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In this video we will talk about time — a topic
covered in the TranslatorsCafe.com Unit Converter website article.
We will discuss the units and devices
used to measure time. Finally, we will discuss the time zones
and the Coordinated Universal Time. To open the Time converter,
click on the link below. This video explains in detail
how to use the TranslatorsCafe.com Unit Converter.
So, what is time?
What are its physical properties? Time can be viewed in two ways:
as a mathematical system, designed to help with better understanding
of the universe and the progression of events, or as a unique dimension,
part of the structure of the universe. In classical mechanics time has a constant rate
of change
and is viewed independently, not in relation to other variables.
Einstein's theory of relativity redefined time
as having a variable rate of change for the objects in movement
relative to one another. This phenomenon
is called time dilation and can be observed at speeds near the speed of
light.
It has been experimentally proven in the Hafele-Keating experiment
where 5 atomic clocks were synchronized, one of them was left stationary,
and the rest were flown around the Earth and back on commercial airplanes.
When the experimenters compared the time
they found a disparity between the stationary and the traveling clocks,
as predicted by the theory of relativity. Increase in both speed and gravity
slow down time.
How is time measured?
Clocks measure the physical movement of time, while calendars consist
of abstract systems that represent longer time intervals
such as days, weeks, months, and years.
The shortest unit of time
is second, which is an SI unit defined as: "the duration of
9,192,631,770 periods
of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the caesium
133 atom". Mechanical clocks
generally measure cyclical events of pre-determined length,
such as pendulum swings, calibrated to oscillate
every second. Sundials track the movement of the Sun
across the sky throughout the day and use a shadow
to display the passage of time on a dial plate. Water clocks,
which were used from the antiquity and throughout the Middle Ages,
measured the time by the flow of water between
several vessels, just as the hourglass uses sand
and other similar materials. A San Francisco-based
Long Now Foundation is designing a clock, the Clock of the Long Now,
meant to survive and remain accurate for 10,000 years.
The project focuses on creating a simple,
transparent, and easy to understand and maintain design,
with parts made from non-precious materials.
Currently the design supposes human maintenance,
including winding. It uses a dual
time-tracking system of an inaccurate but reliable
mechanical torsional pendulum and an unreliable
(due to the weather) but accurate lens that gathers sunlight.
Atomic clocks
are currently the most accurate time-measuring devices, used to ensure
accuracy during radio wave broadcasting, in global navigation satellite systems,
and in global time distribution services. The atoms
used in these clocks are slowed down with lasers and cooled to the
temperature
close to absolute zero. Time is measured
by measuring frequency of the radiation produced by electronic transitions in
atoms,
and the oscillation frequency is dependent on the gravity
and the electrostatic forces between the electrons and the nucleus,
as well as on the mass of the nucleus. Currently
the most common atomic clocks use cesium, rubidium,
or hydrogen atoms. Cesium atomic clocks
are the most accurate long-term, with the error of less than one second
per one million years. Hydrogen atomic clocks are about ten times more
accurate for short periods of time up to a week.
Other measuring devices
include chronometers, which are precise enough
to be used for navigation. They determine the geographic location
based on the position of the stars and the planets.
Today some of the marine professionals are required to know
how to use a chronometer in order to become certified,
and chronometers and sextants are kept on a number of
vessels as a back-up system, but global navigation satellite systems are more
commonly used.
Globally, the Coordinated Universal Time
(UTC) is used as a universal timekeeping system.
It is based on the International Atomic Time
(TAI) system, which uses a weighted average
of the time of over 200 atomic clocks
located across the globe to calculate time.
As of 2012, TAI is 35 seconds ahead of the UTC.
This is because UTC adjusts to the mean solar day
by adding leap seconds, due to the fact that the solar day
is a little longer than 24 hours. Sometimes
these leap seconds can cause problems, especially in software.
Alternatively, to avoid problems with leap seconds,
some institutions, such as the Google server division, use a leap smear,
lengthening a number of seconds preceding the leap second.
