US total solar eclipse glasses
Solar eclipses should be viewed only with the right kind of glassesReuters

Total solar eclipses will not always remain a celestial event in the way we experience them now. Astronomers say that in about 600 million years, total eclipses will not be possible.

A complete eclipse happens only when "there is an exquisite alignment of the moon and the sun in our sky," said Richard Vondrak, a lunar scientist at NASA's Goddard Space Flight Center.

Total eclipses are also possible only because of the moon's position and location. The moon, however, is slowly moving away at a rate of about an inch each year, according to Nasa.

The sun and the moon have the same apparent size viewed from earth as the sun is 400 times the size of the moon, and the moon is 400 times closer to the earth than the sun, reports New Scientist.

Eventually, the moon will be far enough from the earth and will not be able to entirely block the sun out. Its apparent size in relation to the sun will get smaller and will not be able to cover the sun entirely. "About 600 million years from now, Earth will experience the beauty and drama of a total solar eclipse for the last time," said Vondrak.

It has been theorised that the moon will eventually move far enough away from the earth where the sun's influence will push the moon closer to earth than it is now. This will cause the earth's gravity to cause tidal motions in the moon that will eventually rip it apart. The moon will settle as a ring of debris around the earth much like Saturn's rings, according to Lee Anne Willson of Iowa State University in an interview with Space.com.

Total solar eclipses are not rare. In fact, there is an eclipse happening once every 18 months, says Nasa. A partial eclipse happens at least twice a year. The shadow of the moon is relatively small, and for the eclipse to be seen clearly, it has to happen in the day time.

The same spot getting to see a total eclipse is a rare, once-in-a-lifetime event. The "path of totality" – the regions on earth where the eclipse can be witnessed from – passing by the same region happens around once in 375 years, according to Nasa.

A total solar eclipse will be visible to mainland USA on 21 August this year. It will be visible all the way from Salem, Oregon, to Charleston, South Carolina.