Editor's Note: This article is one of three sidebars to "Time to Take Note: The 1752 Calendar Change" by Kip Sperry. Read also "The Complexities of Leap Year" and "When Is George Washington's Birthday?"

This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.

—J.R.R. Tolkien, The Hobbit

What is time? What historical and cultural factors have influenced how we measure the perceived passage of time? Why do different peoples record time and dates in different ways? These and other questions are the basis of horology, the science of measuring time.

Almost without exception, all calendars and time systems now in existence are based on the movement of the earth and/or the moon. The movements of these celestial bodies are obvious to the naked eye and are regular and consistent. The earth rotates once every day. To an earthbound observer, the sun rises in the East and sets in the West. The moon follows a set path through the sky and waxes or wanes on a somewhat regular schedule, repeating itself every 29.5 days (on average). The earth, tilted slightly on its vertical axis, changes its relative position to the sun gradually throughout the year. This change in position accounts for solstices, equinoxes, and the seasons.

In fact, most time systems are categorized as either solar or lunar, based on whether the system relies chiefly on the sun or the moon to measure each passing year. For example, the Babylonians created a lunar system of 354 days, divided into twelve lunar months of either twenty-nine or thirty days in length. Lunar systems do, however, have their drawbacks—they eventually fall "out of whack" with the seasons and the stars. How valuable is a calendar if harvest time or planting time falls in a different month every year?

The Egyptians are thought to have been the first to create a calendar based on the sun. They noticed that Sirius, the "Dog Star," rose in the same place relative to the sun every 365 days. The Roman world followed this system, adopting the Julian calendar in 45 B.C.

Even this system has its flaws, however, as the Julian calendar requires an extra day every four years (leap day) because the earth actually takes a little longer than 365 days in its trip around the sun (an extra five hours, forty-eight minutes, and forty-nine seconds, to be exact). Even the leap day does not completely compensate for these extra hours, as evidenced by the "10-day gap in time" which was the impetus for the change to the Gregorian calendar in 1582. In addition to repairing the gap and changing the start of the year to January 1, the new calendar also decreed that leap years would not occur on years divisible by 100 unless the year was also divisible by 400 (i.e., 1900 was not leap year, despite being divisible by four, but the year 2000 is a leap year after all).

Jake Gehring is a product manager at Ancestry.com and a genealogical researcher and lecturer. He lives in Provo, Utah with his wife and three children.