The period between astronomical sunrise or sunset and the time when the sun's unrefracted center is at elevation h0 = −6°. Local topography above the astronomical horizon will make local sunset occur before astronomical sunset. During a clear evening's civil twilight, horizontal illuminance decreases from ∼585–410 lux to ∼3. 5–2 lux. As is true of nautical and astronomical twilight, civil twilight's length varies greatly with latitude and time of year. At h0 = −6°, the ambient illuminance under clear skies historically was deemed just adequate for doing outdoor work without artificial illumination. However, perceptually demanding tasks, such as driving an automobile, require higher illuminances. Also at h0 = −6°, luminances near the clear sky's zenith are low enough to make the brightest stars visible. Compare astronomical twilight.