Faculty Voice: Watching the total eclipse — when 99.9% just isn’t good enough

APRIL 6, 2024 — Editor’s note: The San Antonio Express-News ran the following op-ed online on (March 10) and in print on (March 11).

The best possible place to view a total solar eclipse is along the path of totality. Watching it anywhere outside the path won’t do it justice. Even when the sun is 99% eclipsed, it isn’t enough. It’s still considered a partial eclipse.

Here’s why.

The path of totality, or PoT, is the strip of the surface of the Earth where the sun will be completely blocked out by the moon. Anyone outside the path of totality does not see a total eclipse. A large area parallel to the PoT will get a partial eclipse, with the locations closest to the PoT getting the most eclipsed sun. But it’s still not 100%. 

“To experience the sudden plunge into deep twilight, followed by the sudden return of daylight a few minutes later and all the other awesome wonders of a total solar eclipse, get into the path of totality.”

The sun’s faint atmosphere, known as the corona, is always present. The brightness of the light emitting from the sun’s main disk often hides this faint atmosphere. The uneclipsed sun is 100,000 to a million times brighter than the corona. 

Imagine being in a dark room, looking at a phone. When the brightness is low, the screen is visible just fine. However, outside in bright sunlight, a phone’s screen brightness must increase for anything to be seen. And even then, it’s hard to see what's on the screen. If the automatic screen brightness adjustment is disabled, the screen isn’t visible until the brightness is adjusted.

Viewers witnessing an eclipse experience a similar effect when they look at the corona, which is significantly fainter than the main disc of the sun. 

Consider that when 99% of the sun is eclipsed, 1% of the sun is still showing. The sun is 100,000 times brighter than the corona, so even that 1% sliver of sun is at least 1,000 times brighter than the corona.

Similarly, if the sun is at 99.9% partial eclipse, 1/1,000 of the sun is still visible. That’s still 100 times brighter than the corona. That’s still too much light to experience the glory of a total solar eclipse. Outside the path of totality — even at 99.9% partial eclipse, it will NOT get dark. 

Those who witness the full awesomeness of a total solar eclipse within the PoT are in for a treat. Here are a few things PoT viewers will experience firsthand:

• Temperature and wind changes.
• Behavioral changes in animals.
• Shadow bands.
• Dramatic changes in brightness and darkness.
• Baily’s beads (beads of sunlight caused by the moon’s topography right before totality).
• Diamond ring effects.
• Chromosphere and prominences.
• The main attraction: the corona.
• Sunset colors around the horizon.
• Bright stars and planets.

To experience the sudden plunge into deep twilight, followed by the sudden return of daylight a few minutes later and all the other awesome wonders of a total solar eclipse, get into the path of totality.

Visit the San Antonio Express-News’ interactive online map to view the path of totality for the total solar eclipse on April 8, and make plans now to truly experience this amazing event.

— Angela Speck