It’s been 50 years since humans walked on the surface of the moon — but our half-century of lunar longing is quickly coming to an end. Space agencies and private companies are preparing a deluge of missions to our celestial neighbor, with the ultimate goal of establishing a base camp on its surface. While scientists are trying to solve the myriad issues presented by living on the moon (food, water, and shelter being chief among them), there’s another not-insignificant question that needs answering: What time is it on the moon? Moon missions in the past have operated on their own various timescales in relation to Coordinated Universal Time (UTC), but the short answer is: We don’t really know, because we haven’t decided yet.
In November 2022, representatives of space agencies around the world gathered at a European Space Agency facility in the Netherlands to answer this very question, but creating a time for the moon isn’t easy. For one thing, a day on the moon lasts 29.5 Earth days. Time itself also flows differently on the moon, as explained in Albert Einstein’s general theory of relativity, which notes that time moves more slowly in stronger gravitational fields. Because the moon’s force of gravity is weaker than Earth’s, a lunar clock would gain an estimated 56 microseconds (a microsecond is one millionth of a second) compared to Earth clocks every Earth day. That may seem like a small discrepancy, but it’s one that would prove disastrous for any lunar GPS system, which would be extremely reliant on accurate timekeeping. One idea is to synchronize the moon to UTC, or to create a wholly independent lunar time — a framework that could also work well for future human settlements on Mars and beyond. For now, the moon’s time remains mysterious, but soon Earth’s only natural satellite will be brought into the temporal fold.
Although several theories attempt to explain why the Earth plays host to just one lonely moon, the leading idea is that a Mars-sized celestial body (nicknamed “Theia”) smacked into Earth some 4.5 billion years ago during the chaotic beginnings of our solar system. The debris from the collision shot up into orbit, and eventually coalesced into our moon. However, there’s one major difference between the moon of today and the moon of 4.5 billion BCE. Scientists estimate the moon was around 14,000 miles away back then, compared to the 250,000-mile distance today. As a result, Earth’s ancient skies would have been dominated by an enormous, dull red moon roiling with magma. Today, the moon’s slow journey from Earth continues, and the sun will go supernova before it ever stops.