PES 105 General Astronomy I - Timekeeping
Measuring time is an essential part of our daily lives. Whether we are counting the time until the next meal or the
next holiday we are making use of a system of timekeeping that has developed over the course of history. How
any culture marks short, medium or long periods of time typically originates as a mixture of physical
measurements and arbitrary traditional choices.
Probably the most fundamental measure of time is the day, which is measured roughly by the alternating cycle of
light and dark (the diurnal cycle). The length of a day can be defined by the time between subsequent sunrises,
but is more accurately defined as the time between meridian passages of the sun. A meridian is a line from the
North Pole to the South Pole that passes through your immediate location. When the sun ceases to rise and
begins to set, it is passing the meridian. Before the meridian it is ante-meridiem (AM) and after the meridian it is
post-meridiem (PM). This would be the exact time of local noon, and every longitude has its own local noon.
However, to keep time systems useful, we group local longitudes together into time zones of approximately 15
degrees each making at least 24 times zones. There are some additional time zones between the basic 24 that
are a half hour different from their neighbors.
Measuring the day in this way, “noon-to-noon”, gives a number very close to 24 hours, but the actual length of the
day still varies up to about 20 minutes over the course of the year due to the eccentricity of the Earth’s orbit,
speeding up near perihelion and slowing near aphelion. To remove this variation, we define the Mean Solar
Day as the average day length over the course of a year, and this number is exactly 24 hours.
The same technique can be used to measure the subsequent passages of a star across the meridian, or a Sidereal
Day. This actually measures how long the Earth takes to turn on its axis once. Recall that the measure of the
solar day also includes the motion of the Earth along its orbit during that day, so the Earth has to turn a little
farther around its axis to get the sun directly overhead again. This is not the case for measuring the “noon-tonoon” using a star, since the stars are so far away. Therefore a Sidereal Day (23 hours 56 min 4 sec) is a bit
shorter than a Mean Solar Day (24 hours).
How should we break up the day into shorter segments? There is no definite physical reason to choose one subday length over another, so we rely mainly on tradition and culture. The ancient Egyptians noted that twilight
(lightening of the sky prior to sunrise or after sunset) took up about 1/24th of a day each, so they decided to
break the day into 24 equal sections, which we call hours and still use today. The ancient Babylonians used a
counting system based on the number 60 (unlike our system based on the number 10), and it is their influence
that cause us to break up hours into 60 minutes and minutes into 60 seconds. Thus a mean solar day consists
of 24 hours, 1440 minutes, or 86,400 seconds. With the beginnings of international trade, however, choosing
how to align the beginning of the day among disparate regions became an issue. Regions were grouped into
“time zones” which shared a common “noon”, and thus a common beginning to their day.
Simply looking at a globe of the Earth underscores the fact that the choice of the beginning of the time zones and
the longitude system is not trivial. While the locations of the north and south poles (and thus the basis for the
measurement of latitude) are easily defined as the intersections of the Earth’s spin axis with its surface, no such
clear choice is available for referencing longitude measurements. The location of zero longitude was decided in
1884 to be the line that runs from north to south that passes through the Royal Observatory in Greenwich,
England. Time measured at this location is called Greenwich Mean Time (GMT). This line of longitude is called
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PES 105 General Astronomy I - Timekeeping
the Prime Meridian, and delineates between eastern and western longitudes. The first time zone west of this
line, GMT, is defined to be the reference time zone. Other time zones have borders defined by convenience
along lines from north to south and spaced approximately 15 degrees apart. The offset of each time zone is
measured as some integer number of hours from Greenwich. Since Greenwich time is at zero offset, it is also
sometimes called “Zulu time”.
On the other side of the Earth from the line of zero longitude is the International Date Line at 180 degrees longitude.
This line determines when one day transitions to the next. Choice of Greenwich as zero longitude means that
the somewhat meandering International Date Line runs through the Pacific Ocean, a region of very low
population, to avoid the extreme inconvenience of some region having a different day than its neighbor.
