Being There: What's the Time?

So, what is Universal Time, and is it the same as UTC? How does it relate to BST? And whatever happened to good old GMT?

Simple Answer

Basically, UT is GMT; British Summer Time (which will be in force in August) is 1 hour ahead of that. So, if the maximum eclipse at Cornwall is at 10:10 UT, that will be 11:10 BST.

Other countries have their own timezones, of course. The following table shows the offset from GMT (UT) for countries along the eclipse path, together with the offset used in summer time. Since the eclipse is in August in the northern hemisphere, I assume that summer time will be in force in all countries which use it.

Please note that all timezone information is non-authoritative. Check locally before comitting to any plans.

Country	Offset from UT			Country	Offset from UT
	Standard Time	Summer Time			Standard Time	Summer Time
Eire	0	+1		Hungary	+1	+2
UK	0	+1		Romania	+2	+3
France	+1	+2		Bulgaria	+2	+3
Belgium	+1	+2		Turkey	+2	+3
Luxembourg	+1	+2		Iraq	+3	+4
Germany	+1	+2		Iran	+3.5	+3.5
Switzerland	+1	+2		Pakistan	+5	+5
Austria	+1	+2		India	+5.5	+5.5

So if you'd like to know more about UT, GMT, and all that, read on...

Universal Times

The time used by astronomers is Universal Time, or UT (I believe it's actually UT1), which represents the time based on astronomical observation of the Earth's rotation, as measured at the Greenwich Meridian. Since eclipse predictions are based on the position of the Earth and Moon relative to the Sun, this is quite natural.

The problem (as far as ordinary mortals are concerned) is that the Earth doesn't rotate evenly; not only is it slowing down overall (don't worry, not by very much!), but the rate of rotation can actually speed up and slow down erratically, due to effects such as tidal forces. That would mean that the length of the second calculated by dividing the day into bits would vary from day-to-day.

Civil time, however, is based on the time standard maintained using atomic clocks, which for the sake of uniformity assumes a constant-length second.

TAI is the international atomic clock standard, but it has drifted from UT1 over the years (due to the aforementioned changes in the Earth's rotation: and TAI is actually based on the length of a day in January 1900), so another standard, UTC, is widely used as the basis for civil time (in the UK and US, for example).

Like TAI, UTC uses constant-length seconds, but to stop it drifting away from astronomical time (we want the Sun to be overhead at mid-day, after all), it has leap seconds inserted every year or two. This process is designed to keep it within 0.9 seconds of UT1; but at the same time, it is synchronised to TAI, in that the difference between UTC and TAI is always a whole number of seconds. (The next leap second is coming at the end of 1988.)

British NonStandard Time

So what about GMT? Well, the term "GMT" seems to have become rather ambiguous; it seems to mean either UT1 or UTC depending on the context (although I think it's "supposed" to be UTC). So if you want to be ultra accurate, it's probably best to talk about UT1 or UTC.

And BST? As I said before, UK civil time is based on UTC: ie. it is UTC in winter, and jumps 1 hour ahead in summer. By that rule, the centre of eclipse will be over the Cornish mainland at 11:12 in the morning, British Summer Time (10:12 UT) (to within a second, anyway).

Earlier incarnations of this site were wary about predicting BST times, and quoted only UT. How come? Well, if you're talking about an event that's happening years in the future, there's a problem; the UK government is almost always, on and off, considering mucking around with BST. Ideas which are, at various times, on the table, include:

• Scrapping BST (just using UTC);
• All-year BST (ie. stay at UTC+1);
• Double BST (ie. UTC in winter, UTC+2 in summer);
• Merge with Europe (ie. UTC+1 / UTC+2).

Nobody in Whitehall seems to care much about Scottish school pupils; I can remember walking to school in pitch darkness the last time they decided to mess around with this, and the count of road deaths inevitably rises as a result. So, as a practical matter, it's hard to say what "BST" will mean in the future. (Although it looks like a safe bet to be UTC+1 in 1999.)

As a side note, even assuming that BST continues as it is, the start and end dates are not based on any published rule that I know of. In recent years, one rule and another has been chosen and discarded on an ad hoc basis. As far as I know, the Admiralty is still responsible for setting the start and end dates, and it is done ad hoc each year. There have been attempts to get in step with Europe; but we seem to have broken away, with the effect that, in some years, Europe has tried getting in step with us! Take a look at the RGO's Summer Time leaflet for more info.

So What Time Is It Anyway?

So what's the bottom line? Well, to within 0.9 seconds at least, UT1 (used in the eclipse predictions) and UTC (what your clocks are based on, with whatever offset for your time zone) are effectively the same. Of course, I don't know what country you're reading this from, so I can't tell you what your local time will be in August 1999. So (apart from the UK page), times here will continue to be quoted in UT; when the time comes, you'll need to convert to your own local (summer) time.

Time Links

As usual, beware; the above information has been compiled by me from various sources, and I may well have got it wrong. However, you can always go and check up with the people who really know. Alternatively, you might just want more background information on all the systems of time in use. These links might come in handy: