Supermoons: In Depth

The term “supermoon” was coined in 1979 and is often used to describe what astronomers would call a perigean (pear-ih-jee-un) full moon: a full moon occurring near or at the time when the Moon is at the closest point in its orbit around Earth.

The term gives preference to the geometric alignment of Sun-Earth-Moon and allows the occurrence of perigee into a wider time period than the actual instant of perigee (up to about two weeks, which is almost half of the Moon’s orbit).

For those looking carefully, there are several things worth noting. Take a look at the lunar images from November 2016 below:

The figure’s left image was taken after moonrise, at about 6 p.m. local time when it had cleared the local, mountainous horizon. The right image was taken with the Moon near its maximum altitude that night, at about 12:30 a.m. The lines across the pair of images show the apparent difference in size: The rising moon is smaller because it was further away when it was rising. At that time, the Moon’s center was about the same distance from Earth’s center and from the observer. At the time of the second picture, Earth had rotated about a quarter-turn and the Moon was highest in the sky. At that time it was almost 4,000 miles closer to the observer because Earth’s rotation has carried the observer more directly under the Moon; Earth’s center was now farther from the Moon than the observer was. Except for the small change in distance between their centers due to the Moon’s orbital motion in those 6.5 hours, the observer’s decrease in distance from the Moon is demonstrated by the pair of photographs.

Some lunar perigees are closer than others. The shape of the Moon’s orbit changes over time (thanks to the gravitational influence of the Sun and the other planets). Extreme perigees and apogees, or the most distant point in the orbit, happen on a predictable basis. Something that’s out of the ordinary: Having a full moon at the same time as an extreme perigee.

An extreme perigean full moon’s angular radius and diameter will appear slightly larger than it does at other full moons. Let's go back to November 13-14, 2016, when the extreme perigean full moon’s radius and diameter were:

Seen from Earth, the area of sky covered by the extreme perigean November 2016 full moon is greater in area by:

Setting the Sun’s illumination of the lunar surface during the extreme perigean full moon in November 2016 to a value of 1.00000, the illumination of the Moon at:

These differences are thanks to the small differences in the Moon’s distance from the Sun. To our own eyes, the differences are indistinguishable.

Illumination of Earth’s surface by the November 2016 extreme perigean full moon was just slightly higher because the Moon was slightly closer to the Sun than usual, and because it appeared slightly larger in the sky than usual compared to other full moons. If the extreme perigean full moon provides an illumination value of 1.00000 on Earth’s surface, the illumination by…

Looking around you when the full moon is high in the sky and there are no other lights around, the differences in ground illumination are indistinguishable to our eyes.

High tides and low tides will be more extreme with a perigean full moon and more so for an extreme perigean full moon. The extremes are greater due to the difference in the gravitational pull of the Moon across Earth’s diameter. When the Moon is close, Earth’s diameter is a slightly larger fraction of the Earth-Moon separation. This means that the Moon’s gravitational pull on the oceans (and Earth’s crust) has a greater difference between the point on Earth closest to the Moon’s center and the point on Earth diametrically opposite it (the antipode, an-tih-pode). This increases the effects of tides. If the extreme perigean full moon in November 2016 caused tides with a force value of 1.000000, the tidal force value by caused by…

Perigean high tides during a full moon and new moon can cause major problems on some coasts, especially if weather adds high waves or a storm surge (due to low atmospheric pressure over the involved area).