The occurrence of a partial lunar eclipse instead of a total lunar eclipse is due to the alignment and positioning of the Earth, Moon, and Sun. Here's why partial lunar eclipses occur:
During a lunar eclipse, the Earth comes between the Sun and the Moon, casting its shadow on the Moon. For a total lunar eclipse to happen, the Earth's shadow must cover the entire Moon, causing it to appear fully dark or reddish-brown in color.
However, a partial lunar eclipse occurs when the alignment is such that only a portion of the Moon passes through the Earth's shadow. There are a few reasons why this might happen:
Geometric Alignment: The Earth, Moon, and Sun are not perfectly aligned in a straight line during every lunar eclipse. The Moon's orbit is slightly tilted relative to the Earth's orbit around the Sun. As a result, during some lunar eclipses, the Moon only grazes the edge of the Earth's shadow, causing a partial eclipse rather than a total one.
Size of Earth's Shadow: The Earth's shadow has two distinct regions—the inner dark shadow called the umbra and the outer, lighter shadow called the penumbra. During a partial lunar eclipse, the Moon may pass through only a portion of the Earth's penumbra, resulting in a partial darkening of the lunar surface. The Moon needs to pass through the umbra to experience a total eclipse.
Atmospheric Conditions: Earth's atmosphere also plays a role in the appearance of a lunar eclipse. During a total lunar eclipse, some sunlight is refracted by the Earth's atmosphere and bent toward the Moon. This causes the Moon to have a reddish hue, often referred to as a "blood moon." However, atmospheric conditions, such as cloud cover or pollution, can affect the amount of light reaching the Moon, leading to variations in the appearance of the eclipse, even if it is a total eclipse.
In summary, the occurrence of a partial lunar eclipse instead of a total one is mainly due to the geometrical alignment of the Earth, Moon, and Sun, as well as the specific regions of the Earth's shadow that the Moon passes through. These factors determine the extent to which the Moon is obscured during the eclipse, resulting in either a partial or total lunar eclipse.