Time:
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What time is it? Well, you look at your watch and say "10:35." Be more precise: "10:35 AM on Wednesday, the 12th of October, 2000."  OK, but how can I really know what time it is? "Check the radio - at noon each day Public Radio counts down through a series of beeps, and at the last beep it's exactly 12:00 noon. Set your clock to that and you're all set."

Almost always a check of your watch is enough for you to tell the time, but for many situations time must be measured very precisely. A case in point: The Global Positioning System (GPS). This system allows you to know your position (in 3 dimensions) anywhere on (or above) the earth. It measures the direction and time of a signal from your transmitter to a system of satellites to establish your position. To do this accurately requires accurate measurement of time. To establish your location to within 10 feet requires measuring time to 10 billionths of a second. That's an accuracy of 1 part in 100,000,000, measured by a clock orbiting in a satellite above the earth. Time measurement is the business of NIST, the National Institute of Standards and Technology (formerly the National Bureau of Standards). Another source of time information is the USNO (The U S Naval Observatory). How accurately can we measure time? To accuracies approaching 1 part in 1018. If you had a watch that accurate, it would lose (or gain) about 1 second every 30 billion years. Click here for an interesting look at measuring time.

The standard of time is the second. Here's how it is defined: We call the time it takes for the earth to go around once on its axis a day. In early times the day was divided into 24 hours, an hour into 60 minutes and a minute into 60 seconds. Of course we need an instrument to measure time - a clock. In 1656 Christiaan Huygens designed a clock accurate to within 10 seconds per day. The time it takes for Earth to rotate once is fairly stable, staying within about 3 milliseconds over several years, but the need for more accurate measurements led to the definition of a second in terms of the oscillations of the light emitted by a cesium-133 atom. Specifically, the time taken for 9,192,631,770 oscillations of a specified wavelength of a cesium-133 atom is equal to one second. One advantage of such a standard is that it can be replicated in any well-equipped lab on Earth, or in any lab on a distant planet, should we chance to communicate with one some day. (It would be impossible to explain the length of our second using a rotating Earth a distant civilization could not see).

Of course, there is also the matter of other time standards, such as the week, month and year. An interesting look into the matter of calendars can be found here.

Times of physical things range from 10-43 s (the so-called Planck time - the shortest time for which the laws of physics as we know them hold) to 1039 s (the predicted time of the proton lifetime - the age of the universe 1017 s, so not many protons have decayed so far).