On Aug. 21, millions of people across the United States will have the opportunity to view a total solar eclipse, an astronomical event that has not happened in St. Louis since 1442.

The sun will disappear and temperatures will decrease, all while revealing streamers of light through the sky around the moon’s silhouette. At 1:06 p.m. the eclipse will enter the state of Missouri from the northwest. The viewing duration will depend on the location. Thirteen minutes later the eclipse will touch Illinois.

What can be a beautiful experience may turn into a dangerous moment if safety is not taken into consideration.

NASA advises those planning to view the eclipse to take proper safety measures while understanding the different periods in which the eclipse is safe to look at without special viewing glasses.

To safely view the eclipse, people must wear special viewing glasses that protect the eyes from the intense rays of the sun during the eclipse. As the moon’s movement covers the sun, a bright spot will appear, looking like a giant diamond ring.

“It is still not safe to look at the sun at this point. Only when that bright spot completely disappears can you safely look at the sun,” NASA’s website says.

Viewers may take their glasses off during the short time when the moon entirely covers the sun, also known as the period of totality.

“It is safe to look directly at the star, but it’s crucial that you know when to take off and put back on your glasses,” NASA’s website said.

Emily Pike, an optometrist in St. Louis and public relations committee chair for the Missouri Optometric Association, said looking at an eclipse at the wrong time can cause serious damage to the eyes.

“The intense brightness of the sun and the radiation from UV rays can damage the retina tissue,” Pike said. “If you are staring at the eclipse at the wrong moment and for too long you can actually cause permanent vision loss.”

Pike said one of the consequences of viewing the eclipse at the wrong moment is solar retinopathy. Patients with this eye injury can suffer from blurred vision, an alteration in the field of vision and headaches.

Pike’s advice is to wear a pair of solar eclipse glasses or create alternative pinhole projectors.

Those hoping to capture the beauty of the eclipse with their cameras and cellphones should take safety precautions before using their devices.

Cellphone users should consider taking their photos during the period where the sun is entirely covered by the moon and not before or after as there could be damage to the phone’s camera.

Camera owners may take photos before and after the period of totality with a special solar camera filter that will protect their equipment from any damage.

For more information about the spectacle, the Solar Eclipse Expo will be held on Saturday at Queeny Park, with local and national eclipse experts in attendance.