What celestial event can occur only when the moon crosses the ecliptic?

The celestial event that can occur only when the moon crosses the ecliptic is both a lunar eclipse and a solar eclipse. This is because the ecliptic is the apparent path of the sun across the sky, and the moon’s orbit intersects this path at two points known as nodes. A lunar eclipse happens when the Earth is positioned directly between the sun and the moon, causing the Earth's shadow to fall on the moon. This event can only take place when the moon is near one of these nodes during a full moon.

Conversely, a solar eclipse occurs when the moon moves between the Earth and the sun, obstructing the sun's light. This alignment also requires the moon to be near one of the ecliptic nodes, specifically during a new moon.

Since both types of eclipses depend on the moon crossing the ecliptic, the correct choice highlights that both lunar and solar eclipses are linked to this unique alignment of celestial bodies.