A micro full moon is a popular term for an astronomical event where the Moon appears visibly smaller in the night sky than its average monthly appearance. This visual effect is not caused by any actual change in the Moon’s physical size but rather by its varying distance from Earth. The phenomenon is a direct result of the mechanics of the Moon’s elliptical path as it orbits our planet.
Defining the Micro Full Moon
A Micro Full Moon occurs when the Moon reaches its full phase at or very near the point in its orbit farthest from Earth, a location astronomers call apogee. Since the Moon’s apparent size is inversely proportional to its distance from the observer, this maximum separation makes the lunar disk appear smaller than usual. Although the difference may be subtle to the casual observer, it is scientifically measurable.
The angular size of a Micro Full Moon can be up to 14% smaller in diameter than a Supermoon. Furthermore, a Micro Full Moon appears significantly dimmer, sometimes up to 30% less bright than its closest counterpart. The perception of size can also be affected by the “Moon illusion,” where the Moon appears larger when closer to the horizon, but this is a trick of human perception.
The popular term “Micro Moon” is not a formal astronomical classification but is used widely to describe a Full Moon that occurs when the center of the Moon is farther than approximately 405,000 kilometers from the center of Earth. This definition provides a measurable threshold for what qualifies as the farthest and smallest full moon event.
The Mechanics of the Moon’s Orbit
The underlying cause of the Micro Full Moon is the elliptical shape of the Moon’s orbit around the Earth. If the orbit were a perfect circle, the Moon would always remain the same distance from us, and its apparent size would never change. The orbital path has an average eccentricity of about 0.054, meaning its distance from Earth constantly fluctuates.
This elliptical path creates two extreme points: perigee, the point of closest approach, and apogee, the point of greatest distance. On average, the distance between the Earth and the Moon is about 384,400 kilometers. The distance at apogee, which causes the Micro Full Moon, can stretch to approximately 406,700 kilometers.
A Micro Full Moon happens when the full phase of the Moon aligns precisely with the apogee point in its orbit. The Moon’s cycle of phases (29.5 days) is slightly out of sync with its orbital cycle from perigee to apogee (about 27.5 days). The occasional synchronization of the full phase with the maximum distance produces the smallest visible full moon of the year.
Comparing Micro Moons and Supermoons
The Micro Full Moon is most easily understood when contrasted with the Supermoon, which is the full moon occurring near perigee. The Supermoon represents the maximum close-approach and largest appearance, while the Micro Moon represents the maximum distant-approach and smallest appearance. These two phenomena are simply the extremes of the Moon’s monthly distance variation.
The difference in distance between a Supermoon and a Micro Moon is significant, averaging roughly 50,000 kilometers from one extreme to the other. A Supermoon must be less than 360,000 kilometers away to qualify, while a Micro Moon is defined as being farther than 405,000 kilometers away. This substantial change in separation results in the noticeable difference in apparent size.
When comparing the two extremes side-by-side, the Supermoon can appear up to 14% larger in diameter than the Micro Moon. Both “Micro Moon” and “Supermoon” are popular culture terms, not official designations used consistently across the entire astronomical community.