Fig. 1. Illuminance and gravimetric cycles that the Moon imposes on Earth.

(A) Schematic of the movements of Moon and Earth around the Sun (sizes of Sun, Earth, and Moon and distances between them are not to scale). (B) The Sun illuminates the Moon, which causes a 29.53-day cycle (the synodic month) of changing nocturnal illuminance on Earth (full, half, and new moon). The synodic month is ~2.2 days longer than the orbital period of the Moon because Earth has moved in its orbit around the Sun and it takes 2.2 days longer until the Moon is again in its full moon position. The Moon also exerts gravitational effects on Earth that are most evident in the tides. High and low tides occur twice a day with a period of 12.4 hours. Spring tides happen every 14.765 days during full and new moons and neap tides during half-moons. (C) The Moon’s orbit around Earth is tilted with respect to the plane of Earth’s orbit around the Sun (the ecliptic). When the Moon is high in the northern sky, the difference in the amplitude between the two daily tides is maximal in the Northern Hemisphere of Earth. This happens every 27.32 days, and the corresponding cycle is called tropical month. (D) The orbit of the Moon around Earth is elliptic with Earth in one of the focal points of the ellipse. Therefore, the Moon-Earth distance varies cyclically: Every 27.55 days, the Moon is close to Earth (in its perigee) and exerts maximal gravitational forces on Earth, resulting in high tidal amplitudes, while it exerts minimal gravitational forces when it is far from Earth (in its apogee). This cycle is called anomalistic month.