Waxing Gibbous

The Waxing Gibbous is the phase between the first quarter and the full moon, when more than half the disc is lit and the illumination continues to increase toward complete. The word gibbous describes the swollen, more-than-half shape. The waxing gibbous Moon rises in the afternoon and is prominent through the evening and much of the night. Traditionally it is read as a phase of refinement and near-completion, the last growth and adjustment before fullness. These are the astronomical facts of the phase together with the meanings traditions attach to it; the shape is simply the sunlit hemisphere of the Moon seen from an angle that reveals most but not yet all of it, as the Moon moves toward the point in its orbit opposite the Sun where the full moon occurs.

Because the Moon's orbit is not a perfect circle, its distance from Earth varies over the course of a month, and a waxing gibbous Moon near perigee, its closest approach, can appear measurably larger and brighter than one near apogee at the same fraction of illumination, an effect popularly called a supermoon when it coincides with the full moon two phases later. The gibbous shape itself, from the Latin gibbus for hump, was a term astronomers had long used for other partially lit bodies before it settled into its modern near-exclusive association with this lunar phase. Traditionally the waxing gibbous is treated as a phase of trust rather than of doubt, the outcome no longer in question but the details still being refined before the fullness two or three nights ahead.

The waxing gibbous covers elongation from ninety degrees to a hundred and eighty, between first quarter and full, with more than half the disc lit and the illuminated portion still growing. The word gibbous means humped, and it describes the outward bulge of the terminator once it has passed the straight line of the quarter.

The phases are produced by the changing angle between the Sun, the Earth and the Moon, and the quantity that defines them is elongation, the Moon's angular distance from the Sun as seen from the Earth. The complete round from one new moon to the next is the synodic month, about 29.53 days, and it is longer than the sidereal month of about 27.32 days in which the Moon returns to the same place among the stars, because the Earth has moved along its orbit in the meantime and the Moon must travel further to catch up.

During this interval the Moon rises in the afternoon and sets in the early hours, so it dominates the evening sky. The proportion of the disc lit does not change at a constant rate through the cycle: it changes fastest around the quarters and slowest around new and full, because the illuminated fraction varies as a function of the cosine of the elongation rather than linearly with it.

The eight-phase division used here is a modern convention of convenience. The underlying quantity is continuous, and traditions that divide the lunation differently, the Panchanga into thirty tithis among them, are not describing different phenomena but cutting the same continuum at different points.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Indian CalendarRobert Sewell and Sankara Balkrishna Dikshit