Further complicating the accounting of hours in separate locations is the concept of Daylight Savings Time. This
idea was proposed by Benjamin Franklin as a means to maximize the number of daylight hours for businesses,
thus minimizing the usage of energy devoted to lighting dark establishments. During the winter months when
days are short, clocks would be shifted backward by an hour, and then shifted forward again in the spring. The
practice was not enacted in America until World War I, at which time individual regions were allowed to decide
whether or not to observe the practice. The result is that some states do not observe Daylight Savings Time.
Even some individual counties near population centers which lie across a state line and time zone border have
chosen to adopt the practice of the nearby city.
Now, what about grouping days together into larger units? The smallest collection of days in our culture we call the
week, and it consists of seven days, each devoted to the god governing one of the visible solar system bodies.
Sunday: the Sun
Monday (Lunes): the Moon
Tuesday (Martes): Mars in the Roman pantheon, Tiew in the Norse pantheon
Wednesday (Miercoles): Mercury in the Roman pantheon, Woden in the Norse
Thursday (Jueves): Jupiter (Jove) in the Roman, Thor in the Norse
Friday (Viernes): Venus (Roman) and Freig (Norse)
Saturday: Saturn in the Roman pantheon
The choice of seven days is arbitrary; the Egyptians chose 10 day weeks (called decans).
Larger groupings of days, however, are less arbitrary. It is logical to measure the passage of time by watching the
phases of the moon cycle through from New Moon to Full Moon and back again. This collection of days is
called, not surprisingly, a month, or moon period. The number of days in a month, however, is not an exact
number of days, but instead includes a fraction of a day (approximately 29.5306 days in a month). We really
shouldn’t expect that the day should be an exact divider of a month since there is no physical connection
between the two phenomena. The result is that months of exactly 30 days will drift away from the actual moon
phase cycle. None-the-less, the month is still a convenient way to measure larger passage of time.
Likewise, the number of days in a year don’t match up to an exact number of days, but include a fraction of a day,
as well. There are about 365.25 days that make up the time for the Earth to orbit the Sun exactly once. Prior to
the Roman era, the year was divided into 10 months of about 30 days each, with a collection of holidays and
official days added in to make the calendar better match the actual seasons which it was designed to measure.
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PES 105 General Astronomy I - Timekeeping
The vestiges of this tradition are still evident in some of our month names: September (seven), October (eight),
November (nine) and December (ten). However, at the time of Julius Caesar it was recognized that the
calendar in use at the time had drifted significantly away from the seasons. The first of the (seasonal) year as
measured by the winter solstice had moved all the way into April, a drift of about 100 days. In order to correct
this deviation, Julius Caesar decreed that the new year would begin properly near the winter solstice and cut
out the extra days. However, this decree took time to disperse to the rural areas, and the people who continued
to celebrate the first of the (old) calendar year in the (new) calendar’s April were considered April Fools.
In creating the new calendar, Julius Caesar set the number of days in each month to be either 30 or 31 days and
added in two extra months, shifting September later in the year. The first of the months was named after Julius
himself (July) and the second of the added months was named after his successor, Augustus Caesar (August).
Of course, a month named after an emperor must have the maximum number of days (31 each), so the two
needed days were removed from February, leaving it only 28 days. In order to prevent further drift, however,
the new Julian Calendar included 365 days in each normal year, but added a day to the calendar every four
years to make up for the approximately 1/4 day difference between the calendar year and the actual seasonal
year corresponding to exactly one Earth orbit about the sun, properly called a tropical year. The structure of the
new calendar meant that every year the 1st of January would occur one day later in the week, except for the
years following the addition of the extra day, causing the 1st of January to “leap” over one extra day of the week,
from which the “leap year” derives its name. The added day is now termed the “leap day”.
