Time is something that most of us take completely
for granted. But have you ever thought about it? Why, for
example, are there 12 months in a year? Why are there 30 days
in September? Why are there time zones and what's with
daylight-saving time? Why are there 86,400 seconds in a day?
In this edition of HowStuffWorks,
we'll help to clarify the subject of time. In the same way
that many of the traditions surrounding Christmas
have totally unexpected origins, so do the traditions
surrounding clocks and calendars!
Webster's New World College
Dictionary (Fourth Ed.) defines time as:
II. a period or interval. 1: the period between two
events or during which something exists, happens or acts;
measured or measurable interval At its core, time is
fairly elusive. We can't see it or sense it -- it just
happens. Human beings have therefore come up with ways to
measure time that are totally arbitrary and also fairly
interesting from a historical perspective.
The day is an obvious starting point for time. A day
consists of a period of sunlight
followed by night. Our bodies are tuned in to this cycle
through sleep, so
each morning we wake up to a new day. No matter how primitive
the culture, the concept of a day arises as an obvious and
We use clocks to
divide the day into smaller increments. We use calendars to
group days together into larger increments. Both of these
systems have very interesting origins that we'll find out
about in the course of this article.
measurement of time covers an incredible range. Here are some
common time spans, from the shortest to the longest.
- 1 picosecond (one-trillionth of a second) - This
is about the shortest period of time we can currently
- 1 nanosecond (one-billionth of a second) - 2 to 4
nanoseconds is the length of time that a typical home computer
spends executing one software instruction.
- 1 microsecond (one-millionth of a second)
- 1 millisecond (one-thousandth of a second) - This
is the typical fastest time for the exposure of film in a
normal camera. A
picture taken in 1/1,000th of a second will usually stop all
- 1 centisecond (one-hundredth of a second) - The
length of time it takes for a stroke of lightning
- 1 decisecond (one-tenth of a second) - A blink of
- 1 second - An average person's heart
beats once each second.
- 60 seconds - One minute; a long commercial
- 2 minutes - About as long as a person can hold
his or her breath
- 5 minutes - About as long as anyone can stand
waiting at a red light
- 60 minutes - An hour; about as long as a person
can sit in a classroom without glazing over
- 8 hours - The typical workday in the United
States, as well as the typical amount of sleep a person
needs every night
- 24 hours - One day; the amount of time it takes
for the planet Earth to rotate one time on its axis
- 7 days - One week
- 40 days - About the longest a person can survive
- 365.24 days - One year; the amount of time it
takes for the planet Earth to complete one orbit around the sun
- 10 years - One decade
- 75 years - The typical life span for a human
- 5,000 years - The span of recorded history
- 50,000 years - The length of time Homo sapiens
has existed as a species
- 65 million years - The length of time dinosaurs
have been extinct
- 200 million years - The length of time mammals
- 3.5 to 4 billion years - The length of time that
life has existed on Earth
- 4.5 billion years - The age of planet Earth
- 10 to 15 billion years - The suspected age of the
universe since the big bang
How long is a day? It's the
amount of time it takes for the Earth to rotate one time on
its axis. But how long does it take the Earth to rotate? That
is where things become completely arbitrary. The world has
decided to standardize on the following increments:
That's a pretty bizarre way to divide a day
up. We divide it in half, then divide the halves by twelfths,
then divide the twelfths into sixtieths, then divide by 60
again, and then convert to a decimal system for the smallest
increments. It's no wonder children have trouble learning how
to tell time!
- A day consists of two 12-hour periods, for a
total of 24 hours.
- An hour consists of 60 minutes.
- A minute consists of 60 seconds.
- Seconds are subdivided on a decimal system into
things like "hundredths of a second" or "millionths of a
Why are there 24 hours in a day? No one really
knows. However, the tradition goes back a long way. Take, for
example, this quote from Encyclopedia
The earliest known sundial still preserved is an
Egyptian shadow clock of green schist dating at least from
the 8th century BC. It consists of a straight base with a
raised crosspiece at one end. The base, on which is
inscribed a scale of six time divisions, is placed in an
east-west direction with the crosspiece at the east end in
the morning and the west end in the afternoon. The shadow of
the crosspiece on this base indicates the time. Clocks of
this kind are still in use in primitive parts of Egypt.
