How could you catch a train when every town and every railroad company kept a slightly different time?

The widespread use of telegraphs and railroads finally forced a change. The problem grew more acute in the 19th century. This didn't matter when travel and communication were slow. 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. At 40° latitude, people living just 13 miles (21 kilometers) east or west of each other had clocks differing by 1 minute until standard time zones were imposed.Īt 40° latitude (either north or south), the difference amounts to one minute of time for every 13 miles (21 kilometers) east or west. The time depends on where you stand - if you’re using Local Mean Time, the system governing star charts, planispheres, and the meridian crossings of the Sun and stars. 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. Because the Earth's surface curves, 'overhead' at your location is a different direction than 'overhead' just a few miles away. An even worse problem results from the fact that the Earth is round 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.īut this adjustment wasn't enough. The number of minutes the real Sun lags behind or runs ahead of the mean Sun was named the equation of time. Noon became the moment when the fictitious mean Sun crossed the meridian. The mean Sun has the average or mean right ascension of the real Sun. 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. To escape the problem, our next time system was invented: This effect arises from the tilt of the Earth's axis and the ellipticity of the Earth's orbit around the Sun. 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. The very word 'meridian' is from the Latin for 'mid-day.'īut when reasonably accurate clocks were invented, careful timekeepers noticed that something was wrong with solar time. 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. 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. Here, then, is a summary of the time systems that a well-informed amateur should know about. Do so, and all the carefully hidden fudge factors erupt back into view. The result is a simple, easy-to-use timekeeping arrangement that serves society well - as long as nobody looks too closely at the sky. Most of these complications have been smoothed out of our everyday civil time system by official edict. 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. For astronomers, however, time in the sky can become quite complex. You assume that everyone else's clock in your time zone reads the same. It keeps apparent solar time, which is out of step with modern clock time on several counts. The first sundial was probably just a stick in the ground this fine porcelain dial, made in France in 1793, is no great improvement.

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