How is the standard time calculated?  \n\nFor instance, an atomic clock is has the uncertainty of 5 x 10-16, which means it would neither gain nor lose a second in more than 60 million years.\n\nWhat are they comparing that to?\nIs there an equation that defines time?  If so, what is it?
137562
This is where convention comes in. hehe.\n\nYou see, today "time" is defined based on a reference. We define one second as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency, replacing the old second that was defined in terms of the Earth's motions.\n\nThe "uncertainity" is due to the fact that our one second here ( where its based on the measurement of this number of oscilations )  is NOT 100% the same as compared to our measurement of one second based on the previous way.\n\nWhat was the previous way? Well, it bascially defines one second the other way around. What i mean here is that in the old days time is defined relative to the motion of the earth around the sun. Ancients defined the total time taken for the earth to finish one complete revolution around the sun as one year. Then they agreed that one year is approximately made up of 365 days. Each day is then divided into 24 hours. 1 hour is then considered as having 60 minutes where 1 minute comprises as 60 seconds.\n\nOf course to achieve the definition of time as how we have it now did not just come overnight. It took some time... hehe. I did not attempt a chronological development of the idea but just intend to shed some light on the subject. You can find further information on the link below.
260803
I think what you're looking for is the "equation of time", which is the difference between local mean time and local apparent time.  The term "difference" is used because "variance" has a very specific mathematical meaning.   Local apparent time is determined by the position of the sun in the sky.  local mean time is based on Greenwich Mean Time.  The difference occurs because GMT is based upon a fixed rotational rate, and local apparent noon is determined is by the maximum altitude of the sun in the sky.\n\nThe two time scales differ by as much as 15 minutes.  The web page below has more details.\n\nThere are formulas for this, but they are complex calculations.  The equation of time changes constantly, although the change is very slow, so a graph of the difference in 1980 is still mostly correct for 2006.  You'll be accurate to within a few seconds.
360404
Imagine the average human life of about 70 years (hypothetically).\n\nMake a time line of that persons life of 70 years.  Just for giggles we will make the time line one meter long, for every year of this person's life.  So you have a time line that is 70 meters long.  \n\nLooks good, right.  70 years is almost three quarters of a century, which in perspective to us as humans, is a relatively long time.  \n\nImagine that somone could actually live for 7000 years.  Now, make a time line that is 7000 meters long for this second person.  This is our "extended time line".  \n\nThis 7000 Meter time line or 7 Kilometers is a rather larger distance in perspective to the first 70 meter line.\n\nNow imagine laying both lines down, parallel to each other in a clear open field somewhere and imagine looking down at the two lines from the sky.  \n\nOf course the shorter line is shorter, but compared to the longer line it will appear extremely small.  The small line may even appear to have ceased to exist when compared to the longer line.  \n\nDoes this help put in into perspective?\n\nKinda like having a single specific drop of water, going to the beach and dropping it into the ocean.  Now go find that specific drop of water that you released into the ocean.  It is still there, but compared to all of the other drops of water that make up the ocean, it is all but non-existent.
609856
Does any body really know what time is?  Does any body really care about time?  As I was walking down the street today, being pushed and shoved by people trying to beat the clock, that day, that day.....does anybody really know? Does anybody really care?
527675
Judge for yourself based on this: Time is a MAN MADE construction. That is, it is not a natural element in life. For those of you who would argue, please answer how it can be set back and set forward (daylight's saving time). If it were natural you wouldn't be able to limit it to the watches you wear or the clocks on the wall. It would be beyond any human control and it would be very hard for anyone to grasp the concept and explain it. As it is, we CAN explain it and grasp it, therefore, it is not natural.\n\nSo my answer is no.\n\n\n\n\n\n\n\n\n\n\n.
117883
Great subject!\n\nI think it is it's own form of energy. \n\nEinstein said if you looked at a clock and moved at the speed of light, away from it, the hands would stay still. But the speed of light is measured by distance over a period of time. You can't divide by 0, so time is not stopped. It's simply an optical illusion.\n\nYes, history repeats itself, but that is just because we could never know all the mistakes that were made in the past. So time doesn't repeat itself, our human nature does.\n\nMy belief on time is that it is relative to the energy of your surroundings. I.E. a peice of meat will go bad in a week, but in a freeezer it is good for a long time. To the piece of meat, time was slower. The time was relative to the heat (energy).
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for every 15 degrees of longitude, there is essentially 1 hour difference in when the sun rises.  360 degrees/24 hours.   -  that set's up the time zones, basically.\n\nbut - it's not like the time just comes in 1 hour chunks :)\n\nevery degree on longitude is roughly 4 minutes away (sunrise, sunset...that is) from the next degree.  and in there....15 arc-minutes (1/4 degree) = 1 minute.   and so on. :)\n\nand...since the earth is constantly revolving, but we assign the same "local time" to a huge swath of the earth. - every spot on the earth sees the sun rise at a different moment as measured by "local time".\n\n:)
11717
Einstein said we are living in the past time, that means we have not "now time"!\nSo, nobody can not answer, what time is it! maybe we can ask, what time was it!!
