How Dark Does It Get During a Total Solar Eclipse?

Before I experienced my first total solar eclipse more than 30 years ago, I used to tell people that the difference between a total eclipse and a partial one was literally the difference between night and day. But once I spent a few minutes in the Moon’s umbral shadow, I realized I was mistaken. It’s more like the difference between twilight and daylight.

What caused me to think otherwise? I’d heard the “night and day” comparison from other astronomers –– probably ones who also hadn’t yet seen a total solar eclipse. I’d also heard that totality is about as bright as the full Moon and that stars and planets become visible. I misinterpreted those statements as implying that totality is about as dark as a night when the Moon is full. I’m sure I also was fooled by the many close-ups of totality I’d seen that show the eclipsed Sun in a black sky.

Now that I’ve viewed and photographed totality myself many times, I understand that it’s the solar corona, not the environment, that’s as bright as the full Moon and that the black sky in photographs is an artifact of the short exposures needed to capture detail in the bright inner corona. Such exposures are too short to capture the fainter blue light of the surrounding sky.

Eclipse Darkness by the Numbers

So, how dark does it get during totality? Let’s imagine what happens at mid-latitudes when a total solar eclipse occurs around noon on an average sunny day. In such a situation, the altitude of the Sun doesn’t change all that much across all phases of the eclipse. Before the Moon takes its first bite out of the Sun, the illuminance measured looking straight up is in the neighborhood of 100,000 lux (1 lux = 1 lumen per square meter; an ordinary household light bulb emits about 1,000 lumens). I’ve confirmed this with my own measurements outside using a professional-quality light meter.

Until the Sun is at least three-quarters obscured by the Moon, even though the light level has dropped to a quarter of its pre-eclipse value, you might not notice much change in the ambient daylight. Why not? Because over the hour or so that the partial eclipse has been progressing, your pupils have been slowly dilating to compensate for the decreasing illumination. A typical adult’s pupils will double in diameter from about 1 to 2 millimeters during this time, increasing their light-gathering power by a factor of 4.

As the Moon continues its advance and the solar crescent dwindles to a sliver, you notice not only the fading light, but also a change in its color. This occurs because of solar limb darkening. Compared with light from the middle of the Sun, light coming our way from the Sun’s limb, or edge, originates in the higher, cooler layers of the photosphere (the visible surface of the Sun). This light peaks at slightly longer wavelengths than the overall white light of the Sun. Many observers describe the color of the sky during a deep partial eclipse as “silvery,” while others perceive it as slightly purple.

When the Sun is 99% obscured, the illuminance at the zenith has dropped to 1,000 lux. Perhaps surprisingly, this is only about as dark as an overcast day –– still quite obviously daytime.

As the advancing Moon increases the solar obscuration to 99.9%, then 99.99%, and ultimately 100% –– which takes barely a minute, too fast for your pupils to accommodate in real time –– the light fades by another two-plus orders of magnitude. But the illuminance doesn’t go to all the way to 0 lux. Not only is there the glow of the solar corona, there’s also still daylight coming from beyond the umbra (the shadow cast by the Moon), where the landscape remains lit by the partially eclipsed Sun.

Measurements made at recent total eclipses put the illuminance at totality around 5 lux, comparable to civil twilight. The sky is still some shade of silvery, purply blue. In contrast, the black sky on a night of full Moon is 10 times darker still, less than 0.5 lux. Yes, bright planets –– especially Venus and Jupiter –– are obvious in twilight, but stars? I don’t like to waste valuable time during totality looking for stars; the only time I saw one was on August 21, 2017, when 1st-magnitude Regulus glimmered just to the left of the totally eclipsed Sun.

It certainly feels like it gets as dark as night during the final minute before totality, but that’s just because your eyes haven’t had time to adjust. The changes in illumination at the beginning and end of totality happen much faster and more dramatically than at dusk and dawn, respectively.

What Matters Most

Of course, the difference between a 99% partial solar eclipse and a total one is about a lot more than the change in illumination. Only at 100% –– totality –– can you gaze in wonder at the solar corona, marvel at hot-pink prominences erupting from the Sun’s limb, and enjoy pastel sunset/sunrise colors all around the horizon. Yes, you might spot Venus and/or shadow bands during 90-plus percent partial eclipse, and you may see Baily’s beads wink on and off and perhaps glimpse an arc of chromosphere at 99-plus percent, but why would you settle for that when moving into the path of totality can provide the most spectacular and awe-inspiring cosmic experience available on this planet?

The difference between a total solar eclipse and a partial one is not literally the difference between night and day, but figuratively, I’d argue it’s way more than that.

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Rick Fienberg served as Sky & Telescope’s Editor in Chief from 2000 to 2008, then spent 12 years as Press Officer of the American Astronomical Society, S&T’s publisher since 2019. On April 8, 2024, he’ll experience his 15th total solar eclipse.