The Julian Calendar was very successful in aligning the calendar with the tropical year, whose length is almost
exactly 365.2422 mean solar days. However, the method of adding in a leap day every four years means that
the calendar is exactly 365.25 mean solar days in length, on average. There remains a slight discrepancy, a
mere 11 minutes per year. But given enough time, even this will eventually cause the calendar and the tropical
year to again drift apart. Over the course of time following the implementation of the Julian Calendar the
calendar did indeed drift, so that by the year 1582 the difference had reached a total of ten days. This had been
recognized by the scientists who served Pope Gregory, who originated a solution to the problem. The ten extra
days were excised from the calendar that year, meaning that October 4 th, 1582 was followed immediately by
October 15th (much to the chagrin of rent-payers, but to the delight of landlords, who collected a full month’s
rent for just the 21 days of that month!) To remove the necessity to repeat this event in the near future, Pope
Gregory proposed that the method of calculating whether to add leap days be modified to the following:
If the year is divisible by 4, then add a leap day,
UNLESS the year is also divisible by 100, in which case DON’T add the leap day,
UNLESS the year is also divisible by 400, in which case DO add the leap day.
For example, using this set of rules means that the year 2000 AD had a leap day added, while the year 2100
AD will not. With this somewhat cryptic method of adjusting for the disconnected human and seasonal
calendars, there would be no need for wholesale addition or removal of a calendar day for another 3,000 years.
This new calendar, called the Gregorian Calendar, is the basis of our modern calendar.
Traditional, religious, and cultural calendars remain in some use even today. These calendars are typically based
on lunar months and add occasional holidays to align themselves with the tropical year. The Persian calendar
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PES 105 General Astronomy I - Timekeeping
begins on the day of the vernal equinox each year and adds days for seasonal alignment. The Jewish calendar
has months which begin exactly at each new moon and adjusts to match the seasons by the addition of an
extra month as needed. The Chinese calendar is the longest continuously used calendar, originated about 500
BC, and its beginning is based on the winter solstice. Its months are based on the lunar cycle, with extra
months added as needed. The Chinese years are also grouped into 60 year cycles with 5 12-year sub-cycles,
associating each year with particular animal.
In the face of this dizzying assortment of calendar rules, astronomers sought a simpler method of accounting for the
passage of time that is not so dependent on arbitrary definitions of weeks, months, leap days, and daylight
savings conventions. Thus the Julian Date system was created. (The name “Julian” is not related to Julius
Caesar, but to the originator’s father, whose name was Julius.) The Julian Date system has a base year of
4192 BC (chosen to predate some of the earliest pre-historic astronomical references in cave drawings), and
simply counts the number of mean solar days that have passed at Greenwich, England, since noon on January
1, 4713 BC. Using this method of timekeeping means that as of midnight on January 1 st, 2000 AD in Greenwich
there had been 2451544.5 mean solar days that had passed since the base reference time. The reason for
beginning the astronomical timekeeping system at noon may seem unusual until it is realized that the majority
of earth-bound astronomical observations occur at night. Having to change from one day to the next in the
middle of set of night-time data collection would be quite inconvenient, so the choice was made to start counting
days at noon, the middle of the observational astronomer’s “night”. Though it lacks the richness of traditionally
named intervals of days, the Julian Date system is a straight-forward, practical timekeeping system which
greatly simplifies the correlation of astronomical observations.
A further complication in measuring time arises from the gravitational interaction of the Moon and the Earth. Ocean
tides due to the moon are constantly slowing the spin rate of the Earth. This change in the time for the Earth to
rotate about its axis is exceeding slow (1.2 milliseconds per day every century), yet is measurable using modern
techniques. The original definition of the time interval of one second used to be 1/86,400 th of the time for the
Earth to rotate once with respect to the Sun. With a changing Earth spin rate it became necessary to replace
this definition with one based on physical observations independent of astronomical events. The modern
(1967) definition of one second is 9,192,631,770 oscillations of a particular wave of light from the caesium 133
atom. With modern international financial, scientific, and information processing requiring exact times, the
Universal Coordinated Time (UTC) system was created based on the atomic clock. Periodically a second must
be added or subtracted from the UTC time to account for the slowing of the Earth’s spin. This is called a “leap
second” and is added or subtracted at an agreed-upon date and time, as needed.
The history of mankind’s effort to account for the passage of time is responsible for an interesting and varied
collection of measures. The independence of the astronomical events upon which the measures are based
have resulted in a variety of methods of resolving discrepancies that accumulate over time. Astronomers
invented a system that makes the measurement of time more direct and scientifically useful. So the next time
someone asks you “What is today?” or “What time is it?”, realize that the real answer to such a question is
anything but simple.
R. Gist