For complete coverage of the Eclipse of the Century go to cnn.com/eclipse. Watch live, in virtual reality, as the eclipse moves coast to coast on August 21st.

(CNN) The solar eclipse, which will happen August 21, only happens about once every 375 years, according to NASA.

If your trips to the library are similarly infrequent, you may want to change that.

More than 2 million pairs of eclipse glasses, which allow you to watch the solar eclipse without damaging your eyes, are being distributed free at public libraries across the country.

During a total solar eclipse, the moon and the sun both appear to be about the same size from the ground.

According to NASA, this is a "celestial coincidence," as the sun is about 400 times wider than the moon and about 400 times farther away.

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