Science Olympiad Solar System Practice Test 2025 – The All-in-One Guide to Master the Competition!

A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, resulting in the Earth's shadow falling on the Moon. However, this alignment does not happen during every full moon due to the tilt of the Moon's orbit relative to Earth's orbit around the Sun.

The Moon's orbit is inclined at an angle of about 5 degrees to the plane of the Earth's orbit (known as the ecliptic). Because of this tilt, during most full moons, the Moon is either slightly above or below the Earth's shadow, which prevents an eclipse from occurring. It is only when the full moon coincides with the points in the Moon's orbit where it crosses the plane of Earth's orbit, known as lunar nodes, that an eclipse can take place. This alignment happens roughly 2 to 3 times a year, which is why lunar eclipses are relatively rare events compared to full moons.

The other options do not address the fundamental geometrical relationship between the Earth, Moon, and Sun that governs eclipses. The stability of Earth's orbit, the changing position of the Sun during the year, or the reflective properties of the Moon's surface do not affect the occurrences of lunar eclipses directly.