291270
I forgot what the longest time from 12 was, but with USSR eased off, it should be max right now.\n\nI have not seen the clock lately either.\nLooks like 7 min till 12 is what it is on. 2006
158921
http://physics.nist.gov/GenInt/Time/time.html
30290
There are even today many different ways of figuring out time. For example this weekend, on 29th Jan the chinese new year starts. It is Year 4703 by the Chinese calendar. And it is the jewish year 5677.\nAnd even the christian calendar has been modified a couple of times. The gregorian calendar reform was adopted in 1582 but not in England and so also not in the US, they only changed in 1752 (so they lost 11 days).\nWhich year we are in is only relative to what you want to measure. If you want to be more explicit you don't just say B.C. (Before christ) but B.C.E (before the christian era). As long as you can figure out how far apart they are you can easily convert one into the other and allow for changes. \nFor time of the year (month/day) it is probably best to go by a system like now, which is constant with and corresponds to the seasons of the year. So while George Washington was born on the 22nd feb 1732, his parents must have thought it was the 11th when he was born. \nI don't know how far historic dates get modernized to reflect calendar changes.
6981
Time is just a concept, it does not really exist. We track time by the movement of celestial objects in the universe and light but time is abstract.
115984
We really do not know what time is. However, here are some comments:\n\n1. The other 3 dimensions of the universe we live in are reversible. You can go forward and back, you can go left and right, and you can go up and down. But you cannot go forward in time and backwards, as far as we know. So it is different than the other 3 dimensions. \n\n2. The units of length, width and breadth are the same. The units of time are different. Physicists use the speed of light to convert between them, but that is semi arbitary, or at least it appears that way.  \n\n3. There are 3 spatial dimensions, but as far as we know, only one temporal dimension. We do not know why. We do not know if there are more. \n\n4. The equations of physics are mostly invariant with respect to time reversal (t---> -t): wave equation, heat equation, Laplace equation, schrodinger equation, etc. However, the phenomena we observe are not. \n\n5. The invariance we observe in nature mainly stems from thermodynamics, which requires the use of Grassmanians to be rigorously described. This is not true of other branches of physics. \n\n6. As far as we know, time is continuous. However, we have no way to tell. There are theories in physics which require time to be discrete. \n\n7. In the Lorentzian metric, time appears with a different sign that the other dimensions. We live in a nonRiemannian manifold because of this. That is, the proper distance between events can actually be an imaginary number!  \n\n8. We do not know if time is linear or not. We do not know if it has some strange topology or not. \n\n9. We use time as a parameter. We observe its passage. But we have no idea what it is, to be honest.
363427
Time is relative.  It doesn't really exist.  Time is effected by the speed of motion, and is not an object.
510096
In a modern clock, a voltage is applied to a crystal such as quartz.  This causes the molecules in the crystal to vibrate at a fixed frequency.  A sensor then counts the vibrations over time and determines how much time has elapsed.
620964
Latitude does not affect time, it just affects daylight length. Longitude affects time\nNow ... the earth is a circle, (360 degrees) it turns once a day, 360 degrees in 24 hours, 15 degrees an hour. The 0 meridian is at Greenwich in London. so 10 degrees east is 40 minutes in front of Greenwich, and 70 degrees west is 280 minutes behind Greenwich. Using this we can say that 70 degrees west is 320 minutes behind 10 degrees east that is 5hrs and 20 minutes take that away from the flight time and you have 4 hours and 40 minutes, this means it will be 06.40 a.m. , local time, when you land
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A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. Most time zones are exactly one hour apart, and by convention compute their local time as an offest from Greenwich Mean Time.\n\nStandard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighbouring zones is then exactly one hour different. However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.\n\nThere are different definitions of time zone which generally fall into two meanings: a time zone can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.\n\nGreenwich Mean Time (GMT) was established in 1675 as an aid to determine longitude at sea by mariners. The first time zone in the world was established by British railways on December 1, 1847 — with GMT hand-carried on chronometers. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Greenwich Observatory. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some old clocks from this period have two minute hands — one for the local time, one for GMT [1]. This only applied to the island of Great Britain, and not to the island of Ireland.\n\nOn November 2, 1868, New Zealand (then a British colony) officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.\n\nTimekeeping on the American railroads in the mid nineteenth century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time. The Pittsburgh main station used six different times! The confusion for travellers making a long journey involving several changes of train can be imagined