Time. It’s an illusion, you know. Something we’ve invented to keep track of greater events in the Cosmos. Earth spins on its axis in what we call a day, which we have chosen to break up into smaller units called hours. Earth moves around Sol, rotating on its axis three hundred and sixty five times. And a bit. Our planet’s moon moves around Earth, reflecting light from Sol onto the Earth. How much light depends upon the geometric relationship between the Moon and Sol, moving from full moon through gibbous, quarter, right through to no light at all in a regular cycle. We call those cycles ‘months’. More or less. Sort of.
We are so obsessed with the minutiae of our existence we forget that time is nothing more than a reference point. (Meet you at Starbuck’s at 10.) And when you’re writing science fiction, time becomes even more problematical. On Earth we have created for ourselves an imaginary line which is the base line for time. It passes through Greenwich in UK. Why? Well, just because. The English started it, so they had first dibs. So when it’s twelve midnight (ie the beginning of the day) in UK, it is 10am of the following day where I live. You think that’s hard? (I know some of you do)
Well, when we talk about more than one planet, we’ll need some sort of celestial equivalent to Greenwich so we can agree on what the ‘time’ is. Planets spin at different rates, so their ‘day’ will be different. For example, the length of a day on Mars is 24 hours 39 minutes. Pretty close to our day. But a Mars year is 687 of our days. Let’s look at mighty Jupiter, much much larger than Earth. In fact, Jupiter is so big that all the other planets in the solar system would fit inside it. Jupiter’s day is 9.8 of our hours. Yes, that is nine point eight. Its year is 11.86 of our years.
So if we eventually colonise Mars, we’ll have to have some way of equating time on the two worlds. Sure, mainly you’ll work in local time. But let’s say somebody signs a mining lease on Earth for property on Mars. When does the lease expire?
So there you are. When you agree to meet someone for coffee, you set a spatial reference – ie latitude and longitude and then you add time. So whether you knew it or not, you’ve understood space-time all along. Easy.