What day is it? Pull out your little desk calendar.

We take these time-keeping devices pretty much for granted, when in reality, many of us base our very lives on this notion of time.

We don't know what time itself is, but we can measure it arbitrarily, and so use it very accurately.

But remember, the passage of time as we are familiar with it, is useful only on the Earth.

As we leave our planet and embark on interplanetary excursions, Earth time becomes useless.

To begin with, if you are travelling in space, there is no natural division between day and night. All you have to do is look out your spacecraft window and the Sun is always shining.

In space, the concept of the year as we know it also becomes meaningless.

In fact, it would be far simpler to establish a rythmic system of time keeping aboard a spacecraft. You could set up any kind of arbitrary system to match your needs — accurate and exact, and not dependant upon the random motions of nature.

Whereas, like the Earth, any other planet would have its own peculiar movements and motions — some of which would make even our cumbersome Earthy system of measuring time look like grade school arithmetic.

For instance, the calculations needed to keep our Earth's calendar in step with the seasons, or compensate for orbital differences in time-keeping, and adjust Daylight Savings Time periods are nothing when compared to some of the bizarre situations that arise elsewhere in the Solar System.

Take the planet Mercury for example. It takes only 88 days to orbit around the Sun (one Mercury year) but requires 59 days to turn once on its axis (one Mercury day). The ratio of these periods is such that from the surface of Mercury — you would, at times, see the Sun rise about halfway up the Eastern sky — halt, and reverse — set again — then rise again to finally complete its cycle.

In the West it would then set — and after a time, rise again, as if for one last look around, and finally set once more. This “dance” of the Sun then, is a result of the interplay between Mercury's motions. How would you like to try telling time by the Sun on Mercury?

Let's take another example — the planet Venus. Not only is this planet upside-down — with the North Pole at the bottom and the South Pole at the top from our perspective — but it also rotates backwards, as compared to all the other planets.

On top of that, Venus takes 243 Earth days to rotate once on its axis — but only 225 days to orbit once around the Sun. So its day is longer than its year — an interesting problem for calendar makers!

Mars is somewhat easier. With a period of rotation only half an hour longer than Earth's, and its axis tilted to almost the same degree, its days are quite similar to ours. But being it takes nearly two Earth years to orbit the Sun, the Martian seasons are twice as long as Earth's

How about a shorter day? Then move to Jupiter, where the entire planet rotates in slightly under eleven hours. Of course since it takes twelve years to revolve once about the Sun, you would have a Jupiter year of over 8,000 days! How many months could you divide that into?

How about a simpler calendar with some longer seasons? Try Uranus, where you have 42 years of sunshine and another 42 years of starlight. Of course the temperatures that far into space would make Antarctica look like a summer resort.

This condition on Uranus arises from the planet's axis being tipped at right angles to normal — in other words, the planet “rolls” on its side as it orbits the Sun — alternating seasons on each hemisphere as it moves through its 84 year course.

Of course when we resume exploring the Earth's moon and set up mining stations there — lunar time will come into wide use. Up there you have 14 days of sunlight and 14 days of darkness — but the Earth will remain fixed in the sky.

This is because the Moon is orbit locked. Its periods of rotation and revolution are equal — with one side always facing the Earth. But the whole Moon, does get sunshine at one time or another as it revolves about our planet.

Of course there may arise a few labor union complaints about a 14-day night shift!

In retrospect then, our Earthly problems of telling time and the seasons are rather mild as things go. Well, it's about time for this column to end, so …!