how do we define time how is time calculated star , rotation of earth how?

Answer 1

Our time divisions were originally derived from the rotation of the earth. One solar day was the standard. But as clocks got more precise, we came to realize that the Earth does not keep perfect time, speeding up and slowing down ever so slightly. We decided to find a more reliable standard. We now use a precise number of vibrations of a Cesium atom as the definition of a second. In the years when the Earth doesn't quite keep up, we add a leap second to the count to keep everything on schedule.

Answer 2

Generally there are three forms of calculation for time. Both are dependent on the rotation of the Earth, however each is in respect to different objects. First, the most common measurement of time is the solar day (a little over 24 hours), which is measured by the rotation of the Earth in relation to the Sun. The second form is called sidereal(pronounced si-der-e-ul) in which the rotation of the Earth is measured with respect to a given star (about 23 hours and 56 minutes). This form is the most accurate of the time charting methods. The third form is using the lunar month, about 29.5 days or 27.3 in sidereal terms. This form is the least accurate as the moon is in motion around us and can only be compared to the solar day, otherwise we would have 13 months instead of the twelve we have with solar days and months. The reason for the difference between the first two forms is because the Earth revolves around the Sun and therefore must take a little extra time for one point on the Earth to face exactly at the sun, whereas the stars, while moving, are so far away that they appear to fixed in the sky for the majority of our lifetimes.

Answer 3

Time is what keeps everything from happening all at once.

Do you mean, how do we define the UNITS of time?

The second is the basic unit. It is defined as the time it takes for a certain number of vibrations of a certain type of light.

The exact words are: The second is 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. This definition refers to a caesium atom at rest at a temperature of 0 K.

The old definitions used to be that a second was a fraction of a day, or a year. But now we use caesium clocks to define the second. (Also spelled cesium.)

Answer 4

time is defined (technically) by counting the oscillations of specific atoms at given temperatures.
I remember something about Caesium or so (?)


talking about time in the universe is a little bit more complex.
Einstein figured that time isn't just a universal ticking clock running always in one direction. He discovered that time is connected to space and can be warped.
Lucky as we are we do not directly have to deal with such effects, except if it comes to the GPS-system, where the clocks need to be corrected for the speed they travel, not to cause faulty results in your receiver.

A simple approach to explain what stars or planets do with time is to state that the gravity of celestial bodies warp space that way that time is running slower near such a body.

not much, but a tiny tiny fraction.