Tip:
Highlight text to annotate it
X
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 five 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
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 or time zones are formed,
having approximately the
same local time.
Times 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--3:30.
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,
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".
Thank you for watching.
If you liked this video,
please don't forget
to subscribe to our channel!