And before time was standardized, trains not only didn't run on time, they sometimes ran into each other, as miscalculations, inaccurate clocks, and local variations in timekeeping occasionally put two trains on a collision course.

Time, that tick-tocking metronome that marks the unrelenting passing of our lives, which many of us track with near-religious fervor, and that regulates our meetings, our partings, our work, and our fun, has not always been ruler and master.

An exhibition at the Houghton Library of the Harvard College Library, "The Art and Science of Finding Time," highlights the pursuit of an accurate means to find and measure time with a display of sundials from the Collection of Historical Scientific Instruments and books on sundials from the Houghton's own collection.

The exhibit takes visitors back to a time when hours weren't the length they are today and spans the development of sundials to an accuracy within minutes. It ends finally with the standardization of time, brought about by increasingly rapid travel and communication.

"One of the purposes of this exhibition is to remind people that the system of time measurement we use today didn't always exist," said William Andrewes, the David P. Wheatland Curator of the Collection of Historical Scientific Instruments. "Time is an arbitrary concept that can be measured in many different ways to serve the needs of a society."

In fact, it wasn't that long ago that the system changed. Before the advent of railroads, time was kept independently in each hamlet and city in the United States. The Harvard College Observatory did its part to help people keep track of time by instituting, in December 1851, the world's first public time service, distributed over telegraph wires. It wasn't until 1883 that the government divided the country into time zones.

With today's atomic clocks and crystal watches, it's easy to forget that the original divisions of time depended upon the motion of the Earth around the sun and the moon around the Earth.

The division of the day into hours was introduced around 1600 B.C. in Egypt, but these hours divided each period of daylight and darkness into 12 equal parts. Therefore in summer, there would be 12 long hours during the day and 12 short hours at night and, in winter, the opposite would occur. Only at the spring and autumn equinoxes would the hours of daylight and darkness be equal.

This system, called temporal hours, continued to be used in Europe until the invention of the mechanical clock around 1300. A similar system dividing each day and each night into six equal parts was used in Japan until 1873.

It was the growth of urbanization that fueled the need for a better method of timekeeping. And with the advent of portable timekeeping devices during the 16th century, there was an increasing demand for time-finding devices -- such as sundials, astrolabes, and nocturnals -- by which clocks and watches could be set, Andrewes said.

To create a useful instrument, a sundial maker had to be knowledgeable about mathematics, astronomy, and geography. Some types of sundials had to be made differently to tell time at different latitudes. Other time-finding instruments, such as astrolabes and nocturnals, used the stars to tell time. Some portable dials incorporated a moveable scale that allowed users to convert lunar time to solar time so they could tell time by moonlight.

"It's just fascinating to see how these things were made, with beauty and precision, by hand. And even more fascinating to see how they work," said Mariana Oller, joint curator of the exhibition and curatorial assistant in the Rare Books Department in the Houghton Library. Oller spent hours poring over the Houghton's collection in search of books that illustrate the sundials in the Collection of Historical Scientific Instruments.

When the first watches appeared in the 1500s, some predicted they would mean the quick demise of the sundial. But, because they frequently needed to be reset, these early timekeepers created an increased demand for portable sundials, Andrewes said.

Even as the accuracy of clocks and watches increased, there wasn't a need to standardize time until the expansion of the railroads in the last century. Before then, noon was whenever the sun was at the top of its trip across the sky, which occurred four minutes later for each degree one traveled west. For example, when it was noon in Worcester, it would be 12:04 in Boston.

Faster communication and transportation systems put an end to that by making it important for everyone to know what time it is, not just where they are, but everywhere else as well.

The exhibit marks the 50th anniversary of the first exhibit of the Collection of Historical Scientific Instruments, which took place in the foyer of the Mallinckrodt Laboratory in February 1949. The collection itself, which contains about 15,000 items that date as far back as 1450, was established by David Wheatland and others in 1947. In addition to donating many pieces to the Collection of Historical Scientific Instruments, Wheatland gave 4,600 rare books and manuscripts to the Houghton Library between 1941 and 1991.

And for those who haven't seen the exhibit yet, don't worry, it's open until Feb. 27.

There's still time.