What causes a lunar eclipse to occur?

A lunar eclipse occurs when the Earth is positioned directly between the sun and the moon, causing the Earth's shadow to fall on the moon. This can only happen during a full moon when the three bodies are aligned in a straight line. As the moon moves into the Earth's shadow, it can take on a reddish hue, often referred to as a "blood moon," which is a result of the Earth's atmosphere scattering sunlight and allowing only the longer red wavelengths to reach the moon.

In contrast, the other options describe different celestial phenomena. The moon’s shadow falling on Earth refers to a solar eclipse, not a lunar eclipse. Additionally, while the sun's light being blocked by Earth is part of the process, it is specifically the Earth's shadow that falls on the moon that defines a lunar eclipse. Lastly, the idea of the moon passing through the Earth’s atmosphere is inaccurate since the moon is not in the same vicinity as the Earth's atmosphere during these events. Understanding the mechanics of how shadows work in relation to these celestial bodies clarifies why the correct answer highlights the relationship of Earth's shadow on the moon.