Greenwich Mean Time (GMT) was also widely
used until recently, but has since been replaced by UTC.
GMT is less accurate than the UTC because it is based on solar day calculations,
which, in turn, depend on the Earth rotation period,
which is not constant. GMT was widely used in the past.
However, now UTC replaced GMT.
Based on Coordinated Universal Time (UTC),
time zones are formed. Time is approximately the same
within each time zone. Time zones
are formed so that the differences in time between them
are a multiple of one hour.
Thus, there are 24 time zones, and the reference point is the Greenwich
meridian. The corresponding time zone is the zone with zero UTC offset.
When moving from one time zone to another,
the number of minutes and seconds is not changed.
Sometimes time zones differ by an integer number
of half-hours or quarter-hours from UTC.
One example is the island of Newfoundland, where the local time
is UTC
minus 3 hours and 30 minutes.
Calendars track single or multiple levels of cycles such as days,
weeks, months, and years. They can be subdivided into lunar,
solar, lunisolar, and other types.
Our description of calendars is accompanied by the video story about
four seasons.
Lunar calendars are based on the phases of the Moon
with one month consisting of one lunar cycle.
A year is 12 months long, which is 354.37
days. The lunar year is shorter
than the solar year, and as a result, lunar calendars are synchronized with the solar
year only once in about every 33 lunar years.
Islamic calendar is one such example. It is used for religious purposes,
and as an official calendar in Saudi Arabia.
Solar calendars are based on the movement of the Sun
and correspond to the seasons. They are based on a solar or tropical year,
which is the time it takes for the Sun to complete one cycle of seasons,
for example from winter solstice to winter solstice.
The mean solar year is about
365.242 days. Because of the axial precession,
which is a slow change in the position of the Earth's rotational axis,
a solar year is roughly 20 minutes shorter than the time needed
for the Earth to orbit around the Sun, as measured against the fixed stars.
This time is known as a sidereal year.
The solar year is gradually becoming shorter by 0.53 seconds per 100
solar years;
therefore some steps may be necessary in the future to synchronize solar
calendars to the solar year. The most well-known and widely-used solar calendar
is the Gregorian one. It is based on the Julian calendar,
which in turn is based on the Roman one. Julian calendar
defines the year to be 365.25 days.
This is 11 minutes longer than the solar year.
As a result Julian calendar ran 10 days ahead of the solar year
by 1582.
Gregorian calendar was established to correct this discrepancy.
Some places still use the Julian calendar, including the Orthodox Church,
but most countries have adapted to use the Gregorian calendar exclusively,
or alongside other calendars. By 2014
there is a 13 day difference between the Julian and the Gregorian
calendars.
The difference between the 365-day Gregorian calendar year
and the 365.2425-day
solar year is adjusted by having
a 366-day leap year every four years,
except for years divisible by 100 but not
divisible by 400. For example, 2000 was a leap year,
while 1900 was not.
Lunisolar calendars are a composite of the lunar
and the solar calendars. They generally use the lunar
phases for months, and often the months alternate between 29 and 30 days,
because 29.53 is the approximate mean length of the lunar month.
Lunisolar calendars adjust to synchronize with the solar year
by adding an extra month every few years.
Hebrew calendar is one example, where a thirteenth month
is added seven times during the nineteen-year period —
a practice referred to as a 19-year cycle or Metonic cycle.
Chinese and Hindu calendars are also lunisolar.
Other types of calendars are based on other astronomical phenomena,
such as the movement of Venus, or historical events
such as the change of an era. For example in Japan
the Japanese era calendar scheme (年号 nengō,
literally meaning "era name") is used in addition to the Gregorian calendar.
It refers to each year according to the name and the year
of reign of the emperor in power. The new emperor's name
becomes the name of the era, and is taken as a posthumous name
later. According to this scheme, 2013
is Heisei 25, the 25th year of rule of emperor
Akihito of the Heisei era. Heisei is intended to mean
"peace everywhere".
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