4].
The 2024 United States memorandum names the proposed operational standard Coordinated Lunar Time and sets a deadline for an implementation strategy [6]Official. Nothing in it sets a date, a month or a year on the Moon.
The International Astronomical Union defined a coordinate time for the lunar reference system in its 2024 resolution [3]Official. A coordinate time is a mathematical scale, not a way of numbering days.
The one place the two subjects touch
The length of a lunar day comes up in both. Sunrise to sunrise at a fixed point repeats about every 29.5 days, while the Moon turns on its axis in 27.322 days [1]Official.
Those intervals matter to mission planning, and they are the reason a calendar built on lunar cycles drifts against a solar one. They still say nothing about the rate of a clock. That contrast is worked through in how long a day on the Moon is.
If you came here for a calendar
This site does not cover lunar calendars or moon phase dates, and it will not add them. NASA publishes a page on lunar phases, which we cite here only for the orbital period of 27.322 days [1]Official.
If you came here for the time standard instead, start with what Coordinated Lunar Time is, then why clocks run faster on the Moon. The Learn hub lists every article we have on the subject, and the glossary defines the terms.
Sources
Last verified