The passage of time

 

            It is eminently clear that the ancient peoples around the world saw that there was an inextricable connection between the heavens and the calendar.  The only real purpose of ancient astronomy seemed to be to predict times for planting, worshipping, sacrificing, or celebrating.  This connection between the stars and the keeping of time as well as the search for more accurate clocks has continued even until today.  In fact, even today, the most accurate clocks in the universe are not built here on earth but are found in pulsars that inhabit regions of space beyond our own solar system.  Mankind’s continued search for a more accurate timekeeper will be addressed later in this analysis.

            Familiar objects in the sky that change their positions over time, specifically the sun, moon, planets, and stars have provided mankind with a reference for measuring the passage of time throughout our existence.  There were three natural clocks that ancient man could discern when he looked up at the sky.  First, the rising and setting of the sun and stars measures the day and night (diurnal motion).  Second, the phase cycle of the moon was used for the month.  Third, the apparent motion of the sun amongst the unmoving background of the stars gave rise to the concept of the year.  Unlike the day, month, and year, we can see that there is no celestial basis for the days of the week.  These seven days seem to have been named after the seven bright heavenly bodies that ancient man saw move across the sky.  The chart below captures the relationship between the names of the days of the week and those moving heavenly bodies.

 

Ancient civilizations relied upon the apparent motion of these “sky objects” through the heavens to determine seasons, months, and years.  The ancient Babylonians had a lunisolar calendar of 12 lunar months of 30 days each, and they added extra months when necessary to keep the calendar in line with the seasons of the year. 

The ancient Egyptians were the first to replace the lunar calendar with a calendar based on the solar year.  The earliest Egyptian calendar was based on the moon's cycles, but later the Egyptians realized that the "Dog Star" in Canis Major, which we call Sirius, rose next to the sun every 365 days, about when the annual inundation of the Nile began.  Based on this knowledge, they devised a 365-day calendar that seems to have begun in 4236 B.C., the earliest recorded year in history.  They measured the solar year as 365 days, divided into 12 months of 30 days each, with 5 extra days at the end.  About 238 BC King Ptolemy III ordered that an extra day be added to every fourth year, similar to the modern leap year. 

In Babylonia, again in Iraq, a year of 12 alternating 29-day and 30-day lunar months was observed before 2000 B.C., giving a 354-day year.  In contrast, the Mayans of Central America relied not only on the sun and moon, but most importantly to them, the planet Venus, to establish 260-day and 365-day calendars.  This culture flourished from around 2000 B.C. until about 1500 A.D.  They left celestial-cycle records indicating their belief that the creation of the world occurred in 3113 B.C.  Their calendars later became portions of the great Aztec calendar stones.  In ancient Greece, a lunisolar calendar was in use, with a year of 354 days. The Greeks were the first to insert extra months into the calendar on a scientific basis, adding months at specific intervals in a cycle of solar years.

Other civilizations, such as our own, have adopted a 365-day solar calendar with a leap year occurring every fourth year.  The evolution of the particular calendar that we use today was not a smooth process since it was first adopted by the Egyptians and Romans.  When Julius Caesar came to power in Rome, he discovered that the alignment of the seasons with the months was all “wrong”.  In 46 B.C., he extended the year to include 445 days to correctly align the months and seasons.  He then established the Julian calendar on the advice from the Alexandrian astronomer, Sosigenes who determined the length of the year as 365 ¼ days.  To account for the extra quarter day each year, an extra day was added every fourth year and thus the leap year was born.

Although the new Julian calendar had a length of 365.2500 days, this differed enough from the actual astronomical year of 365.2422 days that by the time 1,600 years had passed, Pope Gregory XIII had to subtract 11 days from the calendar to align it with astronomical observations.  In 1582 on the advice of the astronomer Calvius, Pope Gregory issued a papal bull ordering the day after Thursday, October 4 to be Friday, October 15.  Thus, the Gregorian calendar that we employ today was born.  To keep the Gregorian calendar from running “fast” like the Julian one did, the Gregorian calendar drops the leap year day from any multiple of 100 years that is not evenly divisible by 400.  This change gives the year an average length of 365.2425 days which is so close to the astronomical one that the error is only 3 days in 10,000 years.

Ancient man has left us very little about the details of his timekeeping, but whatever we have found seems to indicate that in every culture, some people were preoccupied with measuring and recording the passage of time.  Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon.  Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes).  We have no written records of Stonehenge but its alignments show its purposes apparently included the determination of seasonal or celestial events such as solstices and may have included lunar eclipses.

The concept of keeping track of the passage of time within a given day seems to have originated 5,000 to 6,000 years ago in the civilizations of the Middle East and North Africa.  The oldest clocks that have survived were Egyptian obelisks that divided the day into a first and second half.  Later, flat sundials divided the day into partitions similar to hours and then, in an attempt at greater accuracy, curved sundials, called hemicycles were developed about 300 B.C.  By 30 B.C., at least 13 different sundial styles were in use in Greece, Asia Minor, and Italy.  Actual clocks that used a repetitive process to mark off equal increments of time and employed some means of keeping track of the increments of time and displaying the result have been discovered in tombs from around 1500 B.C. 

The oldest clocks used the passage of water to keep track of time.  By the early-to-mid-14th century, large mechanical clocks began to appear in towers of several large Italian cities.  These large clocks were weight driven and were also very inaccurate.  Between 1500 and 1510 Peter Henlein of Nuremberg invented spring-powered clocks.  These clocks required constant winding as they slowed down dramatically as they wound down.  In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock although Galileo Galilei is credited with inventing the pendulum in 1582.  Galileo never did build the clock he designed based upon the pendulum.  Huygens’ pendulum clock began with errors of less than one minute per day and eventually achieved errors of less than 10 seconds a day.

Clock accuracy improved dramatically over the next 200 years and the work of master clock maker John Harrison played a critical role in allowing English sailors to navigate around the world.  Details of this revolution in clock accuracy will be covered in the second section of this report.