On Monday, Aug. 21, millions of people in the continental United States will be looking up at the sky to witness a nearly once-in-a-lifetime event. The moon will cross between the sun and the earth, completely obscuring the sun. It will be the first solar eclipse seen in the U.S. since 1979.
During a total solar eclipse, viewers can see the corona, the outside of the sun’s atmosphere, flickering around the dark disk of the moon. During this event, both the sun and the moon appear to be about the same size because, while the sun is 400 times wider than the moon, it is also 400 times farther away from the earth, according to NASA. For an eclipse to occur, the moon has to be in the same spatial plane with the sun and earth, which only happens twice a month. The rarer part is that the moon also has to be new.
On Monday, all of these elements will fall into place in a 70-mile-wide band across the U.S. While the eclipse will only be visible in full following a path between Oregon and South Carolina, Delawareans will be able to witness part of this rare celestial event. Simply looking up to the sky—with proper eclipse-viewing eye protection—will offer views of a partial eclipse.
For safe viewing of the eclipse, gazers should use extreme caution. Staring at the sun with unprotected eyes will lead to irreparable damage. Sunglasses, cameras and counterfeit eclipse glasses will be insufficient for safe viewing. While most NASA-recommended suppliers of eclipse glasses are sold out, there are still alternatives.
Pinhole projection is a popular and low-tech option. You can observe the sun’s unusual shape by watching projected light through a hole in a card, or quickly create a simple projector out of a cardboard box, as shown in this video from NASA. For an indirect viewing option touted by the American Astronomical Society, simply stand with your back to the sun and cross your fingers over one another to see the sun’s crescent shadows on the ground.
For extra-cautious viewing, witness the phenomenon via livestream. Not only is it guaranteed to protect your eyes, but it will also show the full eclipse. NASA, The Weather Channel and CNN are all offering streaming.
If you miss this celestial event, you’ll have to wait seven years to try again: The next solar eclipse will be on April 8, 2024.
A version of this article originally appeared on MainLineToday.com.