Sidereal Orbital Period of Neptune
| Value | 5,200,329,600 s |
| Status | Conventional / typical value |
| Source | NASA NSSDC Planetary Fact Sheet, Neptune (sidereal orbit period) |
| Categories | Astronomicalsolar-systemplanets |
| Planck time | 9.6458752e+52 tP |
| attosecond | 5.2003296e+27 as |
| femtosecond | 5.2003296e+24 fs |
| picosecond | 5.2003296e+21 ps |
| nanosecond | 5.2003296e+18 ns |
| shake | 5.2003296e+17 shake |
| microsecond | 5.2003296e+15 μs |
| millisecond | 5,200,329,600,000 ms |
| second | 5,200,329,600 s |
| minute | 86,672,160 min |
| hour | 1,444,536 h |
| sidereal day | 60,353.792 sd |
| day | 60,189 d |
| week | 8,598.4286 wk |
| fortnight | 4,299.2143 fn |
| month | 1,977.5026 mo |
| year | 164.7885 yr |
| decade | 16.47885 dec |
| century | 1.647885 cent |
| millennium | 0.1647885 mil |
| megayear | 0.0001647885 Myr |
| eon | 1.6478850e-07 eon |
Learning zone
Neptune's year is so long that the planet has been observed for barely one of them: discovered on 23 September 1846, it returned to the same heliocentric longitude on 11 July 2011, having completed a single circuit while Earth ran 165. Every claim about long-term behaviour — seasonal change, cloud cycles, the waxing of southern summer since Voyager — rests on less than one Neptunian year of data, most of it from Earth-based telescopes and Hubble.
The period's most elegant service predates the discovery itself. Le Verrier's computed orbit, fed to Galle with a position good to one degree, implicitly contained this period through Kepler's third law; the actual 164.8-year figure emerged within months once pre-discovery sightings were recognised — including Galileo's, who plotted Neptune as a fixed star near Jupiter in December 1612 and January 1613, and even noted that it seemed to have moved. He was one careful night from finding a planet 233 years early.