6/7/2023 0 Comments Coordinated universal time utcThe 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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