The Greeks placed darkness at the root of everything. Hesiod's Theogony, the oldest map of the divine world, begins with Chaos, the yawning gap that was first of all, and then in order: broad-breasted Gaia the Earth, dim Tartarus beneath her, and Eros, the love that moves gods and men. And from Chaos came two more: Erebus, the darkness below, and Nyx, the darkness above, black Night. No Titan fathered her, no god preceded her. She belongs to the handful of beings who simply emerged, the primordial foundations on which every later generation of divinity rests. This is the deepest difference between Nyx and the familiar gods of Olympus. Artemis may hunt by moonlight and Hecate may haunt the midnight crossroads, but they are goddesses who move about within the night. Nyx is the night: the actual falling dark that ends each day, the starry ceiling that wheels over every city, the medium in which the world was cradled before there was a world at all. The Greeks were not afraid to say it plainly. Before the light, there was the dark, and the dark was a mother.

Her children are a catalogue of the things that come to mortals in the dark, and Hesiod lists them like a litany. With Erebus she bore two luminous children: Aether, the bright upper air where the gods breathe, and Hemera, the Day, proof that even the Greeks' night was fertile with light. Then, alone, by no father's aid, she brought forth the rest, and the list darkens: Moros, the Doom that is appointed to every man; Ker, the violent death-spirit that drags corpses from the battlefield; Thanatos, Death with his heart of iron, and his twin brother Hypnos, Sleep, who alone of the brood is kind to mortals; the tribe of the Oneiroi, the Dreams that slip out by gates of horn and ivory; Momos, fault-finding Blame; Oizys, Misery; the Moirai, the three Fates, Clotho who spins, Lachesis who measures, Atropos who cannot be turned, and with them the Keres, the dooms of cruel death; Nemesis, the retribution that follows pride; Apate, Deceit, and Philotes, tenderness, for night brings lovers together as surely as it brings terrors; Geras, sorrowful Old Age; and Eris, Strife, who would go on to mother all the griefs of the world and to roll the golden apple that started the Trojan War. Beyond the Ocean stream, says Hesiod, her daughters the Hesperides sing as they guard the golden apples at the western edge of the world, where the sun goes down into her country every evening. Sleep and Death are her twin sons. The Greeks looked at their own nightly unmaking and saw a family.

Her house stands at the edge of the world, and Hesiod describes it in one of the strangest and most beautiful passages in the Theogony. Far in the west, near the place where Atlas stands holding up the sky, there is a dwelling wrapped in dark clouds, and before it two figures pass each other for ever across a single threshold. One is Day, white and shining, going out over the earth. The other is Night, black and veiled, coming home from her journey. They never are within the house together; as one crosses the threshold descending, the other passes out and rises over the lands, and they exchange a word of greeting as they pass, mother and daughter sharing one house in shifts, the machinery of time itself kept turning by their endless relay. Behind that house lies the region where the sources and ends of all things stand in fearful order, the neighbourhood of Tartarus, and the Greeks understood that Nyx's country bordered the place where sky and earth and sea have their roots. To approach Night's house is to approach the foundations of the world.

The measure of her power is given in a single astonishing scene in the Iliad. The goddess Hera, plotting against Zeus, begs Hypnos, Sleep, to lull the king of heaven to rest, and Hypnos answers with a memory. Once before, he says, I did this favour for you, when Heracles had sacked Troy and you set the winds on him; and Zeus woke in a rage beyond telling, and strode through Olympus seizing the gods and hurling them about, searching for me to cast me from the sky into the deep sea, to be seen no more. And what saved him? I fled, says Sleep, to Night, and Zeus, for all his anger, held back and did nothing, for he stood in awe of doing aught that would displease swift Night. It is the only moment in Homer where the thunder-god checks his hand before another power. Zeus rules gods and men, but he rules within an order that Night predates; she is not his subject, she is his elder, and somewhere in the old Greek mind she is nearer to the substance of things than any thunderbolt. The Orphic poets took her higher still. The Orphic Hymn to Night addresses her as the mother of gods and men, and in the Orphic theogonies she dwells in a cave at the world's edge where Zeus himself comes to consult her ancient oracle, and comedy confirms her rank when Aristophanes' birds sing that in the beginning were Chaos and Night and black Erebus, and Night laid a wind-egg in the lap of darkness, from which golden-winged Love hatched and made the world.

Her afterlife has been as long as her darkness. Rome received her as Nox, and the Latin poets kept her majesty, black-winged, star-veiled, drawing her chariot across the sky. In Greek art she drives that chariot or flies on dark wings, her robes spangled with stars, sometimes carrying in her arms her twin sons, Sleep the white child and Death the black, an image that haunted sculptors down to the Renaissance and beyond. Science has written her name across the heavens she once was: an asteroid, 3908 Nyx, carries it through the dark between Mars and Jupiter, and one of the moons of distant Pluto, discovered in 2005, was named Nix in her honour, spelled with an i but spoken with her name. Medicine keeps her in the words nyctalopia, the blindness that comes at night, and nyctophobia, the fear of her. But her truest monument is an idea, one of the oldest and most serious the Greeks ever had: that darkness is not the absence of things but the mother of things, that the day is a visitor and the night is the house, and that the black veil spangled with stars is the oldest face of all.