The Babylonians seem to be the ones who started the six
fetish, but it is not clear why.
Why are there 60 minutes in an hour and 60 seconds in a
minute? Again, it is unclear. It is known, however, that
Egyptians once used a calendar that had 12 30-day months,
giving them 360 days. This is believed to be the reason why we
now divide circles into 360 degrees. Dividing 360 by 6 gives
you 60, and 60 is also a base number in the Babylonian math
What do a.m. and p.m. mean? These abbreviations
stand for ante meridiem, before midday, and post
meridiem, after midday, and they are a Roman invention.
According to Daniel Boorstin in his book "The
Discoverers," this simple division of the day into two
parts was the Romans' first increment of time within a day:
Even at the end of the fourth century B.C., the Romans
formally divided their day into only two parts: a.m. and
p.m. An assistant to the consul was assigned to notice when
the sun crossed the meridian, and to announce it in the
Forum, since lawyers had to appear in the courts before
Modern man bases time on the second. A day is
defined as 86,400 seconds, and a second is officially defined
as 9,192,631,770 oscillations of a cesium-133 atom in an atomic
Everyone on the planet wants the
sun to be at its highest point in the sky (crossing the
meridian) at noon. If there were just one time zone, this
would be impossible because the Earth rotates 15 degrees every
hour. The idea behind multiple time zones is to divide
the world into 24 15-degree slices and set the clocks
accordingly in each zone. All of the people in a given zone
set their clocks the same way, and each zone is one hour
different from the next.
In the continental United States there are four time zones:
Eastern, Central, Mountain and Pacific. When it is noon in the
Eastern time zone, it is 11 a.m. in the Central time zone, 10
a.m. in the Mountain time zone and 9 a.m. in the Pacific time
All time zones are measured from a starting point centered
at England's Greenwich Observatory. This point is known as the
Greenwich Meridian or the Prime Meridian. Time
at the Greenwich Meridian is known as Greenwich Mean
Time (GMT) or Universal Time. The Eastern time zone
in the United States is designated as GMT minus five hours.
When it is noon in the Eastern time zone, it is 5 p.m. at the
Greenwich Observatory. The International Date Line
(IDL) is located on the opposite side of the planet from the
Why is the Greenwich Observatory such a big deal? A bunch
of astronomers declared the Greenwich Observatory to be the
prime meridian at an 1884 conference. What's funny is that the
observatory moved to Sussex in the 1950s, but the original
site remains the prime meridian.
During World War I,
many countries started adjusting their clocks during part of
the year. The idea was to try to adjust daylight hours in the
summer to more closely match the hours that people are awake.
During World War I, the goal was to conserve fuel by lowering
the need for artificial light.
The United States and several other countries still use
some variation on this system. In the United States, by an act
of Congress, daylight-saving time starts on the first
Sunday in April and ends on the last Sunday in October. Clocks
are advanced one hour in the spring and moved back one hour in
the fall ("spring forward, fall back" is a phrase many people
use to remember this). You lose an hour in the spring and get
it back in the fall.
During the winter, the United States is on standard
time. During the summer, the United States is on
daylight-saving time. Even though it's an act of Congress,
some states (like Arizona) ignore it and don't have
daylight-saving time. They are on standard time all year.
As mentioned earlier, the day
is an obvious unit of time for people. But what about weeks,
months and years?
Years are fairly straightforward. Man created the
concept of a year because seasons repeat on a yearly basis.
The ability to predict seasons is essential to life if you are
planting crops or trying to prepare for winter. Most plants
sprout and bear fruit on a yearly schedule, so it's a natural
A year is defined as the amount of time it takes for the
Earth to orbit the sun one time. It takes about 365 days to do
that. If you measure the exact amount of time it takes for the
Earth to orbit the sun, the number is actually 365.242199 days
(according to Encyclopedia
Britannica). By adding one extra day to every fourth year,
we get an average of 365.25 days per year, which is fairly
close to the actual number. This is why we have leap
years that are one day longer than normal years.
