Coordinated Lunar Time: Why the Moon Needs Its Own Clock

The race to return humans to the lunar surface has introduced a strange and complex problem: what time is it on the Moon? In April 2024, the White House Office of Science and Technology Policy (OSTP) officially directed NASA to solve this problem. They must establish a unified standard of time for the Moon, known as Coordinated Lunar Time (LTC), by the end of 2026. This is not just about astronauts setting their watches; it is a critical physics challenge essential for the safety of future space exploration.

The White House Directive

On April 2, 2024, Arati Prabhakar, the head of the OSTP, sent a memo to NASA instructing the agency to work with other U.S. agencies and international agencies to establish a time-centric strategy. The goal is to have a fully operational strategy for Coordinated Lunar Time developed by December 31, 2026.

This deadline is not arbitrary. It aligns closely with the Artemis program schedule. NASA currently plans to send astronauts around the moon on the Artemis II mission in 2025 and land on the lunar South Pole with Artemis III as early as September 2026. Without a unified time standard, these missions face significant technical risks.

The Physics of Lunar Time

To understand why we need LTC, you have to look at the theory of relativity. Einstein discovered that gravity influences the flow of time. The stronger the gravity, the slower time moves. The Earth is much more massive than the Moon, meaning Earth has a much stronger gravitational pull.

Consequently, time actually moves faster on the Moon than it does on Earth. While the difference is imperceptible to a human standing on the lunar surface, it is massive in terms of high-precision computing.

Specific calculations by NASA and the OSTP show that a clock on the Moon would appear to gain an average of 58.7 microseconds per day (0.0000587 seconds) compared to a clock on Earth. Additionally, there are periodic variations causing the time to drift even further.

Why Microseconds Matter

You might wonder why a fraction of a second matters. In the high-stakes environment of space travel, 58.7 microseconds is an eternity. Modern navigation relies on the travel time of light and radio waves to calculate distance.

• Light Speed: Light travels at approximately 300,000 kilometers (186,000 miles) per second.
• The Error Margin: If there is a discrepancy of 58.7 microseconds between a transmitter and a receiver, the distance calculation could be off by roughly 17 kilometers (10.5 miles) in a single day.

If a spacecraft tries to dock with the Lunar Gateway (a planned space station orbiting the Moon) or land at a specific coordinate on the South Pole, an error of several miles could lead to a catastrophic crash or missing the target entirely.

Creating a "GPS" for the Moon

On Earth, we rely on the Global Positioning System (GPS). This system works because dozens of satellites carry highly precise atomic clocks that are perfectly synchronized with Coordinated Universal Time (UTC). Your phone determines your location by measuring how long it takes for signals from these satellites to reach you.

NASA and its partners want to build a similar architecture for the Moon, often referred to as LunaNet. This network will provide communications and navigation services for rovers, landers, and astronauts.

However, you cannot simply synchronize lunar satellites with Earth-based UTC because of the gravitational time dilation mentioned earlier. If a lunar satellite tried to sync strictly with an Earth clock, the signals would drift apart immediately. Therefore, the Moon needs its own reference time (LTC) anchored by atomic clocks placed physically on the lunar surface.

International Implications and the Artemis Accords

The establishment of Coordinated Lunar Time is also a major diplomatic move. Space is becoming crowded. It is not just NASA going to the Moon; China, India, Japan, and Russia all have active lunar programs. Private companies like SpaceX and Blue Origin are also heavily involved.

If every country or company uses its own time standard, communication between spacecraft would be impossible. Imagine a scenario where a NASA astronaut needs to send a distress signal to a nearby Chinese rover. If their time systems are not synced, the data transfer could fail, or location triangulation could be wrong.

The United States is pushing for LTC to be the standard adopted by all nations. This effort falls under the umbrella of the Artemis Accords, a set of principles for cooperation in space exploration signed by 36 nations so far. By establishing the standard now, the U.S. ensures that future lunar infrastructure is built on a transparent, unified system rather than a fragmented one.

How Will LTC Work?

The OSTP memo outlined four key features that Coordinated Lunar Time must possess to be successful:

1. Traceability to UTC: While LTC is distinct, it must have a clear mathematical relationship to Coordinated Universal Time on Earth so that ground control in Houston can communicate effectively with the Moon.
2. Accuracy: It must be precise enough to support navigation and scientific experiments.
3. Resilience: The system must function independently. If contact with Earth is lost, the lunar time system must keep ticking accurately on its own.
4. Scalability: The system must be designed to extend to Mars. The gravitational differences on Mars will present a similar problem, and LTC will serve as a testing ground for future Martian time standards.

The prevailing theory is that LTC will be determined by an ensemble of atomic clocks deployed on the Moon. Just as UTC is an average of hundreds of atomic clocks on Earth, LTC will likely be a weighted average of clocks on the lunar surface and in lunar orbit.

Frequently Asked Questions

Will the Moon have time zones like Earth?

Not exactly. On Earth, time zones (like EST or PST) are based on the sun’s position in the sky. The Moon rotates once every 29.5 Earth days, meaning “day” and “night” last for two weeks each. Coordinated Lunar Time will be a single, universal reference time for the entire Moon, similar to how pilots use Zulu time (UTC) regardless of where they are flying.

Does time actually move faster on the Moon?

Yes. This is a proven effect of General Relativity. Because Earth’s gravity is stronger, it bends space-time more than the Moon does. This causes time to pass slightly slower on Earth compared to the Moon. Over 50 years, an astronaut on the Moon would age about 1 second more than their twin on Earth.

Who decides what the time is?

The definition will likely be handled by international bodies such as the International Bureau of Weights and Measures (BIPM) and the International Telecommunication Union (ITU). These are the same groups that maintain UTC on Earth. NASA is leading the push, but the final standard will require international consensus.

Why can’t we just use Earth time?

Using Earth time (UTC) on the Moon creates a “drift.” Because atomic clocks on the Moon tick faster by 58.7 microseconds a day, they would constantly have to be “slowed down” artificially to match Earth. This introduces errors in navigation software. It is cleaner and safer to let the lunar clocks run at their natural speed and call that “LTC,” then use a formula to convert it to Earth time when necessary.