How We Manipulate Time - North Texas Drifter

THURSDAY, MAY 23, 2013


How We Manipulate Time


NORTH TEXAS DRIFTER

http://northtexasdrifter.blogspot.ca/2013/05/how-we-manipulate-time.html


Why are there 12 months in a year? Or, 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?

Maximum degrees of Earth's orbit.

We have figured out ways to measure time that are based on the rotation of the earth through the cycle of day and night and the course of the earth around the sun. So, a day is the amount of time it takes for the Earth to rotate one time on its axis consisting of two 12-hour periods, for a total of 24 hours. Most schoolchildren know that an hour consists of 60 minutes, a minute consists of 60 seconds, and seconds are subdivided on a decimal system into things like "hundredths of a second" or "millionths of a second."

When you think about it, that's a bizarre way to divide up a day. 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.

But why are there 24 hours in a day? The truth is, no one really knows. However, the tradition goes back a long way. It could have something to do with sundials.

The earliest known sundial still in existence is an Egyptian shadow clock of green schist dating at least from the 8th century BC consisting 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 with a sexagesimal (base 60) numeral system.

So how did it end with 60 minutes in an hour and 60 seconds in a minute? Again, we don't know. It is known that Egyptians used a calendar that had twelve 30-day months, giving them 360 days. This is believed to be the same reason we divide circles into 360 degrees. Dividing 360 by 6 gives you 60, and 60 is also the base number in the Babylonian math system.

Babylonian base 60 number system.

Modern man bases time on the second. A day has 86,400 seconds, and a second is officially defined as 9,192,631,770 oscillations of a cesium-133 atom in an atomic clock.

A day is the obvious basic unit of time for people. But what about weeks, months and years? Certainly seasons have something to do with it, so a year would be a cycle of seasons. The ability to predict seasons is essential if you are planting crops or trying to prepare for winter.

It takes about 365 days for the earth to orbit the sun one time. The exact amount of time 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.

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 lunar cycle.

The lunar cycle 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 not standard. One has 28 or 29 days, some have 30 days and the rest have 31 days. Here is how we got such a funny calendar:

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. So, 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, Caesar 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' death, the Romans renamed Quintilis in his honor, hence July. Similarly, Sextilis was renamed to honor Augustus Caesar, hence August. Augustus also moved a day from Februarius to Augustus so that it would have the same number of days as Julius.

Days, months and years all have a natural basis, but weeks do not. They come straight out of the Bible:

"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 Genesis.

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 Romans: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn.

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. 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.

Also attributed to the Romans is the concept of a.m. and p.m. These abbreviations stand for ante meridiem, before midday, and post meridiem, after midday. 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 noon.

Speaking of B.C., in the modern calendar, 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 -- in this system, the year Christ was born is 1 A.D., and the year preceding it is 1 B.C.

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.

Besides B.C. and A.D., some people, mainly Jesus-deniers, use B.C.E. (for "before common era") and C.E. (for "common era").

So how did global Time Zones come into being? Well, it just makes sense to want the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, that 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 zone.

All time zones are measured from a starting point centered at England's Greenwich Observatory 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 Greenwich Observatory.

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. Oddly, the observatory moved to Sussex in the 1950s, but the original site remains the prime meridian.

This brings us to Daylight-Savings Time.

During World War I, many countries started adjusting their clocks during part of the year in order to more closely match the hours that people are awake. During World War I, one of the goals was to conserve fuel by lowering the need for artificial light. By extension, though, people had more time to work and thus were more productive on a daily basis.

The United States and several other countries still use some variation on this system. In the United States, traditionally, daylight-saving time has started on the first Sunday in April and ended on the last Sunday in October. However, the Energy Policy Act of 2005 mandated a change to the observed dates. Starting in 2007 and going forward, DST now begins at 2 a.m. on the second Sunday of March and ends at 2 a.m. on the first Sunday in November.

To observe DST, 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.

Lastly, for fun and for review, here are some common time spans, from the shortest to the longest. How many are you familiar with?

1 picosecond (one-trillionth of a second) - This is about the shortest period of time we can currently measure accurately.
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 human motion.
1 centisecond (one-hundredth of a second) - The length of time it takes for a stroke of lightning to strike
1 decisecond (one-tenth of a second) - A blink of an eye
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 without food
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 being
5,000 years - The span of recorded history

http://northtexasdrifter.blogspot.ca/2013/05/how-we-manipulate-time.html