First Quarter

The First Quarter is the phase at which half of the Moon's disc appears lit, the right half as seen from the northern hemisphere, and the illumination is increasing toward full. Its name comes not from the shape but from the Moon having completed the first quarter of its monthly circuit around the Earth; the Sun, Earth, and Moon stand at a right angle. The first-quarter Moon rises around midday and sets around midnight, riding high in the evening sky. Traditionally it is read as a phase of decision, effort, and the overcoming of obstacles, the point where growth meets resistance. These are the astronomical facts of the phase together with the meanings traditions attach to it; the half disc is simply our side-on view of the sunlit hemisphere of the Moon.

The right angle the Sun, Earth and Moon form at first quarter is the same geometry astrologers call a square when it occurs between two planets, and the coincidence is not accidental: the classical aspects were originally defined by dividing the circle into simple fractions, a quarter, a third, a sixth, and the Moon's own quarter phases are the same division applied to the single fastest-moving body in the sky rather than to two planets at once. Traditionally minded astrologers sometimes read the first-quarter Moon as carrying something of the square's characteristic tension for exactly this reason, obstacle met and pushed through rather than avoided, treating the lunar cycle as a miniature, monthly rehearsal of the same hard-aspect dynamic that unfolds over much longer periods between the outer planets.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with A History of Horoscopic AstrologyJames Herschel Holden

First quarter is elongation ninety degrees, the Moon a quarter of the way round its cycle, with exactly half the visible disc lit. The name is a frequent source of confusion: it refers to the quarter of the cycle completed, not to a quarter of the disc being illuminated, and the same applies to last 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.

At first quarter the Moon rises around midday and sets around midnight, so it is the evening Moon, high in the sky at sunset. The terminator, the line between light and dark, runs straight down the disc at this moment and is where lunar surface relief shows most sharply in a telescope, because the low sun angle there throws long shadows from craters and mountains.

The quarters are also where the Panchanga and the phase scheme most nearly agree: the seventh or eighth tithi falls close to first quarter, though the correspondence is approximate because a tithi is defined by elongation increments of twelve degrees and varies in length.

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