What is time? Albert Einstein explained that time as we know is an invention when he said "Space and time are modes by which we think, not conditions under which we live." Clockworks explores our notions of time, starting with a history of calendars and timekeeping. This is an excellent site for middle and high-school students, sprinkled with dozens of apt quotes from the likes of Einstein, Ben Franklin, William Shakespeare and Tennessee Williams.
My pick of the day is the beautifully illustrated Calendars through the Ages. Organized into chapters with a horizontal menu at the top, and subdivided into topics with a vertical menu on the left, Calendars through the Ages begins with an in-depth look at the astronomical basis of calendars. Significant historical calendars (such as the Roman and Mayan) and currently used international calendars (Jewish, Chinese, and Islamic) are covered in Various Calendars.
"Since the dawn of civilization man has kept track of time by use of the sun, the moon, and the stars. Man noticed that time could be broken up into units of the day (the time taken for the earth to rotate once on its axis), the month (the time taken for the moon to orbit the earth) and the year (the time taken for the earth to orbit the sun)." But since a month is not a whole number of days, nor a year a whole number of months or day, the task is not simple. "The ways in which these problems were tackled down the centuries and across the world is the subject of this Web site."
"In the 1840's a Greenwich standard time for all of England, Scotland, and Wales was established, replacing several 'local time' systems. The Royal Greenwich Observatory was the focal point for this development because it had played such a key role in marine navigation based upon accurate timekeeping. Greenwich Mean Time (GMT) subsequently evolved as the official time reference for the world and served that purpose until 1972." This fabulous site, produced by the National Institute of Standards and Technology, presents both the history of timekeeping and a peek at its current state. If you want to coordinate your Windows-based computer clock to the NIST clock, you can download a program to do so over the Internet (look under NIST Time Calibration).
World Book editors have created a site for elementary and middle-school students that answers basic questions about the invention of the calendar. What I found most fascinating was the brief discussion of two proposed calendars that would simplify our timekeeping. The fixed (or thirteen month) calendar inserts the month of Sol after June. Each month is exactly four weeks long and an extra day (called a year day) is added at the end of the year. The second option is the world calendar which has twelve months of thirty or thirty-one days, and also has a year day at the end.