Note 2 — IV LOWELL'S BOOK ON MARS
Reading edition
The terminator represents the limit of light on that side of the planet in the shade, in other words, where the light terminates. In viewing the Moon, when at quarter or half, the terminator is seen very ragged on account of the illumination of higher points on the surface. If the Moon was as smooth as a billiard ball the terminator would be clear cut.