The term “micromoon” describes a Full Moon that appears noticeably smaller in the night sky than usual. This is a popular label, not an official astronomical designation, used to describe the smallest possible Full Moon as seen from Earth. The event results from the changing distance between Earth and the Moon, which makes the Moon’s size and brightness appear to fluctuate.
The Astronomical Definition
A micromoon occurs when the Moon reaches its full phase while simultaneously being at its farthest point from Earth in its orbit. Astronomers call this farthest orbital point the apogee, making the technical name for the event an Apogee Moon. While the average Earth-Moon distance is about 382,500 kilometers, during a micromoon, the Moon is typically greater than 405,000 kilometers away. This increased distance causes the Moon’s disk to cover a smaller area in the sky, resulting in a visually smaller and slightly dimmer appearance.
Understanding the Elliptical Orbit
The Moon’s varying distance is a direct consequence of its orbital shape, which is an ellipse rather than a perfect circle. This elongated path means the distance between the Earth and the Moon constantly changes during the orbit. The two extreme points of this elliptical path are defined by their distance from Earth. The closest point is called perigee, and the farthest point is called the apogee.
The Moon travels from apogee to perigee and back roughly every 27.5 days. This regular cycle ensures the Moon’s apparent size changes subtly throughout the month, though the difference is difficult to detect without aid. The full micromoon event requires the Full Moon phase to align closely with the moment the Moon reaches apogee. Gravitational forces from the Sun and Earth cause minor fluctuations in the elliptical path, meaning the exact apogee distance changes slightly.
Micromoon Versus Supermoon
The micromoon is the counterpart to the supermoon, which is a Full Moon that occurs when the Moon is near its perigee (closest point to Earth). The visual difference between these two extremes is significant. A supermoon is approximately 14% larger in apparent diameter and up to 30% brighter than a micromoon. This difference results from the supermoon being about 40,000 kilometers closer to Earth than the micromoon.
Compared to an average Full Moon, a micromoon appears about 6% to 7% smaller. Despite these measurable differences, the subtle change in size between consecutive full moons is often difficult to notice without specialized equipment or photographic comparisons. Both micromoons and supermoons are frequent, with each event occurring three to four times within a year.