A day on Neptune lasts about 16 hours, specifically around 15 hours, 57 minutes, and 59 seconds to approximately 16 hours and 6 minutes depending on measurement methods. This is the time it takes for Neptune to complete one full rotation on its axis

. Neptune rotates faster than Earth, whose day is about 24 hours, so its day is significantly shorter despite its much larger size. The planet's rotation period is roughly 16.11 hours in sidereal time, with some variation due to differential rotation in its atmosphere where equatorial and polar regions rotate at different speeds

. In summary:

• Length of a day on Neptune: about 16 hours
• Exact measured values: approximately 15h 57m 59s to 16h 6m 36s
• Neptune's atmosphere shows differential rotation, causing slight variations in rotation speed by latitude