Clocks and Calendars on Other Worlds

What time is it on the moon?

You may have read that the European Space Agency is proposing that an international organization be responsible for setting and maintaining lunar time. Why might that be necessary? And what’s wrong with good old fashioned Earth time?

Earth

As our ability to measure time in ever smaller increments has improved over the years, there have evolved any number of different systems for doing so. The one with which we are most familiar is Universal Coordinated Time (UTC), which is essentially what used to be known as Greenwich Mean Time. With appropriate offsets for different time zones, this is how we set our clocks.

Even here, UTC is not maintained with any single clock. It is a weighted average of the time kept by over 450 atomic clocks worldwide. And each of these clocks must be corrected for their altitude relative to the International Terrestrial Reference Frame. Clocks at different altitudes will run at different rates due to gravitational time dilation, something predicted by Einstein’s Theory of Relativity and confirmed repeatedly since then.

 

Robot Spacecraft

Robots don’t care what time it is, as long as they know when to carry out their tasks. If a spacecraft is being controlled by the Japan Aerospace Exploration Agency (JAXA) for instance, it makes sense to keep clocks at the Japan Standard Time of their headquarters in Tokyo.

Crewed Spacecraft

If anyone in the world had never heard of Houston before the Apollo missions, they certainly became familiar during that era. Neil Armstrong’s first words after landing on the moon—“Houston, Tranquility Base here. The Eagle has landed.”—made it clear where Mission Control was located. A combination of MET (Mission Elapsed Time) and Central Time of Houston kept the astronauts on task.

Moon

Why would we need a separate time system for the moon?

From a Science Alert article:

In the next few years, several robotic landers will be sent off to the Moon from various space organizations and private companies. What’s more, ESA, NASA, the Japan Aerospace Exploration Agency (JAXA), and the Canadian Space Agency (CSA) are working together on establishing an orbiting lunar station, called Gateway, where future expeditions can launch from.

“These missions will not only be on or around the Moon at the same time, but they will often be interacting as well – potentially relaying communications for one another, performing joint observations or carrying out rendezvous operations,” reads a press release from the ESA.

Historically, every mission that has gone to the Moon has used the atomic clocks on Earth to track their progress, synchronizing their time in space with their time on Earth.

This basically requires ‘radioing home’ and asking people on Earth what time it is, while also accounting for the time it takes to make that call.

A normal old clock on board a spacecraft simply won’t do the trick. The forces of gravity and velocity are different on the Moon, which means they impact time in different ways than the forces nearer our own planet.

Practically, this means that if a lunar astronaut brought a watch with them from Earth, it would run faster than normal by dozens of microseconds a day. How fast depends on whether that astronaut is in orbit or standing on the Moon itself.

Under these complex conditions, stable timekeeping set specifically to the Moon will be tricky to establish, but it could be more accurate and faster than synchronizing with Earth time.

Mars

The teams operating the solar-powered Mars rovers Spirit and Opportunity had to adapt themselves to the Martian day (called a sol), which is 39 minutes and 35.244 seconds longer than a day on Earth.

This meant that if you were a rover driver (Could there possibly be a cooler job description?), you might go to work during daylight hours and then, a few weeks later, in the wee hours of the morning.

How about people who could someday actually live and work on the surface of Mars? As you might imagine, science fiction writers have come up with quite a few different schemes.

One would be to stop all clocks on Mars at midnight for exactly 39 minutes and 35.244 seconds, and then resume. This “time slip” would allow a 24 hour clock without having to redefine any units. The second is defined precisely and is the basis for many other derived units of measurement.

The Martian year is 687 days or 668.6 sols long. Complicating the creation of a calendar is the elliptical orbit of Mars, which means the seasons are not of the same length, and that they vary considerably between the northern and the southern hemispheres. Here is one science fiction author’s suggestion. This is from Kim Stanley Robinson’s superb Mars Trilogy.

Others have elaborated on this to have 24 months, 21 of which would have 28 sols, and 3 of which would have 27. This gives a total of 669 sols, which would require a periodic adjustment similar to our leap year. Except for this calendar, it would require a backward leap!

Set your watches now! Oh, wait—does anyone under 30 actually wear a watch any more?