A "supermoon" refers to a new or full moon that occurs when the moon is positioned near its perigee, the closest point to Earth in its orbit.

In layperson's terms, it appears as a noticeably larger moon to the eye.

It's not an astronomical term but rather a concept in astrology, first proposed by American astrologer Richard Nolle in 1979. It refers to a new or full moon when the moon is positioned near its perigee. However, many mainstream astronomers do not endorse the term "supermoon" because, scientifically speaking, the term "perigee full moon" would be more accurate.

Principle of Occurrence

The average distance from the moon to Earth is about 380,000 kilometers, but the moon's orbit around Earth is not perfectly circular; it's elliptical, and the eccentricity of this ellipse varies periodically.

Measurements show that when the moon is at perigee, its distance from Earth ranges from 356,400 to 370,400 kilometers, with an average distance of 363,000 kilometers. When it's at apogee, the average distance increases to 406,000 kilometers, resulting in a difference of up to 10.41%.

When the moon is closer to us, it appears more significant; farther away, it seems smaller. The difference is noticeable when the distance varies significantly, such as tens or hundreds of kilometers. The difference between perigee and apogee is over 50,000 kilometers, leading to a noticeable contrast.

Since the moon is Earth's only natural satellite, it orbits under Earth's gravitational influence, which is also influenced by the sun's gravity. This complex interaction results in a varied gravitational force experienced by the moon during its orbit around Earth, leading to a complex lunar orbit. The perigee constantly shifts due to these gravitational forces.

The occurrence of a "supermoon" depends on two key factors: proximity and fullness. Suppose the moon is at or near perigee during a full moon. In that case, it appears 12.5-14.1% larger than at apogee, leading to a phenomenon where the moon appears larger and brighter than usual, hence termed "supermoon" by many.

The point on the lunar orbit closest to Earth is called the perigee, while the farthest point is called the apogee. The moon passes through perigee and apogee once each in its orbit around Earth, with an average interval of about 27.55455 days, known as the anomalistic month. The interval between two consecutive full moons is approximately 29.53 days. We can determine when a "supermoon" will occur by calculating the perigee and total moon times.

During a lunar orbit cycle, a "supermoon" doesn't necessarily coincide with the moon's closest approach to Earth. There may be a time difference of up to several hours between the perigee and the full moon. The smaller this time difference, theoretically, the larger the moon appears. However, to the eye, it's challenging to discern such differences.