Time in Astronomy
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 in the sky 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.
Types of Time in the Sky
Here, then, is a summary of the time systems that a well-informed amateur should know about.
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 fictitious 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 Sky-Gazer's Almanac that comes every year with Sky & Telescope's January issue.
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 (either north or south), the difference amounts to one minute of time for every 13 miles (21 kilometers) 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 evening constellation map in Sky & Telescope every month. So does the Sky-Gazer's Almanac that accompanies 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 1° you are west of your time zone's standard longitude, add 4 minutes to LMT to get standard time. For each 1° you are east, subtract 4 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 and Canada (except Arizona, Hawaii, and Saskatchewan) from 2:00 a.m. on the second Sunday in March to 2:00 a.m. on the first Sunday in November (the "spring ahead" and "fall back" dates were changed to these in 2007).