Eclipses occur due to Rectilinear Propagation.

Explanation:
Eclipses are fascinating astronomical events that occur when one celestial body passes through the shadow of another celestial body. They can be categorized into two types: solar eclipses and lunar eclipses.

1. Solar Eclipses:
Solar eclipses occur when the Moon passes between the Sun and the Earth, casting its shadow on a portion of the Earth's surface. During a solar eclipse, the moon blocks the direct sunlight from reaching certain areas on the Earth.

- Shadow Formation:
The shadow of the Moon consists of two parts: the umbra and the penumbra. The umbra is the darkest part of the shadow, where the Moon completely blocks the sunlight. The penumbra is a lighter shadow where only a portion of the sunlight is blocked.

- Path of Totality:
The path of totality refers to the region on the Earth's surface where the Moon completely covers the Sun, resulting in a total solar eclipse. Only the observers within this narrow path can witness the complete blocking of the Sun. Outside this path, the observers witness a partial solar eclipse where only a portion of the Sun is obscured.

2. Lunar Eclipses:
Lunar eclipses occur when the Earth comes between the Sun and the Moon, casting its shadow on the Moon. During a lunar eclipse, the Earth blocks the sunlight from directly reaching the Moon.

- Shadow Formation:
Similar to solar eclipses, the Earth's shadow also consists of two parts: the umbra and the penumbra. The umbra is the region where the Earth completely blocks the sunlight, and the penumbra is the region where only a portion of the sunlight is blocked.

- Types of Lunar Eclipses:
There are three types of lunar eclipses: total lunar eclipses, partial lunar eclipses, and penumbral lunar eclipses. A total lunar eclipse occurs when the Moon passes through the Earth's umbra, resulting in the complete darkening of the Moon. A partial lunar eclipse occurs when the Moon passes partially through the Earth's umbra. A penumbral lunar eclipse occurs when the Moon passes only through the Earth's penumbra, resulting in a subtle darkening of the Moon.

In conclusion, eclipses occur due to the optical phenomenon of rectilinear propagation. During a solar eclipse, the Moon's shadow is cast on the Earth, and during a lunar eclipse, the Earth's shadow is cast on the Moon. These phenomena are a result of the straight-line propagation of light, where the light rays travel in straight lines until they encounter an obstacle (in this case, the Moon or the Earth), causing an eclipse to occur.