terminator
**Definition:** The line that divides the illuminated and dark portions of a celestial body — the boundary between day and night on a planet or moon. On Earth, the terminator is the advancing edge of dawn or the retreating edge of dusk, sweeping across the surface at roughly 1,000 miles per hour at the equator. Through a telescope aimed at the Moon, the terminator is where the most dramatic detail appears: craters and mountains cast long shadows at the boundary of light, revealing topography that vanishes under the flattening noon sun.
The line that divides the illuminated and dark portions of a celestial body — the boundary between day and night on a planet or moon. On Earth, the terminator is the advancing edge of dawn or the retreating edge of dusk, sweeping across the surface at roughly 1,000 miles per hour at the equator. Through a telescope aimed at the Moon, the terminator is where the most dramatic detail appears: craters and mountains cast long shadows at the boundary of light, revealing topography that vanishes under the flattening noon sun.
Etymology
Latin terminator, one who sets boundaries, from terminus, a boundary, an end. The terminator is the boundary between light and dark — the end of day.
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