What's the Time?

Watch out!

Except where noted, the eclipse times quoted in these pages are in Universal Time, not local time; you will need to adjust for whatever your local time is (taking summer time into account, where appropriate).

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 (also known as UTC); all the world's timezones are defined with respect to GMT (except where solar time is used). To find your own timezone, you need to correct for:

• where you are in the world;
• summer time, if yor country uses it.

For example, Britain uses GMT in winter; in summer, it uses BST (British Summer Time) which is 1 hour ahead of that.

To find your own time zone, it's best to check in your country to see what the situation is. Unfortunately, no Web page can provide official information on world time zones because nations are sovereign powers that can and do change their timekeeping systems as they see fit. However, the USNO World Time Zones Page offers a non-definitive guide to the world's time zones.

But 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.

British Summer Time

So what about GMT? Well, GMT was originally a time system based on the actual passage of the Sun over the Royal Observatory at Greenwich. Since the Earth's rotation is erratic, this was averaged, giving a mean, or average, value for the time of noon. This was called Greenwich Mean Time (GMT), and for a long time was the basis of civil timekeeping throughout the world.

The disadvantage of GMT was that it is based on a varying quantity, ie. the Earth's rotation. This means that the length of a second, calculated from GMT, would vary from year to year; whereas scientists need an exact and constant unit of time for measurement. UTC provides this, and so now UTC is the basis for all civil timekeeping. Today, the term "GMT" is usually synonymous with UTC -- the RGO's time leaflet says "... GMT, which in fact nowadays is Universal Time (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. And there's good news on this: the UK has finally adopted a single rule for the start and end dates of BST! To quote the RGO's summer time leaflet:

The European Union has now adopted The Ninth European Parliament and Council Directive on Summer Time Arrangements in which it states that summer (or daylight saving) time will be kept between the last Sunday in March to the last Sunday in October. The changes will take place at 1am GMT.

These dates have since been confirmed by the UK Parliament for the UK and Northern Ireland.

Outside of the UK, many countries around the world use daylight saving time; they include the USA, Russia, most of Europe and the Commonwealth. The rule stated for the USA in the U.S. Naval Observatory's Daylight Time page is:

Daylight time begins in the United States on the first Sunday in April and ends on the last Sunday in October. On the first Sunday in April, clocks are set ahead one hour at 2:00 a.m. local standard time, which becomes 3:00 a.m. local daylight time. On the last Sunday in October, clocks are set back one hour at 2:00 a.m. local daylight time, which becomes 1:00 a.m. local standard time.

This puts the USA and the UK out of step for one week in March-April every year.

British NonStandard Time

As a historical note, the definition of summer time in the UK was, until very recently, a total shambles. How come? Well, although the summer time issue seems pretty simple, there was a problem; the UK government was almost always, on and off, considering mucking around with BST. Ideas which were, 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.

Even when "normal" BST was in force, the start and end dates were not based on any published rule that I know of. The Admiralty was responsible for setting the start and end dates, and it was essentially done ad hoc each year. Even when a "rule" was used, one rule and another was chosen and discarded on an ad hoc basis. There were attempts to get in step with Europe; and then we broke away again, with the effect that, in some years, Europe has tried getting in step with us!

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 during any eclipse. So in general, times here will continue to be quoted in UT; when the time comes, you'll need to convert to your own local 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:

What is Universal Time?

One of a collection of papers at USNO, covering the whole issue of Universal Time versus GMT in great detail.

Summer Time

Interesting leaflet on how the start and end dates of BST are calculated. Calculated! Hah! We should be so lucky.

The Time Service Dept, USNO

The U.S. Naval Ovservatory's The Time Service Dept, the authority for the precise definition of time in the USA. Has a number of fascinating papers on all aspects of timekeeping. Well worth checking out.

World Time Zones

From the Time Service Dept. of the U.S. Naval Observatory.

Royal Observatory Greenwich

Museum of the (now defunct) premier UK observatory, with a superb collection of background info.

ROG Information

Information leaflets available from the ROG, on a range of time-related and astronomical subjects. It's all fascinating stuff!

USNO Master Clock

Information about timekeeping at USNO: papers on systems of time, GPS time, and much more.

worldtimezone.com

An interactive world timezone map.

USNO Astronomical Applications

The Astronomical Applications Department of the U.S. Naval Observatory, with a range of astronomical information.