Early morning on April 15th this year over North America, there is predicted to be an observable total Lunar eclipse over North America and some parts of South America. A total lunar eclipse occurs when the moon passes behind the earth’s ‘shadow’, (more specifically it passes through the Earth’s umbra) and the earth sits directly between the sun and the moon.
Image taken from Wikipedia.org
The terms umbra and penumbra are terms for two different parts of a shadow. A shadow is produced when an opaque object, such as the Earth, intrudes the path of light coming from a light source (the Sun). The umbra is the region directly behind the object that completely blocks out the light source, whereas the penumbra is the region where only part of the light source is blocked out by the opaque object. A partial lunar eclipse is defined as an eclipse where the moon passes just into the penumbra region where it is partially blocked out by the sun and then continues to leave the shadow of the earth and becomes illuminated to its normal white color (the moon shines white because it reflects the sun rays of its surface). A total lunar eclipse still passes into the penumbra, but then continues into the Earth’s umbra where it is completely blocked of direct sunlight.
What makes a total lunar eclipse so stunning is that the moon turns a red color while it is in the Earth’s umbra, which is actually a very puzzling effect. One would think that when the moon passes into the Earth’s umbra, no direct sunlight could illuminate the moon, thus it should simply turn black, and be a dark spot in the sky. Instead, the moon glows red, and this surprisingly has to do with the way sunlight refracts and scatters in the earth’s atmosphere.
Images courtesy of NASA.gov
Anyone whose watched a sunset or sunrise knows of the brilliant colors of pink, red, and orange that streak across the once blue sky. These reddish colors we see at sunset essentially are generated by the green to violet portions of the sun’s rays being filtered out by the earth’s atmostphere more. Opacity for a given fluid, such as our atmosphere, is specific to wavelength. Shorter wavelengths, such as green, blue and violet in the visible spectrum, are scattered more (spread out uniformly) than red, yellow, and orange when the sun rays hits the atmosphere, thus making the fluid have that opaque color. The result is that during the day we view the whole sky as blue, because it is scattered the most thus the atmosphere overhead is opaquely blue. At sunset and sunrise, when the sun is lowest in the horizon, it’s rays have the longest path through atmosphere to your eyes, thus all the light that reaches you at sunset, has been filtered down to be the lower end wavelengths, thus producing the red and orange sunsets we love.
Image from Sky and Telescope Magazine
The result of this, however, is that light from the sunset, continues in a straight path, leaving the atmosphere back into space, thus producing a ring of reddish glow around the earth where the atmosphere is, from the perspective of an observer in the umbra. Thus, when the moon enters the umbra during a total lunar eclipse, an astronaut on the moon would observe this red ring. Because of the reflectivity of lunar soil, this red light from the atmosphere is reflected off the moon during the eclipse and illuminates it as red to our eyes.
I look forward to seeing such a fantastical phenomenon in the coming weeks. Besides this eclipse, 2014 is a tetrad for lunar eclipses, meaning that there will be 3 more lunar eclipses besides this one, totaling to four for the year. Such eclipse tetrads are uncommon, and there hasn’t been one for 10 years, and there won’t be another until 2032.
Sources:
http://earthsky.org/space/why-does-the-moon-look-red-during-a-total-lunar-eclipse
http://spaceplace.nasa.gov/blue-sky/en/
http://en.wikipedia.org/wiki/Umbra
http://en.wikipedia.org/wiki/Lunar_eclipse
