The western horizon corrects a popular illusion: stars do not arrive with sunset; they become detectable when scattered daylight loosens its grip on the eye. Nothing is switched on. During early dusk, Rayleigh scattering keeps the atmosphere bright, and stellar points remain beneath the visual system's contrast threshold.
The correction exposes a plain fact: sky is a screen, not an empty stage. As solar altitude declines, sky luminance drops far more quickly than the flux from distant stars, whose photons have crossed space and atmosphere without waiting for anyone's evening. The shift can feel sudden. Yet the governing measure is signal-to-noise ratio: once a star's light exceeds the surrounding background by enough, it crosses from hidden to seen. No cosmic event occurs. Photoreceptors also move from photopic toward scotopic adaptation, increasing sensitivity as the scene darkens. This apparent schedule varies with humidity, haze, lunar light, and local illumination, which alter background radiance without altering the stars themselves.
City lighting makes this ordinary physics less forgiving. Streetlamps, signs, and lit facades create skyglow, lifting the background brightness that faint stars must overcome; aerosols can scatter that artificial light across a wide portion of the sky. Bright planets appear first. Then the most luminous stars pierce the remaining blue, while dimmer ones demand deeper darkness and, often, distance from the urban glow. By nightfall, the heavens have not changed their inventory. Only conditions have changed. Above the darkening roofs, what looked like absence was only concealment.
