Peak of Eternal Light


Today is the first full day of Autumn 2022 in the Northern Hemisphere. The fall equinox means we are currently experiencing nearly identical lengths of night and day.

We have “day” and “night” because the planet spins on its axis. Earth rotates once every 24 hours with respect to the sun. The figure is a bit shorter – 23 hours, 56 minutes, and 4 seconds – with respect to distant stars. The difference arises because the planet orbits the sun, moving about one degree around the star each day. For the sun to appear in the same spot in the sky, from noon today to noon tomorrow, for example, the planet needs to rotate an extra degree. That degree takes 3 minutes and 56 seconds. Our timetables reasonably arose from the most important body in our lives, the sun, though one could argue the more accurate measure of the planet’s rotation is the one in relation to the distant stars, known as a sidereal day.

We have ruminated multiple times over the years about the changing of the seasons. The reason we experience winter, spring, summer, and fall on the planet stems from its axial tilt. If Earth “stood straight up” as it circled the sun, seasons would not exist. In reality, the Northern Hemisphere tilts toward the sun between the March and September Equinoxes, which provides us with more sunlight, bringing spring and summer. Between September and March, the Northern Hemisphere points away from the sun, the days are shorter, and we live through autumn and winter.

A graphic on Earth's axial tilt by Dennis Nilsson

But our concepts of “night” and “day” are not as steadfast as we might think.

The technical definition of a “day” is, indeed, the time it takes the planet to rotate one time. For most of the world, that period includes a session of daylight and a session of darkness. However, we’ve learned that’s not always the case in every spot across the globe. Because of the axial tilt of the planet, above the Arctic Circle and below the Antarctic Circle, neverending daylight or neverending darkness can comprise entire 24-hour days, depending on the season. During summers, these locations see nothing but the sun; during winters, they experience nothing but the dark side. 

In 1837, two German astronomers pondered a hypothetical. If a celestial body rotated but had no axial tilt in relation to its star, would the poles bathe in constant sunlight? Definitively! However, we currently do not know of any orbs with zero axial tilt. The question then became: can a spot sit in constant sunlight if it is not at a pole? The astronomers, Johann Heinrich von Mädler and Wilhelm Beer, believed these locations could exist.

Such a spot was later dubbed a “Peak of Eternal Light.”

On a spinning planet or minor body, a peak of eternal light would require a few attributes. First, the body would need to have an extraordinarily low axial tilt, making it nearly upright in relation to a star. Second, though it need not be on the pole, the spot would require a high latitude (i.e. near a pole). Lastly, this locale would need to be elevated, hence the mountainous nomenclature.

We actually don’t have to go (relatively) far to search for peaks of eternal light. Though Earth’s tilt is rather pronounced, our satellite’s obliquity is not. The Moon tilts just 1.5 degrees with respect to the ecliptic! In fact, Beer and Madler were convinced we would one day verify that mountain peaks near the lunar poles would actually always sit in the sunlight.

Alas, we have made little headway in the verification of these peaks in the past 200 years. Our relationship with the moon makes observation difficult. Because our satellite is tidally locked, we can only see one side with ground-based telescopes. This fact means we can only spy the poles from the side, which has the effect of flattening any imagery. Not until the past half century, when we sent artificial satellites to the Moon have we been able to map the poles and the dark side of our companinon.

The topography of the Moon's South Pole - NASA/Goddard Space Flight Center

Several lunar missions identified a few spots on the poles that might receive persistent sunlight. Clementine found four candidate sites along the rim of the Peary crater near the North Pole. After the Lunar Reconnaissance Orbiter mapped our satellite in high definition, spots along the rim of the Shackleton crater emerged as potential eternal peaks near the South Pole (see image above).

Unfortunately, to date, we do not believe these zones hit 100%. The Peary crater points remain in light over 89% of the lunar year. The Shackleton areas are far more interesting, however. They find themselves bathed in sunlight a whopping 94% of the time! And, when it comes to peaks of eternal light, these points on the crater rim are the victims of topography. Scientists believe it is highly likely that the darkness these spots experience stems not from the tilting of the Moon, but from shadows cast by other points on the rim and other lunar peaks!

Other than a quirk of geography, what does it matter if peaks of eternal light – or close approximations – exist?

Image of lunar peaks taken by SELENE space probe - edited by Paul D. Spudis

Though we can send probes and limited human missions to the Moon, if we want to establish long-term presences there, we will need to solve the problem of power. As we move into the second quarter of the 21st century, we have started to make real progress in harnessing solar power. Eternal sunlight equates to nearly eternal electricity! These locations could be a boon for space exploration because they would provide a constant source of solar power.

Though a peak of eternal light might not truly exist on the Moon, one could theoretically exist elsewhere in the solar system or universe. Mercury’s axial tilt is just 2 degrees and some scientists believe PELs might reside there. NASA discovered a peak near Mercury’s South Pole that receives sunlight 82% of the time, so a true peak of eternal light might be out there somewhere. 

The sun hits this spot on Mercury 82% of the time - NASA/Johns Hopkins University Applied Physics

Going back to the Moon, we have to become buzzkills.

Even if a peak of eternal light exists on Luna, where the axial tilt of the satellite did not cause it to ever lie in darkness, it would still technically not be a peak of eternal light.

During a lunar eclipse, Earth gobbles up all the sun’s light, leaving the entirety of the Moon dark.

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