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Mobile App Chapter 2: Problem 38
Why does the fully eclipsed Moon appear reddish?
Short Answer
Expert verified
The fully eclipsed Moon appears reddish due to the scattering of blue light and the passing of red light through Earth's atmosphere.
Step by step solution
01
Understand the Earth’s Shadow
During a lunar eclipse, the Earth comes between the Sun and the Moon, casting a shadow on the Moon. The Earth's shadow consists of two parts: the umbra (the central, darkest part), and the penumbra (the outer part). When the Moon is fully within the umbra, it is fully eclipsed.
02
The Role of Earth's Atmosphere
The Earth’s atmosphere filters and refracts sunlight. It scatters shorter wavelengths (blue and violet light) more than longer wavelengths (red light). This scattering causes the red light to bend around the Earth and reach the Moon.
03
Red Light on the Moon
As a result of this atmospheric scattering, the Moon receives predominantly red light during a full eclipse. This red light is then reflected back to Earth, making the fully eclipsed Moon appear reddish.
04
Summary
During a total lunar eclipse, the Moon appears reddish because Earth’s atmosphere scatters blue light and allows red light to pass through and reach the Moon, which is then reflected back to Earth.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Earth's shadow
During a lunar eclipse, Earth comes between the Sun and the Moon. This alignment causes Earth to cast a shadow on the Moon. Earth's shadow has two main parts: the umbra and the penumbra. The umbra is the central, darkest part of the shadow. If the Moon is entirely within the umbra, it becomes fully eclipsed.
The penumbra, on the other hand, is the outer, lighter part of the shadow. The Moon first passes through the penumbra before entering the umbra. This shadow play explains why we sometimes see the Moon partially covered before a total eclipse.
The penumbra, on the other hand, is the outer, lighter part of the shadow. The Moon first passes through the penumbra before entering the umbra. This shadow play explains why we sometimes see the Moon partially covered before a total eclipse.
atmospheric scattering
Earth's atmosphere acts like a filter for sunlight. This phenomenon is called atmospheric scattering, where different wavelengths of light are scattered by particles in the atmosphere.
Shorter wavelengths like blue and violet light scatter more easily, causing them to spread in many directions. That's why the sky appears blue during the day. However, longer wavelengths like red light are less affected and can pass through the atmosphere more directly.
During a lunar eclipse, this filtration process allows more red light to bend around Earth, passing into its shadow.
Shorter wavelengths like blue and violet light scatter more easily, causing them to spread in many directions. That's why the sky appears blue during the day. However, longer wavelengths like red light are less affected and can pass through the atmosphere more directly.
During a lunar eclipse, this filtration process allows more red light to bend around Earth, passing into its shadow.
red light refraction
Besides scattering, Earth's atmosphere also bends light, a process called refraction. As sunlight passes through Earth's atmosphere, the light bends, and red light refraction plays a significant role during a lunar eclipse.
Because red light bends around Earth more easily, it reaches the Moon even when it is in Earth's umbra. This is why observers on Earth see the Moon turn a reddish hue during the total eclipse. The bending of red light is also why we sometimes refer to an eclipsed Moon as a 'Blood Moon'.
To sum up, refraction ensures that even when direct sunlight is blocked, the Moon is still lit by refracted red light.
Because red light bends around Earth more easily, it reaches the Moon even when it is in Earth's umbra. This is why observers on Earth see the Moon turn a reddish hue during the total eclipse. The bending of red light is also why we sometimes refer to an eclipsed Moon as a 'Blood Moon'.
To sum up, refraction ensures that even when direct sunlight is blocked, the Moon is still lit by refracted red light.
umbra and penumbra
Understanding the umbra and penumbra is crucial to grasping lunar eclipses. The umbra is the inner part of Earth's shadow, where the Sun is completely blocked by Earth. This part of the shadow is responsible for the darkest portion of the eclipse.
The penumbra is the outer part of the shadow, where only a portion of the Sun is obscured. When the Moon passes through this area, it experiences a partial shadow, causing a penumbral eclipse.
During a total lunar eclipse, the Moon moves from the penumbra into the umbra. When it is entirely in the umbra, observers see a noticeable change in the Moon's color and brightness.
The penumbra is the outer part of the shadow, where only a portion of the Sun is obscured. When the Moon passes through this area, it experiences a partial shadow, causing a penumbral eclipse.
During a total lunar eclipse, the Moon moves from the penumbra into the umbra. When it is entirely in the umbra, observers see a noticeable change in the Moon's color and brightness.
total lunar eclipse
A total lunar eclipse occurs when the Moon travels completely through Earth's umbra. This event happens when the Sun, Earth, and Moon are perfectly aligned. Importantly, during a total lunar eclipse, the Moon does not completely disappear.
Instead, Earth's atmosphere filters the sunlight, allowing predominantly red light to pass through and be refracted into the umbra. This makes the Moon appear reddish. The duration of totality, where the Moon is entirely within the umbra, can last for over an hour, providing a spectacular celestial show.
Watching a total lunar eclipse helps us better understand the dynamics between light, shadow, and atmospheric effects.
Instead, Earth's atmosphere filters the sunlight, allowing predominantly red light to pass through and be refracted into the umbra. This makes the Moon appear reddish. The duration of totality, where the Moon is entirely within the umbra, can last for over an hour, providing a spectacular celestial show.
Watching a total lunar eclipse helps us better understand the dynamics between light, shadow, and atmospheric effects.
