Scattering of sunlight produces patterns of partially linearly polarized light in the sky throughout the day, and similar patterns appear at night when the Moon is bright. We studied celestial polarization patterns during the period of twilight, when the Sun is below the horizon, determining the degree and orientation of the polarized-light field and its changes before sunrise and after sunset. During twilight, celestial polarized light occurs in a wide band stretching perpendicular to the location of the hidden Sun and reaching typical degrees of polarization near 80\% at wavelengths >600\text{\hspace{0.17em} nm} . In the tropics, this pattern appears \text{∼}1\text{\hspace{0.17em} h} before local sunrise or disappears \text{∼}1\text{\hspace{0.17em} h} after local sunset (within 10\min after the onset of astronomical twilight at dawn, or before its end at dusk) and extends with little change through the entire twilight period.