On July 27, a big part of Earth’s population will be able to witness a very special event – the longest total lunar eclipse of the 21st century. From the evening of July 27 until July 28, the moon will pass through the center of Earth’s shadow, being left completely without the sunlight. However, our planet’s satellite will not turn black but instead will be colored orange-red.
Why does the moon turn red?
Even though a total solar eclipse and a total lunar eclipse are quite similar, they are completely different when it comes to their appearances. The shadow of the moon during a solar eclipse is colorless due to the fact that Earth’s satellite does not have an atmosphere that could refract or scatter the sunlight. Earth, however, contributes to the colorful appearance of the moon during the total lunar eclipse.
Earth’s atmosphere as a big lens
Because Earth’s atmosphere is full of nitrogen, it captures white sunlight and then scatters blue. This is why the sun is yellow and the sky blue. However, at the time of the sunrise and sunset, the light is being scattered, even more, resulting in an orange or red sun and sky. This is exactly how Earth would look like during the total lunar eclipse if you were some 240,000 miles away on our planet’s satellite.
Moon’s backscattering ability
The orange-red light, while passing through the Earth’s atmosphere, is focused on the moon in a shadow that has a shape of a cone, known as umbra. Additionally, the moon is covered with regolith, an ultra-fine and glassy dust, which has a unique property called “backscatter”. Thanks to this special ability, most of the light bounces back in the same direction that it came from. Therefore, the red light of the moon during the total lunar eclipse is actually the Earth’s sunrise-sunset light that was refracted and bounced back at us.
Where to see and when?
The total lunar eclipse will be visible in Eastern Africa, the Middle East and Central Asia, while the rest of the world, with the exception of North America, will be able to observe a partial lunar eclipse. This event will start at 19:30 UT and will last 1 hour 43 minutes, just 4 minutes away from the longest total lunar eclipse possible.