Why you can’t see the Moon during a total solar eclipse

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The new Moon is brighter than every star in the sky. But you’ll never see it during an eclipse.

“Even though the reason for taking the photographs was science, the result shows the enormous beauty of nature.” –Miloslav Druckmuller, eclipse photographer

The moments of totality during an eclipse are something rare and special for anyone who gets to experience it. They’re only possible when the Sun, Earth and Moon perfectly align at a moment when the Moon is close enough to block the entire Sun’s disk, from an Earth-bound observer’s perspective. As the shadow of the Moon falls on the Earth, the sky darkens, and stars, planets, and the Sun’s corona appear visible to human eyes during the day. Yet to those of you who’ve seen a thin crescent Moon before, one sight is missing: the face of the Moon itself, normally illuminated by Earthshine. To someone who’s viewed the Moon routinely, it may come as a baffling surprise.

A thin crescent moon, just one day after the new moon, sets in the west. The remaining disk is still illuminated by the light reflected from Earth that’s then incident upon the lunar surface. Image credit: Neal Simpson of flickr.

The Moon is very close to a perfect sphere in shape, and like all spheres (including Earth), precisely 50% of it is illuminated by the Sun at any given time. When the Moon is very closely aligned with the Sun, it appears in a “new” or “almost new” (crescent) phase, as the hemisphere that’s lit up mostly faces away from Earth. But you can still see the non-illuminated portion thanks to the phenomenon of Earthshine. Even when direct sunlight doesn’t fall on the Moon, light from the Earth does.

The Moon takes a little over 27 days to orbit 360º around Earth, and a little over 29 days to go from new Moon to new Moon again. As the phases cycle, the portion of the Moon illuminated by the Sun — and the amount of Earthshine the Moon receives — change. Image credit: Wikimedia Commons user Orion 8.

Imagine the Earth as though it were a mirror. It may be a fairly lousymirror, reflecting only about 30% of the incoming sunlight, but it’s also large enough that a significant portion of the sunlight reflected by the Earth winds up falling onto the Moon. If you stood on the surface of the Moon during its “new Moon” phase and looked towards the Earth, you’d see an object that appeared to be 1/10,000th the brightness of the Sun.

The Earth, as seen rising over the lunar limb in a location where the Sun is just barely incident on the Moon’s surface. Image credit: Japan Aerospace Exploration Agency, JAXA / NHK, Kaguya (Selene).

This light would be more than 40 times brighter than the full Moon appears from Earth. The light that the Earth reflects onto the Moon is so great that even during a perfectly new phase, even during a total solar eclipse, the Moon is still 150% brighter than Sirius, the brightest star in the entire night sky. At the maximal moment of a total solar eclipse, dozens (if not hundreds) of stars are visible to the naked eye, all of which are are fainter than the new Moon.

There will be plenty to see all around you during totality, from the light around the horizon to the Sun’s changing corona to stars in the daytime sky. But the Moon’s face will not be one of those sights. Image credit: Luc Jamet.

But despite all that you can see, the face of the new Moon isn’t visible to human eyes, even during a total eclipse. In fact, only if you take quite a long-exposure photograph with a camera can you see anything other than total blackness where the Moon resides in the sky. Yes, the light is really there, but your eyes are completely unable to perceive it, even when the sky gets so dark that objects hundreds of times fainter can be seen.

The sights of the eclipse are spectacular during totality, but even though stars and the corona are visible, the face of the Moon is not. Image credit: Beawiharta/Reuters.

Why is that? The same reason you can’t see stars at night if they’re located too close to the full Moon: there’s something else that’s much brighter than the new Moon right by it. That’s the Sun’s corona! At temperatures of over 1,000,000 degrees, the corona is never visible when there isn’t an eclipse, due to the fact that the Sun itself is shining right next to it. But the Sun’s corona is approximately as intrinsically bright as the full Moon; if it weren’t located so close to the Sun itself, you’d be able to see it even during the day.

The Sun’s atmosphere is not confined to the photosphere or even the corona, but rather extends out for millions of miles in space, even under non-flare or ejection conditions. Thanks to the masking technology of the coronagraph, we can view it from either Earth or space. Image credit: NASA’s Solar Terrestrial Relations Observatory.

Astronomically, if you were to build a device that blocked out the disk of the Sun but not the corona beyond it, you can view that hot, outer atmosphere of the Sun during the day. Such a device is called a coronagraph, and astronomers have used them since the 1930s to view the solar corona. Without one, however, the brightness of the Sun overwhelms the brightness of the corona; it’s 400,000 times brighter and too close in proximity for you to view it. The situation is very analogous to what happens during totality between the corona and the new Moon.

During partial phases of the eclipse, the Moon is very clearly not visible when compared to the much-brighter Sun. But even the fainter corona drastically outshines the Moon. Image credit: Matt Hecht.

Yes, there is Earthshine illuminating the face of the Moon. And yes, the new Moon’s face is 150% brighter than even the brightest star in the sky. But the corona is 10,000 times brighter than the new Moon, and is located only a quarter-of-a-degree from even the new Moon’s center. Your eyes may be sensitive enough to see something as bright as the new Moon, but not if something 10,000 times brighter is that close to it. If you stood 20 feet (6 meters) away from a 25 Watt light bulb, you couldn’t see a firefly light up one inch (2.5 cm) from the bulb, which is to take nothing away from the firefly.

Fireflies may give off light and put on a spectacular show, but at only 0.025 lumens apiece, they need to be far away from any other, much brighter light source to be seen. Image credit: Otto Phokus / Flickr.

Proximity matters, and in the case of the new Moon, it’s the solar corona that washes it out. The Moon is still illuminated, and that’s why a camera can pick up the details of its face. But the same corona that provides such spectacular sights is also what makes it so difficult to see anything around it. This applies not only to the full Moon, but to the stars closest to the Sun’s limb itself: essential for testing Einstein’s relativity!

32 images of the 2016 eclipse were combined in order to produce this composite, showcasing not only the corona and the plasma loops above the photosphere with stars in the background, but also with the Moon’s surface illuminated by Earthshine. The illuminated new Moon, with the Sun’s corona surrounding it, cannot be seen with the naked eye alone. Image credit: Don Sabers, Ron Royer, Miloslav Druckmuller.

There are some astronomical observations, even with a powerful telescope, that can only be made with the help of photography. To gather enough light to stand out against the brightness of other objects is something that goes beyond what human eyes can deliver. Thankfully, that technology is widespread today, and enables us to enjoy a whole slew of sights our eyes cannot deliver. It may be dark during the eclipse, but darkness, as we perceive it, is relative. Our Sun’s corona, during totality, will become the brightest thing in the sky, and is the reason the Moon, to human eyes, will be completely invisible.

The pink ‘fringe’ around the circumference of the Moon is actually due to plasma loops rising above the photosphere of the Sun. This hot plasma links up with the corona, eventually, and extends great distances into space. The corona itself is 10,000 brighter than the new Moon. Image credit: Upice observatory, Petr Horalek, Jan Sladecek, Miloslav Druckmuller.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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