Domain: Natural Science — Astrophysics & Astronomy
Target concept: The planet moves faster at perihelion and slower at aphelion, sweeping out equal areas in equal time intervals.

Video generation prompt: A planet completes one full orbit around a star along an elliptical path.

Evaluation guide:
(i) Target concept: Kepler"s Second Law (equal-area law) in elliptical orbital motion.
Minimum mechanism: Gravitational central force producing planar, bound, periodic motion with conserved angular momentum, requiring star at focus (not center) and non-uniform angular velocity.
(ii)Storyline: Frame 1–2: Static setup showing star at elliptical focus and planet at perihelion start position. Frame 3–6: Planet accelerates smoothly along tightening curvature toward perihelion apex, covering larger angular displacement per unit time. Frame 7–8: Planet decelerates while traversing widening curvature away from star, covering smaller angular displacement per unit time. Frame 9–10: Planet completes full loop back to start; entire motion observed from fixed frontal 2D viewpoint with no camera movement or perspective shift.

Manual:
Experimental Procedure
1. Place a bright central star at one focus of a static, visible elliptical orbit path.
2. Position a planet on the orbit path and animate it moving continuously along the full ellipse.
3. Ensure planet moves faster when near the star (perihelion) and slower when far (aphelion).
Observation of Experimental Phenomena
Planet traverses entire elliptical path around star; speed visibly increases near star and decreases at farthest point; star remains fixed at one focus—not center—of ellipse.
Summary of Experimental Principles
A planet orbits a star along an elliptical path with the star at one focus; due to conservation of angular momentum, it sweeps equal areas in equal times—hence faster at perihelion, slower at aphelion.

Prompt Grounding rubric:
level_1: No star and planet are shown together, no orbital motion is shown, or the path drawn is a plain circle or unrelated shapes rather than a star with an orbiting planet.
level_2: A star and a planet appear in the same scene, but the path between them is not elongated into an ellipse (it's circular, jagged, or broken), or the planet doesn't move around the star at all.
level_3: The star and planet share a scene, and the planet follows a closed curved path around the star that is visibly elongated (tighter curve at one end, flatter at the other) rather than a plain circle, completing at least one full loop in a consistent direction.
level_4: The path is unambiguously an ellipse (one end curves noticeably tighter than the other), and the star sits visibly off-center, closer to one end of the ellipse than to its middle, consistent with sitting at a focus rather than the center; the planet travels the full path continuously with no pausing, jumping, or reversing.
level_5: The ellipse is rendered as one smooth, continuous line of even thickness throughout; the star stays fixed at its off-center focus position for the entire sequence; the planet keeps a consistent size and shape as it travels; there is no flicker, warping, or clipping at any point, and the full loop completes without interruption.

Scientific and Causal Correctness rubric:
level_1: The planet moves at the same speed regardless of how close it is to the star, or it moves faster when far from the star and slower when near it, or there's no indication that speed relates to position at all.
level_2: The planet's speed does change, but not in a way tied to the ellipse's geometry: for instance, it peaks somewhere in the middle of the long axis rather than nearest the star, or the star sits at the ellipse's center (removing any near/far distinction), or there's no visible cue for which stretches of the orbit are fast versus slow.
level_3: The planet is visibly quicker when it is near the star and visibly slower when it is far from the star (it covers more distance per unit of time close in and less far out), and the star's off-center (focal) position makes the near/far regions identifiable.
level_4: The fastest stretch of the orbit lines up exactly with the region nearest the star, and the slowest stretch lines up with the region farthest from the star, with no unexplained speed-up or slow-down anywhere else along the path; the speed changes smoothly within each region rather than jumping.
level_5: Comparing any two equal-time segments of the orbit (for instance the first quarter versus the last quarter), the planet visibly covers a longer arc when near the star and a shorter arc when far from it, and the star-planet-planet triangles implied by those two segments look like they cover comparable area (wide, short slices near the star balancing narrow, long ones far away), with this balance holding everywhere around the loop, not just in one spot.

Spatiotemporal Consistency rubric:
level_1: The planet suddenly jumps to a new position on screen; the star's position shifts; the orbit path breaks apart or vanishes mid-motion; the scene's brightness or shadowing changes abruptly with no cause; the ellipse splits into disconnected arcs.
level_2: The planet flickers out of view for a moment; the orbit line fades in and out intermittently; the star appears to wobble or change size inconsistently; the planet's motion stutters, visibly freezing or reversing direction at least twice.
level_3: The star stays fixed in place, the orbit path stays whole and continuously visible, and the planet glides along the path without jumping, resizing, or spinning oddly; the scene's brightness and shadow direction stay visually constant, and the camera viewpoint doesn't move.
level_4: In addition, the planet's speed changes without any sudden jolts: no instant jump from slow to fast; the orbit's curve bends smoothly everywhere with no sharp corners where perihelion transitions to aphelion; the planet's own orientation stays consistent, without unintended tumbling or flipping as it travels.
level_5: The whole sequence reads as perfectly fluid to the eye: no stutter or jitter in the planet's motion, no visible warping or tearing of the planet's or the orbit's shape, no moment where the planet appears to pass in front of or behind the star in a way that looks physically wrong, and no shift in the background suggesting the camera moved; the planet's speed increases and decreases gradually through the near-star and far-from-star portions of the orbit, with no abrupt change anywhere.