A “day” is roughly 24 hours long, but if you want to be exact, there are two slightly different answers depending on what you mean by “day.”

Two main kinds of “day”

  • Solar day (our everyday 24‑hour day)
    This is the time from one noon to the next (or from midnight to midnight) as Earth spins once relative to the Sun.

    • By definition in civil time, this is exactly 24 hours = 86,400 seconds.
* In reality, the length of a solar day wobbles a bit because Earth’s rotation is not perfectly uniform, so some days are a few milliseconds longer or shorter than 86,400 seconds.

  • Sidereal day (relative to the stars)
    This is the time it takes Earth to rotate once relative to distant stars, not the Sun.

    • A sidereal day is about 23 hours, 56 minutes, and 4 seconds, roughly 23.934 hours.
* It’s shorter because while Earth spins, it is also moving along its orbit, so it has to turn a little extra for the Sun to appear in the same place in the sky again.

Why your clock says 24 hours

  • The “day” used in clocks and calendars is defined to be 24 hours long for practicality and global coordination, even though Earth’s true rotation is slightly irregular.
  • Timekeepers quietly adjust for this mismatch using techniques like leap seconds, which occasionally add one extra second to keep atomic time aligned with Earth’s actual rotation.

So, in everyday life a day is exactly 86,400 seconds by definition, but in terms of Earth’s physical motion, a “day” is only approximately that long and subtly changes over time.

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