The Moon’s movements are governed by two distinct and slightly mismatched cycles. This leads to the observation that the Moon reaches its closest point to Earth, the perigee, shortly before or after the New Moon phase. This timing difference arises because the cycle of the Moon’s distance and the cycle of its phases are measured against different astronomical reference points, ensuring these two cycles rarely align perfectly.
Understanding the Moon’s Two Primary Cycles
The Moon’s phases are dictated by the Synodic Period, the time it takes for the Moon to complete a cycle relative to the Sun, as seen from Earth. This period, from one New Moon to the next, averages approximately 29.5 days. The Moon must orbit Earth and also “catch up” to the Earth’s position around the Sun, which makes this cycle longer than a simple orbit relative to background stars.
The Moon’s distance from Earth is governed by the Anomalistic Period, the time it takes for the Moon to travel from perigee back to perigee. Since the Moon’s path is an ellipse, its distance constantly changes, reaching a closest point (perigee) and a farthest point (apogee) each cycle. This cycle averages about 27.5 days, making it roughly two days shorter than the Synodic Period.
These two cycles, one controlling light and shadow and the other controlling physical distance, are inherently different lengths because they measure separate phenomena. The Synodic cycle is tied to the Sun’s position, while the Anomalistic cycle is tied to the orientation of the Moon’s elliptical orbit.
The Shifting Orbit: Precession of the Perigee
The Anomalistic Period is not a fixed measurement because the Moon’s elliptical orbit is not stationary. This orbit slowly rotates in a process known as apsidal precession, specifically the precession of the perigee. External gravitational forces, primarily the pull of the Sun, cause the elliptical path to gradually shift its orientation in the same direction as the Moon’s motion.
This rotation means the perigee point is constantly moving forward around the Earth. The line connecting the perigee and apogee completes a full 360-degree rotation in approximately 8.85 years. This constant movement slightly lengthens the Anomalistic Period compared to a simple sidereal orbit.
The movement of the perigee point means the Moon reaches perigee sooner than if the orbit were fixed. This constant, forward rotation of the orbit is the fundamental cause of the timing offset relative to the Synodic cycle, which is governed by the Sun’s position.
How the Cycles Interact to Create the Offset
The difference in the two cycles provides the direct answer to the timing question. The Synodic cycle, which dictates the New Moon, requires 29.5 days to realign with the Sun. The Anomalistic cycle, which dictates the perigee, is completed in about 27.5 days.
Because the Anomalistic cycle is approximately two days shorter than the Synodic cycle, the Moon reaches perigee roughly two days faster than it completes its phase cycle. This differential ensures that the distance cycle is almost always “ahead” of the phase cycle.
The timing offset means the Moon’s closest approach often occurs just before the New Moon, or sometimes just after, depending on the 8.85-year precession cycle of the perigee point. This constant slight misalignment is a natural consequence of the Moon’s orbital distance being controlled by the rotating ellipse, while its phase is controlled by its position relative to the Sun.