Sidereal Rotation Period of Neptune
| Value | 57,996 s |
| Status | Conventional / typical value |
| Source | NASA NSSDC Planetary Fact Sheet, Neptune (sidereal rotation period, 16.11 h; magnetic field, Voyager 2) |
| Categories | Astronomicalsolar-systemplanets |
| Planck time | 1.0757437e+48 tP |
| attosecond | 5.7996000e+22 as |
| femtosecond | 5.7996000e+19 fs |
| picosecond | 5.7996000e+16 ps |
| nanosecond | 57,996,000,000,000 ns |
| shake | 5,799,600,000,000 shake |
| microsecond | 57,996,000,000 μs |
| millisecond | 57,996,000 ms |
| second | 57,996 s |
| minute | 966.6 min |
| hour | 16.11 h |
| sidereal day | 0.67308782 sd |
| day | 0.67125 d |
| week | 0.095892857 wk |
| fortnight | 0.047946429 fn |
| month | 0.022053841 mo |
| year | 0.0018377823 yr |
| decade | 0.00018377823 dec |
| century | 0.000018377823 cent |
| millennium | 0.0000018377823 mil |
| megayear | 1.8377823e-09 Myr |
| eon | 1.8377823e-12 eon |
Learning zone
Before Voyager 2, estimates of Neptune's day scattered between 15 and 20 hours, tracked from clouds that — as on every giant planet — drift with the winds rather than the interior. The flyby settled it: periodic radio emission locked to the magnetic field gave 16.11 hours, and the comparison with the cloud-tracked speeds revealed the astonishing part. Near the equator the atmosphere lags the interior's rotation by up to 2000 km/h — the fiercest differential winds in the solar system, retrograde relative to the spin, on the planet with the least sunlight to drive them.
Like Uranus's, Neptune's field turned out badly off-centre and tilted 47° from the spin axis, so the radio clock wobbles usefully — but it has broadcast to no listener since 1989. The 16.11-hour figure is a single-epoch measurement; whether Neptune's radio period drifts the way Cassini found Saturn's does is one more question waiting on a return.