The actual time for one day is 23hrs, 56mins, 4.1secs right? Then how can the clocks and watches can show perfect time? I mean, if I observe the sun rise at 6:00am this day, tomorrow I should observe it before 6:00 or I can say at 5:56 approximately as our measuring devices follow 12+12 i.e., 24hr day. But in practice again it will be 6:00am when the sun rises. How it is possible?


1 Answer 1


Those 23hrs, 56mins, 4.1secs are actually the sidereal rotation period of Earth. It is how much time the Earth needs to perform one rotation relative to the stars. But that is not exactly the same as solar time. By the time the Earth has completed one rotation, it has also moved a little forward in its orbit around the Sun. Thus, it needs a few more minutes of rotation to catch up so the Sun is at the same location of the sky.

solar time

  • $\begingroup$ True, but the solar day isn't exactly 24 hours either except for a few times a year, if at all. Due to the earth orbit's eccentricity a solar day varies between about 23:45 and 24:15. Only the "mean solar day" (i.e. The average) is 24:00. Lookup the "Equation of Time" $\endgroup$ Apr 4, 2016 at 0:12
  • $\begingroup$ @JimGarrison A much larger cause of different daylengths than the eccentricity, is the axial tilt of Earth. $\endgroup$ Apr 4, 2016 at 3:55
  • $\begingroup$ We're talking about a full 24 hour cycle, not sunrise to sunset. $\endgroup$ Apr 4, 2016 at 5:00
  • $\begingroup$ @JimGarrison But then you have forgotten that there is always a rate of change, for instance, the sunset comes minutes earlier each day during spring. $\endgroup$ Apr 4, 2016 at 5:24

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