A transitofMarsacross theSunas seen fromNeptune takes place when the planet Mars passes directly between the Sun and Neptune, obscuring a small part of the Sun's disc for an observer on Neptune. During a transit, Mars can be seen from Neptune as a small black disc moving across the face of the Sun.
Naturally, no one has ever seen a transit of Mars from Neptune, nor is this likely to happen in the foreseeable future. Nevertheless, the next one will take place on 6 May, 2026 and on 11 June, 2024 there will be a close miss by Mars.
A transit could be observed from the surface of one of Neptune's moons rather than from Neptune itself. The times and circumstances of the transits would naturally be slightly different.
The Mars-Neptune synodic period is 694.911 days, or 1.90 years. It can be calculated using the formula 1/(1/P-1/Q), where P is the sidereal orbital period of Mars (686.98 days) and Q is the orbital period of Neptune (60,190 days). The mutual inclination of their orbits is 2.38°.