KNOWING TIME is simple in everyday life. You look at a clock. You assume that everyone else's clock in your time zone reads the same. And that's that.

For astronomers, however, time can become quite complex. The reason is that our units of time measurement -- the day and its subdivisions of hour, minute, and second -- are based on astronomical phenomena that are themselves more complex than you might think.

Most of these complications have been smoothed out of our everyday civil time system by official edict. The result is a simple, easy-to-use timekeeping arrangement that serves society well -- as long as nobody looks too closely at the sky. Do so, and all the carefully hidden fudge factors erupt back into view.

Here, then, is a summary of the time systems that a well-informed amateur should know.

Local Apparent Time (LAT), also called apparent solar time or sundial time, is what everyone used long ago when they told time by the Sun. Noon was what most people still think is noon: when the Sun crosses the meridian -- that is, when the Sun is due south (for people at north temperate latitudes), at its highest point of the day, and halfway between sunrise and sunset. The very word "meridian" is from the Latin for "mid-day."

But when reasonably accurate clocks were invented, careful timekeepers noticed that something was wrong with solar time. The Sun sometimes runs up to 16 minutes fast in its daily travels across the sky, and sometimes as much as 14 minutes slow, depending on the season.

This effect arises from the tilt of the Earth's axis and the ellipticity of the Earth's orbit around the Sun. To escape the problem, our next time system was invented.

Local Mean Time (LMT). Astronomers created an imaginary, well-behaved mean Sun that travels along the celestial equator at a uniform rate to make its annual circuit around the constellations. The mean Sun has the average or mean right ascension of the real Sun. Noon became the moment when the mean Sun crossed the meridian.

The number of minutes the real Sun lags behind or runs ahead of the mean Sun was named the equation of time. Its value for any date can be looked up in an almanac or can be read from the graphic Skygazer's Almanac in the center of Sky & Telescope's January issues.

But this adjustment wasn't enough. An even worse problem results from the fact that the Earth is round.

Standard time. Because the Earth's surface curves, "overhead" at your location is a different direction than "overhead" just a few miles away. Similarly, when the Sun or a star is on your meridian it has not yet reached the meridian of someone to your west, and it has already crossed the meridian of someone to your east.

At 40° latitude the difference amounts to one minute of time for every 13 miles east or west. To a person watching the sky 13 miles west of you, the time seems to be 11:59 when you swear it's 12:00 and someone 13 miles east insists it's 12:01. This is why Local Mean Time is local. It depends on your location.

This didn't matter when travel and communication were slow. The problem grew more acute in the 19th century. The widespread use of telegraphs and railroads finally forced a change. How could you catch a train when every town and every railroad company kept a slightly different time?

In 1883 the United States was divided into standard time zones; the rest of the world soon followed. In each zone, all clocks are set to the Local Mean Time of a standard longitude: 75° west for Eastern Standard Time, 90° for Central, 105° for Mountain, and 120° for Pacific. Each time zone differs from its neighbors by one hour because these longitudes are 15° apart -- 1/24 of the way around the Earth.

Standard time was a great advance for society. But not for skywatchers. Planispheres (star wheels) still work in Local Mean Time (LMT). So does every all-sky map that shows horizons, such as the one in Sky & Telescope every month. So does the Skygazer's Almanac in our January issues; the "Local Time of Transit" scale on our Sun, Moon, and Planets This Month chart; and every other map, device, or calculation that shows astronomical objects with respect to your horizon, zenith, or meridian without taking your local longitude explicitly into account.

Luckily, correcting for LMT is simple. For every degree you are west of your time zone's standard longitude, add four minutes to LMT to get standard time. For each degree you are east, subtract four minutes.

To make sure you don't do it backward, use this formula: Standard time = LMT + Correction, where the correction is positive west of your time zone meridian, negative east of it. Find and learn your correction; you'll use it forever.

To get daylight saving time, of course, add an hour to standard time. Daylight saving time is currently used in the United States (except Arizona, Hawaii, and a few Midwestern counties) from 2:00 a.m. on the first Sunday in April to 2:00 a.m. on the last Sunday in October.

Universal Time (UT). Standard time (and its daylight-saving variant) serves fine within a given time zone. But when a time applies worldwide, such as in an astronomical almanac, which time zone should be favored?

Logically enough, the "universal" time zone that was agreed upon is that of 0° longitude. This longitude is, by definition, that of a line engraved in a brass plate in the floor of the Old Royal Observatory at Greenwich, England. Hence UT was long known as Greenwich Mean Time (GMT).

By tradition UT is stated in the 24-hour system, whereby noon is called 12:00, 1 p.m. is 13:00, 2 p.m. is 14:00, and so on. Midnight is called 0:00.

