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Solar eclipse 2017

What are solar eclipses, why do they occur, and the types of solar eclipses.
On August 21, 2017, the mainland United States will experience an event that hasn’t happened across the entire country in about a century - a total solar eclipse!
A solar eclipse is one of the most spectacular events you can ever see. The sky goes dark, and the stars come out just like it is nighttime. Ancient people who witnessed these events viewed them with reverence and considered them signs from the gods. There are many important dates connected with these events, such as the birth of the prophet Mohammed and the death of Jesus.
People will travel for hundreds of miles to view this phenomenon.

What is a solar eclipse?

As the Earth orbits the Sun, the Moon orbits the Earth. This creates the different phases of the moon.
Sometimes when the three objects line up we get an eclipse. When the Moon is between the Sun and the Earth during the New Moon phase, we can get a solar eclipse (see Figure 1 below).
Figure 1: Orientation of Sun, Moon, and Earth for a solar eclipse. Image from Wikimedia Commons, CC BY-SA 3.0
It seems like we should get a solar eclipse every month, but we don't. That's because the Moon’s path takes it a little bit above and below the Earth-Sun plane. The spots where it crosses are called nodes.
Figure 2: The Moon’s path through the sky is tilted 5 degrees from the ecliptic. The nodes are where the two paths cross. Image from Wikimedia Commons, public domain.
It is only when the Moon passes through one of these nodes while between the Earth and the Sun that we have a solar eclipse.

Types of solar eclipses

There are several different types of solar eclipses, which are determined by how far the Moon is from Earth and how precisely the bodies are aligned.
You might be wondering how the Moon's distance from the Earth changes. The Moon orbits the Earth not in a perfect circle, but in an elliptical path.
This means that sometimes the Moon is closer to the Earth than other times, giving us the following types of eclipses.
  1. Annular - This type of eclipse happens when the Moon is too far from the Earth to completely block out the Sun, (the
    umbra
    tip in figure 1 doesn’t reach the Earth) and we see a ring of the Sun surrounding the Moon. We would see the following:
Figure 3: Annular eclipse image from Wikimedia Commons, CC BY-SA 2.0
  1. Partial eclipse - This is the type we see if we’re in the
    penumbra
    (see figure 1). We would see something like this:
Figure 4: Partial solar eclipse. Image from Wikimedia Commons, CC BY-SA 2.0
  1. Hybrid eclipse - This is the rare type of eclipse that is annular for some people and a total solar eclipse for others, because of the curvature of the Earth.
  2. Total solar eclipse - This is the type that we will have on August 21, 2017. The Moon will be passing between the Earth and Sun at just the right distance that it will block out the entire Sun for those in the path of the umbra. These occur about every 18 months somewhere on the Earth. The band of the eclipse is narrow because the Moon is much smaller than the Sun (by about 400 times!). This type of eclipse only lasts a few minutes at most because it requires an extremely precise geometry. Check out the images below.

Can I look directly at the solar eclipse?

Never look directly at the Sun. Never. NEVER! Doing so will cause blindness. However, you may view the solar eclipse using special solar eclipse glasses or by making a solar eclipse viewer yourself at home. Check for recommendations from NASA.
With your solar eclipse glasses, you can see some pretty cool features. One is called “Bailey’s beads”. As the Moon eclipses the Sun, some rays of sunlight can remain visible because of the rugged lunar topography.
Figure 5: Bailey’s beads image from Flickr, CC BY-NC-SA 2.0
In addition, there is the “diamond ring” effect, which is visible at at the beginning and end of a solar eclipse. The last bits of sunlight passing over the Moon’s edge combined with the faint outer part of the Sun create an effect that looks like a shining diamond ring.
Figure 6: Diamond ring effect. image from Wikimedia Commons, CC BY 2.0
Scientists have learned a lot about the Universe by studying solar eclipses. We know more about the structure of the sun, and also that Einstein’s theory of general relativity was correct!
People who have seen a solar eclipse say you'll forever be changed. If you are lucky enough to see the eclipse, please share your experiences in the comments below!

Learn more

For more information about the scientists mentioned in this article, check out some of our other resources!

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