Being the occasionally interesting ramblings of a major-league technophile.
Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.
The passage of time has been considered an invariable constant since long before any surviving records of its measurement were created. Therefore, when humans finally started keeping detailed records they were rather shocked to discover that it wasn't invariable.
Everyone assumed, of course, that the heavenly clock was perfectly in synchronization. Only, it wasn't. There are twelve months in a year, but not twelve even lunar cycles. There is not an even number of days in the year, but 365 and roughly a quarter (not even a proper quarter). Some stars moved, and they moved in different ways. (These, of course, being the planets, whose apparent motion against the background of stars varies not only with the size of their orbit but whether said orbit is inside or outside the Earth's.)
In short, making a proper calendar proved to be a major challenge. It required painstaking record keeping and frequent modification, until it finally ran right for not just this year and the next, or even a few decades, but many centuries. This was largely done by noting how the distribution of heavenly events - including the Sun’s behavior - through the year slowly drifted with the passage of the ages. This, in turn, required a society which was relatively stable for a thousand years or more. The Roman Empire didn’t last quite long enough, their Julian calendar being fairly good but having some long-term problems. Several ancient societies had calendars better than those of Europe until the modern variation on the Julian was developed.
Even when long-term good data was available there were disagreements about what a calendar should represent. In fact, there still are such disagreements, and about days and other units of time as well as years. Does the year begin with the Winter Solstice? Or some arbitrary day remotely connected with it? Or do you begin with the first full (or new) Moon of Spring? Does the day begin at dawn or at midnight? All of these and more have their proponents. And simply agreeing to disagree sometimes isn't possible.
For instance, most people use a solar day, marking time by the Sun. Astronomers use the sidereal day, marking time by the stars. Because the Earth moves in orbit around the Sun, sidereal days are a bit shorter than Solar (at twenty-three hours, fifty-six minutes and about 4 seconds). Likewise, the sidereal year is about 20 minutes longer than the solar year.
Which standard to use? Well, the solar day and the solar year are what affect the seasons, so that's the standard for most of us. However, sidereal time is important to astronomers because that marks when they know a particular star will be where. (And pity the poor NASA workers who are on Martian time, where each day is over half an hour longer than either solar or sidereal, and the year about double Earth’s.)
Complicating matters further, because the Earth's orbit is an ellipse (though a very nearly round one) and not a circle, its speed around the Sun varies. The Earth's rotation, however, is more regular, and independent of the orbital motion. Result? Noon by the clock is not always Noon by the sundial. Not something obvious to the casual observer, but someone making careful measurements and keeping notes will spot the difference.
Because the Earth's axis is tilted with respect to the Sun, our primary traces an odd path through the sky. Photograph the Sun at Noon each day (or mark the position of a sundial's shadow at the same times) and after a year has passed you’ll have a lopsided figure-eight. This is the analemma.
The patterns traced by the stars are similar, though, since the Earth does not orbit around them not quite the same. However, because the planets move independently around the Sun their paths across the sky are more complicated. Many of them seem to reverse direction for part of their orbits, because the Earth's orbit is inside theirs, and our planet moves faster. So, during part of an outer planet's orbit we are "catching up" to it, making it appear to move backwards in its path.
Trying to account for these movements drove many early astronomers to distraction, especially in Europe. The Church had decreed that stars and planets were fixed in concentric crystal spheres, with the Earth at the center. How could you reconcile the rotation of these spheres with the actual observed movements? Many tried. Those who finally succeeded did so by rejecting the crystal spheres entirely. Many of those who did and published their results were punished for doing so.
Even when placing the Earth and other planets in circular orbits around a central Sun became acceptable, that didn't end the problem. Circles did not properly represent the actual paths as observed. Another blow to the perfection of the universe came with the realization that ellipses did.
For many decades the ellipses did the trick. Astronomy could predict planetary motions with astounding accuracy, even accounting for the slight variances caused by the mass of one planet acting on the others. (Indeed, these variances allowed accurate estimates of planetary masses.)
Accompanying this more realistic understanding of planetary motions was the development of more accurate clocks. Early on, even the most accurate observatory floor clock had to be set by astronomical observations every day or so. During this period the concept of the universe as a giant clockwork mechanism became popular.
As mechanical clocks were improved, however, the standard for setting them became more stringent. The results of several different observations, often of different stars or planets, were averaged together for one observatory, and observatories compared results. Eventually, the Greenwich Observatory in England became the de facto standard.
One benefit of the understanding better timekeeping brought was accurate prediction of the tides. Knowing roughly when the tides will take place and how extreme they will be isn't difficult for today or tomorrow. Simply living near the sea and being observant is enough for that. But longer range predictions were far more difficult. The movement of the tides is quite complicated, and the pattern is not easily obvious. Neither is the reason for their variation. Even Galileo got the cause of tides wrong.
Accurate record keeping which included time and date according to a known standard, combined with better knowledge of the movements of the Moon around the Earth and the pair around the Sun helped find the pattern in the tides. Not only could tables be prepared showing, for example, the highest tide of next June, but clocks were built which mechanically replicated the process. Many of the fancier timepieces - especially large floor or tower clocks - also displayed the phase of the Moon, the date, and other things.
But as the clocks got better, the heavens again revealed new imperfections. The orbit of Mercury was slightly different from what extrapolations based on previous observations stated it should be. Astronomers predicted the presence of another planet, named Vulcan by some, inside the orbit of Mercury. Some actually recorded observations of it. Observations which could not be replicated.
The prediction was reasonable. Similar predictions had worked for finding undiscovered outer planets. But Vulcan turned out to be a phantom, and another cause for Mercury's variance would have to be found.
Einstein found it. Mercury is close enough to the Sun that the distortion in space-time caused by our central star’s mass makes the planet’s orbit detectably different from what it otherwise would be. And with that came a whole new slew of problems related to measuring time.
For centuries, scientists had assumed the passage of time was not only constant, but constant everywhere, even when studies of the heavens showed that the apparent motions of planets weren't. Now, even time could flow differently. Though only under extreme conditions.
Modern time standards take into account this variation in the flow of time. This works well enough that time signals from orbiting satellites can be used to find locations on the Earth. (Contrary to the concept many have of how GPS works, the units don't send signals to the satellites. They only receive the time signals from them, and by comparing and doing a bit of trigonometry calculate position.) Our clocks are now so accurate and precise that they have to be slightly adjusted about once a year to keep them in synch with the Earth. Which leads some to wonder “Why bother?” Others insist that the way we measure time should remain connected to something “real.” Like the time it takes the Earth to rotate, or orbit the sun. Even though those events are less constant than our best clocks.
When (I’m an optimist) we have colonies on other worlds the problem will be exacerbated, and complicated. The Moon won’t be so bad. Its rotation is so slow that the day-night cycle is beyond our ability to adapt to, and the need for protection from the harsh environment will mean most people living there will spend all or most of their time underground, anyway. Mars, with its slightly longer day, will cause more problems. Anyone spending time there - even if only a few days - will probably use the Martian day. Which will put them out of synch with Earth time. For short-term missions those here who have to work with those there will probably go on Mars time, just as the managers of the Spirit and Opportunity rovers did. For interacting with long term missions, and actual colonies, those on Earth will most likely just stay on this world’s time and have careful shift management.
Technology makes knowing the “right” time much easier. I have a wristwatch with a built-in shortwave receiver which sets itself by time broadcasts. Computers can be automatically set to get the correct time online. And modern clocks are so accurate that they don’t really need to be set that often.
Still, the problem remains; what do you set them to?
This document is Copyright 2005 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: email@example.com