What's so special about a supermoon? Turns out, it's a bit more subtle than it sounds—but for the interested observer, there's plenty to see.

The name “supermoon” was coined by an astrologer in 1979 and is often used by the media today 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 its 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:

SuperMoon Comparison
Which of these views of the Moon is a SuperMoon? Both! This is a size comparison of the Moon on the “supermoon” night of November 13-14, 2016. On the left, the Moon was nearer the horizon, and on the right, the Moon was very high in the sky. Note the lower blue line cutting across the Moon’s south pole on the right image. The full frame of each picture is reproduced.

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.

So, what makes a supermoon different?

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:

  • Less than 2 percent larger than an average perigean full moon.
  • Less than 8 percent larger than the full moon’s appearance at its average distance from Earth.
  • Less than 14 percent larger than the average apogean full moon. For comparison, the width of the fingernail on your smallest finger (your “pinkie”), when held at arm’s length, is about the same diameter as the full moon in the sky.

Could you measure any of these differences by eye?

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

  • Less than 4 percent compared to the average perigean full moon.
  • About 14 percent compared to its appearance at its average distance from Earth.
  • Less than 23 percent larger than the average apogean full moon.

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

  • An average perigean full moon is 0.99996.
  • An average distance of the full moon is 0.99981.
  • An average apogean full moon is 0.99967.

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…

  • The average perigean full moon is 0.99991.
  • The average distance full moon is 0.99963.
  • The average apogean full moon is 0.99934.

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…

  • The average perigean full moon is 0.946493.
  • The average distance full moon is 0.797740.
  • The average apogean full moon is 0.678594.

Perigean high tides during 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).

diagram of different apparent moon sizes It's nearly impossible to compare the apparent size of the supermoon with a micromoon from memory, but when seen side-by-side as in this graphic, it becomes clear. Credit: NASA/JPL-Caltech

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