Last Quarter

The Last Quarter is the phase at which half the disc appears lit, the left half as seen from the northern hemisphere, and the illumination is decreasing toward the dark. Its name marks the Moon's completion of three quarters of its monthly circuit; again the Sun, Earth, and Moon stand at a right angle, the mirror of the first quarter. The last-quarter Moon rises around midnight and sets around midday, seen in the late night and morning sky. Traditionally it is read as a phase of letting go, reckoning, and clearing away in preparation for the new cycle. These are the astronomical facts of the phase together with the meanings traditions attach to it; the half disc is our side-on view of the sunlit hemisphere, lit now on the opposite side from the first quarter.

Islamic, Hebrew and other lunisolar calendars pay closer attention to the crescents that open and close the month than to the quarters, but the last-quarter Moon has its own long-noted calendrical role: in the Hebrew calendar it falls near the midpoint between the full moon that fixes major festivals and the new moon that opens the next month, and its right-angle geometry, the same configuration astrologers call a square, has made it a natural marker of transition in traditions that track the sky closely. The last quarter rises around midnight and is visible through the second half of the night into morning, the mirror image of the first quarter's evening prominence, so that a culture without artificial light experienced the two quarters as opposite halves of the twenty-four-hour day rather than as simply two points partway through a month.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Calendar and Community: A History of the Jewish Calendar, Second Century BCE to Tenth Century CESacha Stern

Last quarter is elongation two hundred and seventy degrees, three quarters of the way round the cycle, with half the disc lit and the lit half on the opposite side from first quarter. It is also called third quarter, and both names refer to the fraction of the cycle elapsed rather than to the fraction of the disc illuminated.

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.

At last quarter the Moon rises around midnight and sets around midday, so it is the morning Moon, visible in the daylight sky of the forenoon. The terminator again runs straight down the disc, and again this is when relief shows most sharply in a telescope, though the region under low sun is the opposite half of the surface from that seen at first quarter.

Because the quarters divide the lunation into four intervals of about seven and a third days, they are the most likely astronomical origin proposed for the seven-day week, although the week ran independently of the Moon in every historical system that used it and the connection is an inference rather than a documented derivation.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens