The total eclipse will only be visible to residents in the western half of the United States (as well as people throughout the Pacific, East Asia, and Oceania). It starts right at 4:58 am Pacific and will last just five minutes. The total eclipse will only be visible to residents in the western half of the United States (as well as people throughout the Pacific, East Asia, and Oceania). It starts right at 4:58 am Pacific and will last just five minutes.
1) What is a lunar eclipse?
Normally, light from the sun directly hits the moon, which is why the moon is illuminated at night. Every so often, though, the sun, Earth, and moon align. When this happens, our planet blocks the sun’s light, causing an eclipse.
When only part of the moon enters the core part of the Earth’s shadow (the umbra), just part of it darkens. This is a partial eclipse, and it’s what US residents on the East Coast will be able to see if they wake up early.
When the entire moon enters the umbra, nearly all the sunlight is blocked from hitting it. This is called totality, and it’s during this stage that the darkened moon will glow red — which is why lunar eclipses are sometimes called blood moons.
2) Why will the moon turn red during the eclipse?
When the sun, Earth, and moon are aligned perfectly, not all of the sun’s light will be completely blocked out by Earth — slight amounts of it will pass through Earth’s atmosphere and then hit the moon.
As the light passes through the atmosphere, some of it reflects off nitrogen and oxygen molecules and bounces away. Longer wavelengths of light are more likely to make it through without being bounced away and arrive at the moon, and red is the longest wave, so the moon ends up looking reddish. (This mechanism is the same reason sunrises and sunsets look red — basically, the more light is filtered through our atmosphere, the redder it gets.)
3) How common are these eclipses?
Because they depend on a precise alignment of the sun, Earth, and moon, eclipses happen in sporadic yet predictable sets of four closely timed eclipses, called tetrads. This is the third eclipse in a tetrad — the previous one was during October 2014 — and it’ll be followed by another eclipse on September 28, 2015. This particular tetrad is pretty special, as all four of its eclipses are total eclipses. (During partial or penumbral eclipses, which are more common, only the earlier, less-cool stages of the eclipse occur — there is no total blockage of the moon, and no eerie red glow.)
As it happens, the 21st century as a whole is going to feature eight tetrads (that’s 32 eclipses), an unusually high number. So in terms of lunar eclipses, this is a pretty good time to be alive.
Source : PTI