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Why Does the Moon Turn Red During a Lunar Eclipse?
On September 28, 2015, the Moon will turn a shade of red in many parts of the World as the Earth moves between the Sun and the Moon, casting its shadow on the Moon's surface.
A totally eclipsed Moon can turn a shade of red.©bigstockphoto.com/ prajit48
Earth Casts its Shadow
The Moon does not have any light of its own – it shines because its surface reflects sunlight. During a total lunar eclipse, the Earth moves between the Sun and the Moon and cuts off the Moon's light supply. When this happens, the surface of the Moon takes on a reddish glow instead of going completely dark.
The red color of a totally eclipsed Moon has prompted many people in recent years to refer to total lunar eclipses as Blood Moons.
Why Red?
The reason why the Moon takes on a reddish color during totality is a phenomenon calledRayleigh scattering. It is the same mechanism responsible for causing colorful sunrises and sunsets and the sky to look blue.
Colorful Sunlight
We all know from our school science textbooks that even though sunlight may look white to human eyes, it is actually composed of different colors. These colors are visible through a prism or in a rainbow. Colors towards the red spectrum have longer wavelengths and lower frequencies compared to colors towards the violet spectrum which have shorter wavelengths and higher frequencies.
Earth's Atmosphere
The next piece in the puzzle of why the sky looks blue and a totally eclipsed Moon turns red is the Earth's atmosphere. The atmosphere is about 300 miles (480 kilometers) thick and is made up of different gases, water droplets and dust particles.
When sunlight entering the Earth's atmosphere strikes the particles that are smaller than the light's wavelength, it gets scattered. Not all colors in the light spectrum, however, get equally scattered. Colors with shorter wavelengths, for example those towards the violet spectrum, are scattered more strongly than those with longer wavelengths like red and orange, which pass through the atmosphere. This light then gets bent or refracted back to the surface of the Moon and gives it the reddish-orange glow that total lunar eclipses are famous for.
Light blue band visible on the Moon's surface.©bigstockphoto.com/Medardus
Look for a Blue Band
Veteran eclipse watchers will tell you that if you look really hard right at the beginning and just before the end of totality, you may detect a light blue or turquoise band on the Moon's face. This happens because the Earth's Ozone Layer scatters red light and lets through some of the blue light that gets refracted to the Moon.
Many Shades of Red
The Moon can take on different shades of red, orange or gold during a total lunar eclipse, depending on the conditions of the Earth's atmosphere at the time of the Eclipse. The amount of dust particles, water droplets, clouds and mist can all have an effect on the shade of red. Volcanic ash and dust in the atmosphere can also lead to the Moon turning dark during an eclipse.
Fun fact: If you were lucky enough to see a total lunar eclipse from the Moon, you'd see a red ring around the Earth. In effect, you'll be seeing all the sunrises and sunsets taking place at that specific moment on Earth!
The Danjon Scale was created by French astronomer, André-Louis Danjon.
Measuring the Eclipsed Moon's Brightness
Scientists measure the appearance and brightness of a total lunar eclipse using a five point scale – ranging from 0 to 4 – called the Danjon Scale. A lunar eclipse is ranked and assigned a value on the Scale at the time of mid totality.
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