Time on Earth is a fascinating concept—we mark it in so many ways, using such a wide variety of divisions. And, of course, most humans are somewhat obsessed with their own personal time on Earth. We are so used to the concepts of time that we rarely think about them in the largest sense—rather, we tend to focus upon the smallest divisions of time that affect our daily lives, in terms of meeting times, appointments, starting and stopping the workday, and so forth. And, because we take the existence of common measures of time for granted, we often lose track of how our measurements of time are tied to things well beyond the Earth itself.
Consider the basic units of time measurement. One year is equivalent to the amount of time it takes for the Earth to complete an orbit of the sun. It can be measured from any point on that orbit, as is our habit when we celebrate the date of our birth, regardless of where in the Earth’s orbit that it occurred. Months, on the other hand, are tied to the lunar orbit—although most modern calendars have subdivided the year into 12 portions, while lunar calendars reflect that the moon’s orbit requires approximately 27 days. Religions often maintain a lunar calendar for calculating their holiest periods—it is a method that provides a link to the past, before the calendar was standardized into our modern system of months. Not surprisingly, as divisions of time get smaller, their source becomes more local. As such, days are related to the rotation of the Earth itself—each day on the calendar represents one such revolution of our home.
Why such an explanation of the astronomical sources of our timekeeping? NASA recently announced that it expected to create a manned lunar outpost in the next decade. If the endeavor is successful (and on time), it will be the first human habitat beyond the orbit of the Earth—and its inhabitants will need to keep time, especially if they wish to remain in close contact with the rest of humanity. But, lunar time does not match Earth time—in many ways, the very concept of months and days are reversed on the surface of the moon. The general expectation is that astronauts on the moon would simply follow one of the time zones of the Earth, most likely Greenwich Mean Time (GMT). That is the current practice aboard the International Space Station (ISS), after all, and it works well enough to maintain contact and coordinate communications. Of course, astronauts aboard the ISS do not experience time in the same fashion as terrestrial humans—their spacecraft orbits the earth every 90 minutes, and as such, they witness 16 sunrises and sunsets in a 24-hour period. This has created a whole host of biological effects upon the astronauts—in addition to being in a weightless environment, their natural circadian rhythms are disrupted by the continual shifts from light to darkness.
The time lag in communications between the Earth’s surface and the ISS is negligible, and in astronomical terms, the lunar surface is not much further away. Contact between the Earth and the moon is only delayed by 1.25 seconds—enough to be irritating, but unlikely to be catastrophic. In comparison, when and if humans ever decide to send missions to Mars, the time lag varies between 3 and 21 minutes, depending upon where each planet is in relation to one another in their separate orbits around the sun. Given such a long period between sending a message and receiving the response, any mission headed to Mars requires either an enormous amount of patience, or a substantial degree of autonomy—waiting for instructions from Earth will probably result in disaster. And, if humans are involved in Martian missions, they will still need a mechanism to track time.
From a daily standpoint, Mars revolves almost as quickly as Earth—each Martian “day” lasts approximately 45 minutes longer than one on Earth. It might be possible for humans to adapt to such a change, and continue to follow normal patterns of daily and nightly activity. But, Mars requires nearly twice as long to orbit the sun—approximately 687 Earth days. It cannot rely upon its moons for months, given that one of those moons, Deimos, completes its orbit every 30 hours, and the other, Phobos, orbits Mars three times per local day. Initially, manned missions to Mars will likely carry timepieces reflecting GMT, given that those early missions are unlikely to result in permanent settlements. But, the inherent difficulties associated with maintaining precise time will be compounded by the distances between Earth and Mars. Consider: a timekeeping error of one millionth of a second can result in a GPS error of several hundred meters on Earth—such a timekeeping problem on Mars could result in the complete loss of communications between the Red Planet and mission support personnel back home.
Of course, the further one gets away from Earth, the less likely that Earth’s timekeeping systems will remain the norm. Beyond Mars, an enormous solar system beckons. The asteroid belt offers potential industrial resources far beyond what is easily accessible on the Earth’s surface—and once it becomes economically feasible to reach the asteroid belt, those resources will likely be harnessed as a means to support far greater explorations and colonization efforts. The moons of Jupiter might offer another potential basing point for human endeavors, and Saturn’s moon Titan looks particularly promising in that regard—but each is so distant from Earth that it would require its own system of time in order to be a functional home for humans. Naturally, for most of us, efforts to move beyond the immediate vicinity of the Earth will not be a practical concern—after all, each of us has a finite amount of time to spend on this planet. But, for a lucky few, moving beyond the constraints of Earth’s concepts of time will be a real possibility in the near future—and by being the first to do so, they will likely be remembered for all time.