To get even closer to the actual number, every 100 years is
NOT a leap year, but every 400 years IS a leap year. Putting
all of these rules together, you can see that a year is a leap
year not only if it is divisible by 4 -- it also has to be
divisible by 400 if it is a centurial year. So 1700,
1800 and 1900 were not leap years, but 2000 was. That brings
the average length of the year to 365.2425 days, which is even
closer to the actual number. Occasionally, we have leap
seconds added or subtracted to keep things exact.
The problem with the concept of a year is that it is hard
to determine the exact length of a year unless your society
has fairly good astronomers. Many cultures that lacked
astronomers relied on the cycles of the moon instead. A moon
cycle lasts approximately 29.5 days (29.530588 days is the
exact number), and it is easy for almost anyone to track the
moon's cycle simply by looking at the sky every night.
The moon is where the concept of a month comes from.
Many cultures used months whose lengths were 29 or 30 days (or
some alternation) to chop up a year into increments. The main
problem with this sort of system is that moon cycles, at 29.5
days, do not divide evenly into the 365.25 days of a year.
When you look at the modern calendar, the months are
extremely confusing. One has 28 or 29 days, some have 30 days
and the rest have 31 days. According to the "World Book
Encyclopedia," here is how we got such a funny calendar:
This little history explains why we have 12
months, why the months have the number of days they have, why
leap day falls at such an odd time and why the months have
such funny names.
- The Romans started with a 10-month calendar in 738 B.C.,
borrowing from the Greeks. The months in the original Roman
calendar were Martius, Aprilis, Maius, Junius, Quintilis,
Sextilis, September, October, November and December. The
names Quintilis through December come from the Roman names
for five, six, seven, eight, nine and 10. This calendar left
60 or so days unaccounted for.
- The months Januarius and Februarius were later added to
the end of the year to account for the 60 spare days.
- In 46 B.C., Julius Caesar changed the calendar. Ignoring
the moon but keeping the existing 12 month's names, the year
was divided into 12 months having 30 or 31 days, except
Februarius at the end with 29 days. Every fourth year,
Februarius gained an extra day. Later, he decided to make
Januarius the first month instead of Martius, making
Februarius the second month, which explains why leap day is
at such a funny point in the year.
- After Julius' untimely death, the Romans renamed
Quintilis in his honor, hence July.
- Similarly, Sextilis was renamed to honor Augustus, hence
August. Augustus also moved a day from Februarius to
Augustus so that it would have the same number of days as
What about weeks? Days, months and years all have a
natural basis, but weeks do not. They come straight out of the
Remember the sabbath day, to keep it holy. Six days
shalt though labor, and do all thy work but the seventh day
is the sabbath of the Lord thy God. (Exodus 20:8) This
fourth commandment, of course, echoes the creation story in
The Romans gave names to the days of the week based on the
sun, the moon and the names of the five planets known to the
These names actually carried through to
European languages fairly closely, and in English the names of
Sunday, Monday and Saturday made it straight through. The
other four names in English were replaced with names from
Anglo-Saxon gods. According to Encyclopedia
Tuesday comes from Tiu, or Tiw, the Anglo-Saxon name for
Tyr, the Norse god of war. Tyr was one of the sons of Odin,
or Woden, the supreme deity after whom Wednesday was named.
Similarly, Thursday originates from Thor's-day, named in
honour of Thor, the god of thunder. Friday was derived from
Frigg's-day, Frigg, the wife of Odin, representing love and
beauty, in Norse mythology.
B.C. and A.D.
We label all years with B.C.
(before Christ) or A.D. (anno domini, or
"in the year of our lord"). There is no "zero" year -- the
year Christ was born is 1 A.D., and the year preceding it is 1
This practice was first suggested in the sixth century
A.D., and was adopted by the pope of that time. It took quite
a while for it to become a worldwide standard, however. Russia
and Turkey, for example, did not convert to the modern
calendar and year scheme until the 20th century.
One interesting side note: Because of a variety of changes
and adjustments made to the calendar during the middle ages,
it turns out that Jesus was most likely born in what we now
think of as 6 B.C., and likely lived until 30 A.D..
For more information on time and related topics, check out
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