The Supermoon

Image description: The Supermoon is a term coined by astrologer Richard Nolle in 1979. The Supermoon occurs when the Moon is closest to Earth, making it appear larger and brighter than at any other time. The Supermoon is even more spectacular when it coincides with a full moon, at which point a moon of great luminosity and magnificent brilliance is observed. Image source: Astronoo.

Difference between Supermoon and Full Moon

The Moon moves at ≈1 km/s on an elliptical orbit and its distance from Earth varies significantly by about 50,000 km. At its apogee (farthest point from Earth) it is on average 405,700 km away, while at its perigee, it is on average 356,400 km away. The term Supermoon was coined in 1979 by astrologer Richard Nolle (1950-).

The Supermoon occurs when the Moon is closest to Earth, making it appear larger and brighter than at any other time. The Supermoon is even more spectacular when it coincides with a full moon. At this time, a moon of great luminosity and magnificent brilliance is observed. Of course, when it is at its perigee, it appears 14% larger. 14% is also the difference between the two apsides (closest and farthest points relative to Earth) of the Moon's orbit.

The Moon has an elliptical orbit around the center of the Earth, but the reality is a bit more complex. The double system formed by the Moon and Earth rotates around the common center of mass called the barycenter. Their barycenter, which is not fixed, varies depending on their average distance (≈384,400 km).

The barycenter is located on the Earth-Moon line inside the Earth at ≈1,708 km from the surface and ≈4,670 km from the center of the Earth, the radius of the Earth is ≈6,378 km. Since the center of mass of the Earth/Moon system is located inside the Earth, we simply say that the Moon orbits the Earth. The Earth therefore rotates around a point that moves inside itself. Thus, it describes tiny spirals in its orbit, in other words, it waltzes with the Moon around the Sun.

Given the distance from the Sun, its gravitational influence on the Earth/Moon barycenter is negligible, because from the Sun's perspective, the stable Earth/Moon pair forms a uniform gravitational field. The gravitational field surrounding the pair is considered as the field of a single object. However, there is also a calculable Sun/(Earth/Moon) barycenter with the formula below.

Supermoons occur frequently (every lunar month) i.e., every 27 days 7 hours and 43 minutes. This corresponds to the Moon's revolution period, also known as the orbital period. It is exactly the time it takes for the Moon to complete one full orbit around the Earth or the time interval between two lunar perigees (on average 27.55455 days).

Full moons occur every 29 days 12 hours 44 minutes. This is exactly the time it takes for the Moon to return to the same apparent configuration relative to the Earth and the Sun.

The coincidence between the Supermoon and the full moon occurs 1 to 5 times per year. In 2014, the Supermoon was 14% larger and 30% brighter than a regular full moon according to NASA records.

What are the effects of the Supermoon?

Besides folkloric effects such as increased births, suicides, illnesses, crimes, social unrest, and other natural disasters, the main effect of the Supermoon is obviously the tidal effect. At this time, the amplitude of ocean tides is maximal. In some places on Earth, the tidal range, i.e., the difference in height between high tide and low tide, exceeds 15 meters. The effects of a Supermoon on Earth are therefore minor.

However, according to studies by seismologists and volcanologists, the combination of the Supermoon (closest to Earth) and the full moon (alignment with Earth and the Sun) should not affect the internal energy balance of the Earth because there are lunar tides every day. The Earth has a huge amount of internal energy and the small variations in tidal forces exerted by the Moon are not enough to fundamentally alter the internal balance that drives plate tectonics.

Calculation of the Earth/Moon Barycenter

The Earth/Moon Barycenter = (ML/ML+MT) x DTL = 4670.78 km
ML = Mass of the Moon = 0.073477 x 1024 kg
MT = Mass of the Earth = 5.9736 x 1024 kg
DTL = Earth/Moon Distance = 384,400 km