Lunar eclipses occur when the Moon passes through the shadow cone of Earth
lunar eclipse, Moon, shadow cone, penumbra, umbra, full moon, orbiting of the Moon, synchronous rotation, orbit of the Moon, orbital plane, Sun, Earth, astronomical object, astronomy, geography
- How long does it take for the Moon to complete one orbit around the Earth?
- What color does the Moon appear during a lunar eclipse?
- How often do lunar eclipses occur?
A lunar eclipse occurs when the Moon passes into the Earth’s shadow. For a thorough understanding of the phenomenon we need to clarify the positions of the Moon and Earth relative to each other.
The Moon is the Earth’s companion and its sole natural satellite. It orbits the Earth, more exactly, it revolves in an elliptical orbit around the common center of mass of the Moon and Earth. The center of mass of the Earth-Moon system lies inside the Earth. The moon completes one orbit in 27.3 days. It takes the same amount of time to complete one orbit as it does to rotate on its axis. Therefore, we always see the same side of the Moon. This is called synchronous rotation.
The Moon has no light of its own, but it reflects the light of the Sun. The Earth blocks the sunlight, thus a shadow is formed behind it. Since the Sun is larger than the Earth, the shadow of the Earth is composed of two cone-shaped components. The inner cone, or umbra, is a region of complete shadow which receives no direct sunlight. In contrast, the external part of the Earth's shadow is a region where sunlight is only partially blocked. This part is called the penumbra.
A lunar eclipse occurs when the shadow of the Earth falls on the Moon. Two conditions must be met for a lunar eclipse. First, the Moon must be on the opposite side of the Earth from the Sun, that is, it must be in the Full Moon phase. This is the only way it can be in the shadow cone. The second condition is that the Moon must be located on or near the Earth's orbital plane. Because the Moon's orbital plane is tilted by 5 degrees with respect to the Earth's orbital plane, i.e. to the ecliptic, full moon phases usually occur when the Moon is above or below the Earth's shadow. This explains why lunar eclipses do not occur every full moon, but only two-three times a year.
A total lunar eclipse occurs when the entire Moon passes through the Earth's umbra. A partial lunar eclipse occurs when only a portion of the Moon passes through Earth's umbra. The third type of lunar eclipse, known as a penumbral lunar eclipse, occurs when the Moon passes through the Earth's penumbra, although this is more difficult to observe with the naked eye. Both total and partial lunar eclipses can be observed from the entire nighttime hemisphere of the Earth. The spectacle can last up to 107 minutes. It can take up to six hours between the moment the Moon passes into the Earth’s shadow until it leaves.
It is not even during a total lunar eclipse that the Moon disappears completely from the night sky; it usually appears to be a deep red color. This is because the Earth's atmosphere refracts the sunlight, and reflects part of it into the Earth's shadow. Since the long-wavelength red light is less absorbed in the atmosphere, the moon appears red. The color of the moon and the degree of its darkening depend on atmospheric conditions, but sometimes also on the dust content of the atmosphere.
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