A civil (clock) day is defined as exactly 24 hours, which is exactly 86 400 seconds, by human convention—but Earth’s actual rotation makes both “day” definitions a bit different from that.

Two main kinds of “day”

  • A solar day (the basis of calendar days) is the time from noon to the next noon, on average about 24 hours or 86 400 seconds, but real days wander by a few milliseconds longer or shorter due to small changes in Earth’s spin.
  • A sidereal day (one full turn of Earth relative to the distant stars) is about 23 hours, 56 minutes, 4.09 seconds, so roughly 23.934 hours, which is shorter because Earth also moves along its orbit while it rotates.

Why it isn’t exactly 24 hours

  • Earth’s rotation speed slowly changes because of tidal friction with the Moon and other geophysical processes, so the average length of a solar day drifts by about a couple of milliseconds per century.
  • Over very long times, that adds up: models suggest the day lengthens by roughly 1–2 milliseconds per century and used to be under 19 hours about 1.4 billion years ago.

How precise modern measurements get

  • Sensitive measurements show that individual days can differ from exactly 86 400 seconds by a few milliseconds, thanks to things like earthquakes, core–mantle interactions, and atmospheric flows.
  • Because of these tiny differences, timekeepers occasionally add or consider adding a “leap second” to keep atomic time aligned with Earth’s slightly irregular rotation.

Quick numeric summary

  • Civil/defined day: exactly 24 h = 86 400 s (by definition).
  • Mean solar day now: about 86 400 s, fluctuating by a few milliseconds day to day.
  • Mean sidereal day: about 23 h 56 min 4.09 s ≈ 86 164.09 s.

Information gathered from public forums or data available on the internet and portrayed here.