Final answer:
Eclipses occur when the Sun, Moon, Earth, or the Moon temporarily blocks sunlight from reaching the other, with the correct answer being the Moon for solar eclipses and Earth's shadow for lunar eclipses.
Step-by-step explanation:
Eclipses happen when the Sun, Moon, Earth, or other objects temporarily blocks the sunlight from reaching the another. The correct answer to fill in the blank is the Moon. This is because an eclipse of the Sun can occur when the Moon is somewhat nearer than its average distance to the Earth, allowing the Moon to completely hide the Sun, producing a total solar eclipse. This happens when the umbra of the Moon's shadow reaches the surface of Earth, causing the area within the umbra to experience darkness in the middle of the day.
Additionally, a lunar eclipse occurs when the Moon moves into Earth's shadow. This can be seen from anywhere on the night side of Earth and results in the Moon appearing to darken as it passes through Earth's shadow. Both solar and lunar eclipses are natural astronomical events that illustrate the fascinating dynamics within our solar system.