From sundials to the atomic second, Encyclopedia Britannica's Clockworks offers a most comprehensive horological examination of time tools with easy-to-read passages and eye-catching imagery. Equally intriguing and visually appealing is the Franklin Institute Science Museum Journey in Time. Enthusiasts will likely find the former more interesting, while schoolteachers may find the latter more useful.

History buffs, trivia collectors, and anyone who enjoyed picture books as a child may enjoy the National Institute of Standards and Technology (NIST) presentation of The Evolution of Time. You only thought you didn't have time to learn about ancient calendars, early clocks, world time scales and time zones, the revolution in timekeeping, the atomic age, and lots of suggested reading in the bibliography.

NIST's laboratories in Boulder, Colorado, developed NIST-F1, a cesium fountain atomic clock. This clock is about as close as you can get to the end-all-be-all of timekeeping as it is "the nation's primary time and frequency standard" and "contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time."

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Everything you wanted to know about time travel is just moments away. For your perusal is PBS' companion Web site to the NOVA program "Time Travel," and "Time Travel for Beginners" by John Gribbin, science writer and visiting Fellow in astronomy at the University of Sussex. If you're thinking about taking that fateful trip, Scientific American presents the schematic-free "How to Build a Time Machine."

If your horological interest in travel is less about dimension and more about destination, there are a few clocks you may find interesting. The Astronomical Clock (Orloj) (3) of the Old Town Hall in Prague, Czech Republic, continually provides the full range of astronomical data. The clock was installed in 1410 and rebuilt in 1490 by the Master Hanuš. Of its three parts — the procession of Apostles, the astronomical clock, and the calendar — its most popular attraction is the hourly procession of the 12 Apostles. The tower stands about 226 feet (approximately 69 meters).

The 1547 Horloge Astronomique of Strasbourg, Alsace, France (4) offers up four levels of timekeeping activity. Among the most fascinating things this clock does is show the different stages of life. The fourth level shows the Apostles pass by, bow, and receive Christ's benediction. Also, a rooster spreads its wings and sings. This used to happen when the clock struck twelve noon, but it upset the clergy that many churchgoers were leaving mass before it ended to see the clock at its most active glory. The clergy delayed the passing of the Apostles until half past noon.