The Moon

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"Dead leaves crunch underfoot. Something rustles ahead. You are soon far from the path and deep in the undergrowth. You turn a corner and find the trees ahead coated in silver. Hanging in the sky above is the moon, fierce and bright."

The Moon is the sole natural satellite of the Earth. Its moonlight can show visions of an alternate version of London known as the Upstairs.

High in the Night Sky

"Even by night, the Port of Naples is alive with shouts and laughter and work-songs, the roar of engines. Brick warehouses loom over the moon-dazzled bay."

Moon-pearls.

The Moon is an enormous celestial body that orbits the Earth. It's well known for illuminating the night sky of the Surface, and it periodically cycles through various phases, which in some cultures are used to determine the passing of months. These phases are reflected in mysterious trinkets called moon-pearls; how exactly moon-pearls work is unknown, but they're precise enough to function as timekeeping devices in Fallen London and the Unterzee. Moonlight is reflected sunlight, and it can reveal strange visions of alternate timelines, most notably an alternate version of London that never fell to the Neath, known as the Upstairs.

The Moon is the "sister-self" of the Creditor, a being of earth that forms the ground of the Hinterlands. In the past, the Creditor and the Moon were once a singular being that served a Judgement, likely the Sun. However, they were separated violently in a "cleaving birth" - an event now referred to as the Cataclysm of Silver. The Creditor's inner "flesh" strongly resembles the Moon, and it too has a strange effect on sunlight as well as on moon-pearls.

Moonlight

"The floor is silvered with spilling moonlight. It makes a ghost of your flesh and your eyes a lens to elsewhere."

A Mirrorcatch Box.

Moonlight in the Neath is not seen from the sky; rather, it is an illegal commodity smuggled in from the Surface. Moonlight has a unique, silvery appearance, and it is transported in devices called Mirrorcatch Boxes that use an array of mirrors to capture moonlight within. The woods of Balmoral are bathed in perpetual moonlight, as per the Empress' demand. Exposure to moonlight in the Neath is considered risky, however, as moonlight can have bizarre, sometimes maddening, effects on those exposed to it.

Moonlight also has a tendency to grant visions, affecting multiple senses, of an alternate timeline where certain events either did not occur or occurred differently. These hallucinations are based on location; for instance, moonlight released in Ladybones Road would depict Marylebone (specifically near the Marble Arch) in a version of London on the Surface. Moonlight visions are also possible outside of London, such as within the Marigold Station in the Hinterlands.

The divergences of moonlight visions from reality are generally significant; it is not made clear whether or not they are a manifestation of any particular being's ideals or goals. The Upstairs is the collective term for the imagined version of London that never Fell and remained on the Surface; this version of London also underwent a political upheaval by revolutionaries, as well as other divergences from the known timeline. The use of moonlight at Marigold Station, meanwhile, conjures a world where the Grand Devils were never defeated and deposed as rulers of Hell.

For reasons currently unknown, moonlight may have a reduced effect near the City in Silver. When this is the case, more distant locations like London will continue to show notable divergences in history. Even under these circumstances, however, some things may remain the same.

The Moonlight Trade

"London is home to an illicit, lucrative trade in Surface sunlight. Its quality varies, as can its effects. Be careful of sunshine laced with moonlight."

A Sun-Stamped Mirrorcatch Box.

London is home to a expansive sunlight trade that is as lucrative as it is illegal. However, sunlight often must be harvested directly from the Surface, which can be both dangerous and expensive. Thus, using a method pioneered by the Cheery Man, sunlight traders may "cut" their sunlight with moonlight, which can have varying effects on the product. Sunlight cut with moonlight is cheaper to import, can last longer in the box, and is safer to transport as well. Relying on this method can be risky, however, as customers who purchase such sunlight may still feel the effects of moonlight, visions and all.

Certain charlatans and false-spiritualists utilize raw moonlight in their trickery to deceive their marks, as the settings and people seen in moonlight visions can easily be mistaken for those of the realm of the deceased.

Lunacy

"Your friend has a bad case of the Upstairs, lads. Harmless enough now, at least for you. But what happens when he starts thinking you're all Constables? Or murderers?"

"He's gone all the way Upstairs, this one."

The Upstairs

Besides causing visions, exposure to moonlight can cause other, sometimes dangerous, side effects. Objects in moonlight visions are palpable, but are not truly real or tangible; thus, one cannot drown in water seen only in moonlight visions, and objects perceived by a hallucinating individual will not be seen by outsiders, and vice versa. Moonlight visions can also be difficult to "see through", and can cause extreme disorientation if the individual attempts to view or interact with the real world with any sort of clarity.

Furthermore, moonlight exposure can induce false memories, and sometimes, the individual may be unable to distinguish the two worlds at all. This can result in a similar effect to being honey-mazed, and chronic exposure may make this ailment seem permanent. Finally, like sunlight, moonlight can be lethal in extremely large quantities. Those who overdose on moonlight may die with their eyes wide open, their gaze appearing burnt, but perhaps also revelatory.

Scientific Inspirations

While there may be other potential explanations, the Cataclysm of Silver fits into the giant-impact hypothesis. This hypothesis, which is currently the most thoroughly evidenced theory for the formation of the Moon, poses that an ancient planet called Theia collided with Earth, and the debris accumulated to form the Moon. A more recent investigation indicates that the remains of Theia are present on both Earth and the Moon.