Mean Solar Day
| Value | 86,400 s |
| Status | Exact by definition — no uncertainty |
| Source | SI definition of the day (BIPM); mean solar day from IERS length-of-day series |
| Categories | Astronomicalearthtimekeeping |
| millisecond | 86,400,000 ms |
| second | 86,400 s |
| minute | 1,440 min |
| hour | 24 h |
| day | 1 d |
| week | 0.14285714 wk |
| year | 0.0027378508 yr |
Learning zone
The second was originally 1/86 400 of a mean solar day, so by construction the day was exactly 86 400 of them. When the second was redefined in 1967 in terms of caesium-133, the definition was pinned to the rotation rate of the mid-nineteenth century, and the planet has been slowly losing ground ever since: the mean length of day currently runs about 1–2 milliseconds over 86 400 s, mostly because tidal friction with the Moon is braking the spin.
Those milliseconds accumulate, which is why UTC has needed 27 leap seconds since 1972 to stay within 0.9 s of Earth's actual rotation. The "mean" also hides the equation of time: because the orbit is elliptical and the axis tilted, real solar days vary by up to 30 seconds through the year, so sundials run up to 16 minutes ahead of or behind clocks — the analemma printed on old globes.