How long is a day on the Moon?
Key facts
- The Moon travels around Earth once every 27.322 days, in an elliptical orbit. [1]Official
- Sunrise to sunrise at a fixed point on the lunar surface repeats about every 29.5 Earth days. [1]Official
- The Moon's surface gravity is about one sixth of Earth's, and it keeps the same side turned towards us. [2]Official
“A day on the Moon” has at least three answers, and they differ by days. Which one is right depends on what you are counting: a turn on the axis, a cycle of sunlight, or the working unit a mission actually schedules against.
The three answers are given below, with the source for each.
Three different days
| Which day | Length | What it measures | Source |
|---|---|---|---|
| Sidereal rotation | 27.322 Earth days | one turn of the Moon on its axis, measured against the stars | [1]Official |
| Solar day, sunrise to sunrise | about 29.5 Earth days | the interval from one sunrise to the next at a fixed point on the surface | [1]Official |
| Earth day used by mission clocks | 24 hours | the unit lunar operations actually run on today | [3]Official |
The Moon turns on its axis in the same time it takes to travel once around Earth, which is why the same side always faces us [2]Official.
Why the two differ
The Moon does not stand still while it turns. It keeps moving along its orbit around Earth, so by the time it has completed one turn against the stars, the Sun is no longer where it was in the lunar sky.
The Moon therefore has to turn a little further before the Sun comes back to the same point above a landing site. That extra turn is what separates the two figures: about 29.5 Earth days minus about 27.3 Earth days, or roughly 2.2 extra days of turning [4].
Earth has the same gap between its sidereal day and its solar day, and it is small, because Earth covers only a tiny part of its orbit while it turns once. On the Moon the gap is measured in days instead, because one trip around Earth takes 27.322 days [1]Official.
Daylight and night at a landing site
If the solar cycle at a fixed point lasts about 29.5 Earth days, then a site on the equator gets roughly 14.8 Earth days of sunlight followed by roughly 14.8 Earth days of darkness [4]. That is the number that drives battery sizing, thermal design and mission planning.
The halves stop being a useful description near the poles, where the Sun stays close to the horizon and the terrain decides what is lit. We do not have a sourced figure for polar sites in our registry, so we do not print one.
Why this has nothing to do with the time standard
The length of a lunar day is a fact about sunlight. A time standard is a definition of how a clock ticks and what it is compared against. The two answer different questions, and mixing them is the most common mistake on this subject.
Nothing in the proposed lunar time work redefines the second or ties a scale to sunrise. The IAU resolution of 2024 defines a coordinate time for the lunar reference system, not a calendar [5]Official. Our lunar clock shows a model value in seconds and microseconds, and it would read the same whether the landing site were in sunlight or in darkness.
Two related questions are answered elsewhere on this site: whether the Moon will have time zones, where the length of the solar day is the reason zones are a poor fit, and what Coordinated Lunar Time is, which sets out the proposals themselves. Both are listed in the Learn hub.
The lengths on this page are facts about sunlight, and they are what a calendar is built from. Why that is a different subject from a time scale is set out in why lunar time is not a calendar.
Sources
Last verified