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Measuring Darkness During Totality

How dark does the sky get during a total solar eclipse? During the 2017 event, I and others used Sky Quality Meters (SQMs) to quantify the light level. Our values ranged from 12.4 to 13.3 magnitudes per square arc second. The meter readings will be contributed to a project by Globe at Night that asked others to measure how dark it got. The video shows our eclipse experience with three observers calling out the SQM numbers during totality.

Visually I could readily see Venus and a second star to the west of the sun-moon pair, but in the excitement of my inaugural total solar eclipse I didn't spend much time scanning to determine fainter stars. Also, seconds prior to totality I admittedly took some furtive glances at the sun, so my eyes were far from dark adapted.

The week before the big event I had addressed nighttime aspects of the solar eclipse. Days before August 21, I noticed Globe at Night had announced they were officially collecting observations to measure the darkness.

In the video from a site along the North Branch of the Payette River in Lowman, Idaho, you can hear the meter readings being called out by three people in this order:

Chuck: 12.7

Teddy: 13.33

Marty: 12.4

Teddy: 13.14

Teddy: 13.29, 13.21

Teddy: (after third contact) 7.32

Marty later suggested his reading may indicate a brighter sky, for he had aimed his SQM toward the eclipsed sun--my fault for lack of clarity in last minute instructions. I was pointing my SQM overhead near mid-eclipse, with the sun's altitude about 46 degrees. While I didn't monitor Teddy's measurements, he had practiced the night before with the Sky Quality Meter. His final reading (7.32) was taken after the sun had reemerged from behind the moon.

Later in the evening August 21, after everyone had left the site and the commotion of the eclipse had settled down, I waited until the twilight seemed about the equivalent darkness of totality. I then took more SQM readings, which were in the upper twelves, so our group's SQM values seem appropriate. While our SQM numbers don't define how dark it can get during a solar eclipse, they suggest the sky was around 13 magnitudes per square arcsecond in one location near the centerline during the 2017 solar eclipse.

As an aside, may I muddy the water? The SQM is a handheld instrument that measures how many photons land on its sensor. It represents that amount of light in terms of a magnitude X star across every square arcsecond of sky. The smaller the number, the brighter the sky. For example, an SQM reading of 16 ("magnitudes per square arcsecond") would be akin to a light polluted urban night sky, like having a 16th magnitude star across every square arcsecond of sky. A really dark site might have an SQM reading of 21, like having a 21st magnitude star across every arcsecond of sky. FYI, a 21st magnitude star is danged faint, yet a square arcsecond is a danged small area of sky.

During totality at our site in the Sawtooth Mountains, twilight encircled the horizon, with Venus and the brightest of stars visible. An SQM reading around 13 is plausible. Once the sun reemerged, Teddy's daylight SQM reading was 7.32. Oftentimes the SQM won't even give a readout if the light is too bright.

[Note: for another experiment we did during the 2017 eclipse, see]

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