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).

Moon-Related Words & Phrases: A Glossary

Apogee (ap-uh-jee). This is the most distant point (ap-) on an elliptical orbit around Earth (-gee). The equivalent point for Earth’s orbit around the Sun is called aphelion (ap-he-lee-un or af-he-lee-un).

Blood Moon. This is a recently coined term for a full moon during a total lunar eclipse. When the Moon is inside Earth’s shadow, with the Sun completely blocked as seen everywhere on the lunar surface, the Sun’s rays bend as they go through Earth’s atmosphere. The atmosphere acts like a lens and bends the rays far enough that Earth’s sunrises and sunsets all illuminate the Moon. Each lunar eclipse is different. The color ranges from bright orange to brick to the complete disappearance of the Moon in the sky (temporarily).

Crescent. This is the phase of the Moon when the Earth-facing hemisphere is illuminated over less than 50 percent of its area. We view a crescent between new moon and first quarter (half moon) in the evening sky and between third quarter (also a half moon, facing the other direction) and new moon in the morning sky.

Dark Moon. This is a recently coined term for the new moon each month.

Dark side of the Moon. Dark Side of the Moon is not just an album by the English rock band Pink Floyd—it’s also an incorrect description used by some when referring to the hemisphere of the Moon invisible from Earth. “Far side of the Moon” or “lunar far side” are correct phrases to use. “Dark side of the Moon” could refer to the Moon’s night side, but this is not common usage.

Earthshine. Also called earthlight. Most easily observed when the Moon is a relatively narrow crescent, sunlight reflecting from Earth’s day side is the earthshine illuminating the Moon’s night side. Scientists have used the brightness of earthshine to gauge the cloudiness of Earth’s dayside visible from the Moon. Earthshine is not related to moonshine, which refers to illicitly manufactured alcoholic beverages.

Full Moon. A full moon occurs in a syzygy wherein the order of alignment in space is Sun-Earth-Moon. The Moon is exactly 180 degrees opposite the Sun, as measured in celestial longitude, though its angular separation from the Sun may be less than 180 degrees because of its tilted orbit. The Sun is almost fully illuminating the hemisphere facing Earth; a telescope can show the slight “phase defect” around the “limb” (disk edge) of a full moon. The phase defect is best seen when the Moon’s orbit carries if off the ecliptic circle (the Sun’s apparent “orbit” in the sky, over the course of a year) and we see a little around to the night side, near either the Moon’s north pole or its south pole.

We never get to see a brightly illuminated, perfectly full moon. That’s because Earth blocks the sunlight falling where the Moon would have to be to be seen perfectly full. Earth’s shadow is eclipsing the Moon when it is in that position. A full moon rises around local sunset, is highest around midnight, and sets around local sunrise.

(Note: Celestial longitude is measured along the ecliptic, a circle in the sky defined by the apparent position of the Sun’s center as it changes through the year due to Earth’s orbital motion. The angle measurement is not made using Right Ascension, which is defined by projecting Earth’s meridian lines into the sky.)

Gibbous. This is the phase of the Moon seen when the Earth-facing hemisphere is illuminated over more than 50 percent of its area. We view a gibbous from just after the evening half moon (first quarter) until the night of full moon and then after full moon until third quarter (also a half moon).

Micromoon. This is a recently coined term to describe the full moon near apogee. Apogean full moon is more descriptive of “micromoon” events but is, perhaps, more limited in time by its specification of apogee.

Near side of the Moon. This phrase describes the hemisphere of the Moon facing Earth.

New Moon. A new moon occurs in a syzygy wherein the order of alignment in space is Sun-Moon-Earth. The Moon is exactly aligned with the Sun, as measured in celestial longitude, though it may actually pass a few degrees above or below the Sun because of its tilted orbit. The Moon is completely shadowing its own Earth-facing hemisphere, while its farside is (almost) fully illuminated. A thin crescent is not visible at new moon because the roughness of the Moon’s surface prevents us from seeing any illuminated surfaces. The silhouette of the new moon can be seen during a solar eclipse but the eclipse may actually not be occurring at exactly when the Sun and Moon have the same celestial longitude. Much more often, the Moon’s orbit carries if off of the ecliptic circle so there is no eclipse and we simply cannot see the Moon for roughly 24 hours (and usually longer) centered on the moment of new moon.

(Note: Celestial longitude is measured along the ecliptic, a circle in the sky defined by the apparent position of the Sun’s center as it changes through the year due to Earth’s orbital motion. The angle measurement is not made using Right Ascension, which is defined by projecting Earth’s meridian lines into the sky.)

Perigee (pear-ih-jee). This is the closest point (peri-) on an elliptical orbit around Earth (-gee). The equivalent for Earth’s orbit around the Sun is perihelion (pear-ih-hee-lee-un).

Phase. “Phase” is commonly used as a stand-in generic reference for the apparent shape and position of the Moon or its age (= number of days since the most recent new moon). Astronomers often use it as “phase angle,” a numerical value that indicates the angular separation in the sky of the Moon (or another object) from the Sun as seen by the observer. New moon has a phase angle of about 0°, first quarter has a phase angle of about 90°, full moon has a phase angle of about 180°, and last quarter has a phase angle of about 270°.

Quarter Moon. Usually used with the modifier “first” or one of either “third” or “last.”

A first quarter moon has traveled one-quarter of the way around its orbit circling Earth, starting from the new moon position (explained above). From Earth’s surface we see 50% (half) of the Moon illuminated by the Sun, half in darkness, in the shade of its own sunward hemisphere. A first quarter moon rises around local noon, is highest at sunset, and sets around local midnight.

“Second quarter” is almost never used; it would refer to the full moon, explained above.

“Third quarter” and “last quarter” mean the same thing and are used interchangeably. The Moon has traveled three-quarters of the way around its orbit. We see the other half of the Moon illuminated by the Sun, half in the darkness of self-shade. A third quarter moon rises around local midnight, is highest at sunrise, and sets around local noon.

Supermoon. This term was coined by an astrologer in 1979 to describe periods when the new moon or the full moon occur near perigee. The definition reads “…a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, moon and sun are all in a line, with moon in its nearest approach to Earth.” Not surprisingly, only full moons near perigee receive attention since new moons in this configuration are invisible (except during a solar eclipse). Perigean syzygies (singular: syzygy, an alignment more completely defined below) including perigean new moon and perigean full moon, are more descriptive of “supermoon” events but are, perhaps, more limited in time by their specification of perigee. The media use “supermoon” only for perigean full moon.

Syzygy (sih-zih-jee). Alignments of the Sun, Earth, and Moon (or Sun, Moon, and Earth). Ocean tides are near maximum during syzygean perigees (new or full moon). The highest tides occur during syzygean perigees when the Earth is at perihelion.

Waning. “Waning” describes the decreasing amount of illumination of the Moon’s face after full moon as it moves to new moon.

Waxing. “Waxing” describes the increasing amount of illumination of the Moon’s face before full moon, starting from new moon until full moon.

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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