One of the first things a beginner must learn is how to turn UT into standard time. It's easy. To get Eastern Standard Time, just subtract 5 hours from UT. For CST subtract 6 hours, for MST 7, for PST 8. Other time zones have their relations to UT listed in many places. (To get daylight saving time, remember to subtract one hour less than these values.)

Of course the date must be given in the same system as the time! If you get a negative time by subtracting from UT, add 24 hours. In this case the result is on the date before the UT date. For instance, 2:00 April 15th UT is 10:00 p.m. Eastern Daylight Time April 14th. These instructions and an example are in the Calendar Notes section of Sky & Telescope every month.

Many amateurs find it easiest just to remember when 0:00 UT (often written 0^h) happens in their time zone. For example, 0^h UT is 7 p.m. EST (8 p.m. EDT) on the previous date.

Ephemeris Time; Dynamical Time. Once the worldwide system of time zones was in place, with UT proudly heading up the list, all should have been well forever after. But such was not to be. Astronomers working with solar system dynamics noticed something very disturbing. The day itself varies in length.

The Earth's rotation slows down and speeds up by small amounts unpredictably, while undergoing a very long-term slowing trend. The gradual slowing is caused by the friction of tides raised by the Moon and Sun. Slow, irregular changes are thought to involve motions of material in the Earth's fluid interior. Changes in winds, air masses, snow packs, and other factors cause shorter-term variations.

Faced with this problem, astronomers in 1960 instituted Ephemeris Time (ET). This time system runs perfectly steadily regardless of the Earth's rotation, almost as if the Earth didn't exist. It is used for most celestial calculations and almanac (ephemeris) predictions, especially those having to do with the motions of the Moon, planets, and other solar system bodies in space.

Ephemeris Time matched UT around 1902. Since then UT has gradually drifted away from it, so that now (as of 1996) UT lags behind by about 62 seconds.

In 1984 ET was renamed Terrestrial Dynamical Time (TDT or TT); also created was Barycentric Dynamical Time (TDB), which is referred to the solar system's center of mass. For amateur purposes they can be considered identical, since they differ by only milliseconds.

If you encounter a time given in ET or Dynamical Time, and if one-minute accuracy matters, you need to know the difference from UT. Almanacs list this difference, which is known as Delta T. Use the formula UT = Dynamical Time - Delta T. It is impossible to forecast Delta T precisely because the Earth's fitful rotation rate is too unpredictable.

Coordinated Universal Time (UTC). Civilization at large, not just astronomers, needs a smoothly running time system like Dynamical Time. But most of humanity is also tied to the natural cycle of the day, variable though it may be. What to do?

Part of the solution has been to redefine the basic time unit, the second. No longer is a second exactly 1/86,400 of a mean solar day. Since 1967 the second has been defined as how long cesium-133 atoms take to emit 9,192,631,770 cycles of a certain microwave radiation in an atomic clock.

With the second no longer defined astronomically, the Earth can spin as it pleases without upsetting the world's clocks. But there is a price to pay. A day no longer has 24 hours. In 1983 there were an average of 24.00000063 hours in a day, and in 1986 there were 24.00000034.

To keep our clocks in close step with the turning of the Earth, a leap second is inserted into Universal Time every year or so when required. A leap second may be added at the end of June 30th or December 31st UT, giving the last minute of the chosen day 61 seconds.

The result is Coordinated Universal Time or UTC (its acronym in French), the system by which all the world's clocks are set. UTC is the basis for all time-signal radio broadcasts and other time services. In non-astronomical circles it is sometimes called World time, Z time, or Zulu.

But the occasional leap-second jerks in UTC go unfelt, of course, by the Earth, planets, and stars. Almanac predictions given in "UT" are actually in a system known as UT1, which is always within 0.9 second of UTC. Therefore, when specifying "UT" to better than 1-second accuracy, you should state whether you mean UTC or UT1 unless this is obvious from the context -- such as if the time came from a radio time-station signal.

There is also a UT0, which is nearly the same as UT1 but includes the tiny effect of the Earth's crust moving with respect to its axis (polar motion), and a UT2, which is obsolete.

Sidereal time. This is simply the right ascension of stars on your local meridian at any moment. Sidereal time runs about 4 minutes a day faster than all the time systems described above. An old amateur astronomer's trick is to adjust a wind-up clock to run 4 minutes a day fast, set it to local sidereal time, and use it to tell what constellations are on the meridian and what star charts to use. For instance if the clock reads 5:30 a.m., right ascension 5^h 30^m is on your meridian, and there you'll find Orion.

Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.