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The value of the selenographic co-longitude increases from 0° to 360° in the direction of the advancing terminator. Sunrise occurs at the central meridian when the Moon reaches first quarter, after one fourth of a lunar day. At this location the selenographic co-longitude at sunrise is defined as 0°. Thus by full Moon the co-longitude increases to 90°; at last quarter it is 180°, and at new Moon the coordinate reaches 270°.
Due to the low angle of the arriving sunlight, the area near the terminator is usually the most favorable for viewing or photographing lunar features through a telescope. Thus the observer will need to know the location of the terminator to plan when to observe selected features on the Moon. The selenographic co-longitude at sunrise is used for this purpose.
The selenographic longitude of the evening terminator is equal to the co-longitude plus 180°.
Moon Physical quantity