Time (physics): Who started counting our current time or is it just "set" by some scientific measure?

Civil time, UTC, Coordinated Universal time, is ultimately based on UT, Universal Time. UT is based on the rotation of the earth relative to the sun.

The basis for civil time is UTC (Coordinated Universal Time). The spelling of the abbreviation is explained in the 2nd reference below.

But on a much smaller scale, UT1 fluctuates in speed just a tisk due to large mass movements, like cold and warm air, mass distirubtion changes due to ocean currents, and even earthquakes. While these changes have a very small effect, it’s not too small to cause problems. And in the long run, the earth’s rotation is slowing down due to tidal friction, we use leap seconds as needed to allow the earth (UT) to catch up so UTC stays within 0.9 seconds of UT. We need a uniform time scale. This given by a weighted average of atomic clocks around the world. The result is International Atomic Time (TAI).

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We have two competing needs for civil time

We’d like a uniform scale, so that every second has the same duration of any other second.

Now we have to mix the two.

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The earths’s rotation is slowing down due to tidal forces. The Sun and the Moon create the tides, which involves dragging water from the sides of the earth to the near and far side of the earth, which, through friction, slows down the rotation of the earth. So this means the earth’s rotation has slowed down since 1948. So UT is therefore based on longer seconds. Thus, UTC must be paused every now and then to allow the Earth, and hence UT, to catch up. It was decided that a leap second would be added or subtracted as often as twice a year to accomplish this pause. Not surprisingly, a negative leap second was never needed.

Originally, a second was defined as 1/86,400th of a mean solar day. I think it was based on Jan 1, 1948. It is now defined as some huge number of certain transitions in Cesium and stuff, which I assume was pretty close to that “original” second.

First of all, there are many time scales: terrestrial time (TT), the successor to ephemeris time. Local solar time. Mean solar time. Universal time (abbreviated UT or sometimes UT1), UT0, UT2, daylight saving time. GMT, which has 3 meanings! (1) The mean solar time at the Royal Observatory in the UK, (2) UT1, or most commonly, (3) UTC. Thus the term GMT should be phased out, but is still in use by a lot of people.

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UTC is a certain number of seconds behind TAI. It was decided to keep UTC within 0.9 seconds of UT1. This is done using leap seconds as needed. The reason for leap seconds is that the earth is turning slower on average than in the past. This is due to tidal friction. The Moon and Sun make tides, which slow down the earth. So instead of 86,400 seconds in a day, there are 86,400 plus a tiny bit more seconds in a day. Eventually that builds up to about a second, and a leap second is introduced at some point as needed. Some committee somewhere decides, I think.

Civil time is based on the mean solar day. Because of the tilt of the earth (relative to the plane of its orbit) and the fact that the earth’s orbit is an ellipse, not a circle, meaning it moves a little faster when it is closer to the sun. It’s pretty close to a circle, though. The result is the analemma, which is the figure-8 you see on some globes. If you mounted a camera and took a picture of the sun at the same civil time every day, or every week over an entire year, without ever moving the camera, you will get a figure-8 curve called the analemma. This means the sun can be as much as 16 min. early or late compare to the mean sun. So we go by what’s called mean solar time, which is the average, or mean, of the local solar time over a period of a year. Why? Because we live our lives by when it’s day and when it’s night, and we don’t want seconds and minutes and hours to “stretch and shrink” or have noon slide back and forth to follow the analemma. The local mean sun is on the merdian at 12:00 local mean solar time.

and

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Unfortunately, Wikipedia articles vary greatly in quality. The article on TT is definitely confusing. The ones here could be rewritten to be in a better order, just like mine! But I have only so much time and am only 1 person.

So we define Coordinated Universal Time (aka UTC). This is given by TAI plus an offset. But we need occasional leap seconds.

See also

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But seconds in UTC are based on the SI definition using atomic clocks. But the earth is slowing down, and being slower than before means that leap seconds needed to be added to UTC from time to time to allow the earth to catch up. (UTC runs at the same rate as TAI, which is based on atomic clocks around the world.)

But I can comment some.

No. 2 is accomplished by defining Universal Time (aka UT, aka UT1). Unfortunately, this time does not pass in a uniform manner. Because of two causes: (1) the tilt of the earth relative to the plane of the earth’s orbit, and (2) the non-uniform speed of the earth as it goes around the sun. The result is that the sun can be as much as 15 min. early or late compared to the “mean sun”, which is based on the average rate of the time given by the actual position of the sun. Universal Time is then the mean solar time at 0 deg. longitude.

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Time is a very complex issue, and you’re question is kinda on the vague side.

We need to stay in sync with the sun

No. 1 is given by International Atomic Time (TAI). It is a weighted average of the time given by atomic clocks around the world.

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I can say civil time (aka UTC) is ultimately is based on Universal Time (aka UT1), which is based on the rotation of the earth relative to the sun. But we use atomic clocks. Since the earth is rotating slower then earlier, we have to add leap seconds from time to time to allow the earth (UT) to catch up to UTC. Adding a leap second basically “pauses” for 1 second.

Time (physics): Who started counting our current time or is it just "set" by some scientific measure?