Approach perihelion
Mercury accelerates along its eccentric orbit while its rotation remains nearly constant.
A geometric solar-day simulator
Watch the Sun change its mind.
Scrub Mercury past perihelion and inspect the instant orbital motion overtakes rotation, making the Sun appear to stop and reverse.
WestThe Sun is turning backEast
The three-minute mental model
Rotation stays steady. Orbital speed does not.
Cross zero
The apparent solar rate reaches zero. From the surface, the Sun seems to pause.
Reverse briefly
Orbital motion wins for a short interval, moving the Sun backward before normal motion resumes.
→ ← →
Durable output
Leave with an observation brief.
Capture the selected state, the model’s finding, its assumptions, and the next phase worth inspecting.
Mercury observationReversal window
ReadyThis is an educational normalized model of Mercury’s 3:2 spin-orbit resonance. It illustrates the sign change in apparent solar motion near perihelion; it is not an ephemeris, an observer-location forecast, or mission-planning software.