Are the times a-changin'?

How do you tell the time? Most people will answer that question by looking at their watch, clock or computer, but how do those devices know the time.  If you are like me, they were probably set from the TV, the speaking clock or those on-the-hour 'pips' broadcast on the BBC World Service. For most people this is good enough. You usually don't need to know the time to better than a few seconds as it usually doesn't matter if you are one second late to a meeting. However, there are a lot of things in the modern world which do need to know the time quite accurately. One application of accurate time is in the global positioning system which is being used more and more these days.

Time also has a lot of applications in astronomy because the Earth rotates. As it rotates, the Sun, Moon and stars appear to move across the sky - rising in the east and setting in the west slightly less than every 24 hours. So, if you want to take a pretty picture of your favourite object, you need to know where it will be and when, otherwise you won't catch it. This is just the sort of information you need to catch a train, although that doesn't necessarily help for the local trains that I use.

In astronomy, if you get the time wrong, you could miss the star, galaxy or nebulae that you are trying to observe. In your backyard this isn't really a problem, as you can have a fish around the area where you think it should be. However, big professional telescopes cost a lot of money to operate and if you are paying for it you don't want to waste time searching for things - you just want to point and shoot. This is especially true when you are trying to observe faint objects that you can only see with a long exposure time.

For a number of years astronomers have made use of Universal Coordinated Time (UTC) or Universal Time (UT) to know precisely what the time is to within a small fraction of a second. UTC is set by a number of atomic clocks around the world which measure oscillations in caesium atoms and gives a time accurate to roughly 1 part in a thousand, thousand, billion. But, the Earth doesn't behave nicely. It slows down over time and occasionally speeds up as happened after last December's tsunami. The result is that over time, the time as measured by the position of the Sun (due to the rotation of the Earth) gradually goes out of step with atomic time. To fix this problem, leap seconds have been occasionally added over the last thirty years to keep solar time and atomic time together.

Now, a proposal to remove leap seconds has been put forward to the International Telecommunications Union (ITU) and will be discussed next month. Abolishing leap seconds has the advantage of making computerised time systems simpler to implement, but at the cost of the time getting out of step with the position of the Sun.

Ultimately, over a very long time, this could mean that the Sun may end up rising at 8pm and setting at 8am. Although this would take a long time at the current rate (21 seconds over 30 years), professional astronomers would have to give up on UTC a lot sooner than that. As you might imagine, the Royal Astronomical Society (the professional body for astronomers in the UK) is not happy about these plans and recently released a response to the International Telecommunications Union (ITU). Can they take on the vested interests of the computer industry and the military and win? It doesn't seem likely.

Talking of time, the last Sunday in October (this weekend) sees the end of summer time in Europe, so remember to put your clocks back one hour and have an extra hour in bed. Other parts of the world change at different times - Tasmania changed a few weeks ago - so make sure you have the right time.

Posted in astro blog by Stuart on Monday 24th Oct 2005 (23:52 UTC) | Permalink
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