Transit of Uranus from Neptune
A transit of Uranus from Neptune occurs when Uranus passes directly between the Sun and Neptune. Uranus can then be seen from Neptune as a small black disk edging slowly across the disk of the Sun. A central transit lasts about 42 hours.
This is the rarest of all single transits involving the eight planets, owing to the long synodic period of 172 years (of Uranus from Neptune), the very small apparent diameter of the Sun (1.07 arc-minutes, close to the limit of human visual resolution) as seen from Neptune, and the mutual inclination of the two orbits, 1.5 degrees, which is less than that of most planetary pairs.
The next transit of Uranus from Neptune will occur in October, 38172 CE.
Notes and references 
- Meeus, Jean (1989). Transits. Willmann-Bell.
- SOLEX 9.1