Home Page

Astronomy Articles

Stars Page

Astronomy Links




SuperMoon Ambiguity & Inconsistency Explained





We can safely say that the September 2014 full Moon is a SuperMoon. It appears less certain, though, that we can rightly call the October 2014 full Moon a SuperMoon. Let us investigate the meaning of SuperMoon, because the term, as presently used, appears to be in dire need of clarification.


Origin Of The Term SuperMoon


The astrologer Richard Nolle is credited for coining the term SuperMoon. He describes a SuperMoon as 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.


The Moon's closest approach to Earth in a given orbit is called perigee, and its farthest point is called apogee. So whenever a new or full Moon aligns with perigee, we say the Moon is at 100% of its closest approach to Earth. And when a new or full Moon aligns with apogee, we say the moon is at 0% of its closest approach to Earth. So far, so good, but an inherent complication lurks . . .


90% Based On Closest Perigee & Farthest Apogee


Richard Nolle gives an example on how he figures 90% of the moon's closest approach to Earth for the year 2011. (See the next to the last paragraph in Nolle's SuperMoon article.) He bases 90% of the Moon's closest approach on the year's closest perigee and farthest apogee. We'll figure likewise for the year 2014, finding out 90% of the moon's closest approach to Earth for this year.


This year, in 2014, the moon comes closest to Earth on August 10 (356,896 kilometers), and swings farthest away some two weeks before, on July 28 (406,567 kilometers). That's a difference of 49,671 km (406,567 - 356,896 = 49,671 km). Ninety percent of this 49,671-figure equals 44,703.9 kilometers (0.9 x 49,671 = 44,703.9). Presumably, any new or full moon coming closer than 361,863.1 kilometers (406,567 - 44,703.9 = 361,863.1) would be "at or near (within 90% of) its closest approach to Earth."


90% of Moon's Closest Approach For The Year 2014


Farthest apogee: 406,567 km

Closet perigee: 356,896 km

Difference: 49,671 km


90% x 49,671 = 44,703.9 km

406,567 - 44,703.9 = 361,863.1 km = 90% of moon's closest distance to Earth


Thus, any new or full Moon coming closer than 361,863.1 km counts as a SuperMoon in 2014.


90% of Moon's Closest Approach Based On Mean Perigee and Mean Apogee


By all indications, Richard Nolle's SuperMoon tables for the 21st century are based upon the years' closest perigees and farthest apogees. In other words, the year's closest perigee = 100% of the Moon's closest approach, and the year's farthest apogee = 0%.


However, Richard Nolle does second computation (in the same next to the last paragraph), which seems to negate the first. This time around, he uses the Moon's MEAN perigee and apogee distances to figure out the limiting distance for the SuperMoon. This procedure does not and cannot lead to the same result, as is evident in the calculation below:


Mean apogee: 405,504 km

Closest perigee: 363,396 km

Difference: 42,108 km


90% x 42,108 = 37,897.2 km

405,504 - 37,897.2 = 367,606.8 km = 90% of moon's closest approach to Earth


Thus, using this alternative method, any new or full Moon coming closer than 367,606.8 km in 2014 qualifies as a SuperMoon. This broadens the definition, additionally conferring the SuperMoon title to the full Moons of June 2014 and October 2014.


So Do the June 2014 And October 2014 Full Moons Qualify As SuperMoons?


Richard Nolle lists the full moons of July, August and September as SuperMoons. Fred Espenak, in his Full Super Moon Table for the 21st century, additionally includes the June and October 2014 full Moons. Fred Espenak gives a limiting distance for the super moon at 367,607 km; presumably, Richard Nolle's limiting distance is 361,863 km.


Reviewing Richard Nolle's Definition


So who's right and who's wrong? Ironically, Fred Espenak seems to more strictly adhere to Richard Nolle's definition than Richard Nolle himself does. Once again, Richard Nolle describes a SuperMoon as "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."


Emphasis is given to the phrase in a given orbit, because Fred Espenak does exactly that: calculates the distance of the full Moon relative to a given orbit. For example The relative distance of the June 2014 full Moon (365,038 km) is based upon the June 2014 apogee (404,954 km) and the June 2014 perigee (362,065 km). And the relative distance of the October full moon is based upon the October 2014 apogee (404,897 km) and the October 2014 perigee (362,476 km). Without a doubt, the full Moons of June and October 2014 exceed 90% of the Moon's closest approach to Earth in a given orbit.


To find the relative distance of June 2014 full Moon: subtract the June 2014 full Moon's distance from the June 2014 apogee's distance to get 39,916 km (404,954 - 365,038 = 39,916 km). Then subtract the June 2014 perigee distance from the June 2014 apogee distance to get 42,065 km (404,954 - 362,065 = 42,889 km). Finally, divide the full Moon's distance (from apogee) by the perigee distance (from apogee) to find the Moon's relative distance: 39,916/42,889 = 0.93068 (93.068%) = the full Moon's relative distance. Whew!


Full Moon's Relative Distance for June 2014


Apogee: 404,954

Full Moon: 365,038

Difference: 39,916


Apogee: 404,954

Perigee: 362,065

Difference: 42,889


39,916/42,889 = 0.93068 (93.068%) = relative distance for June 2014 full Moon


Full Moon's Relative Distance for October 2014


Apogee: 404,897

Full Moon: 365,659

Difference: 39,236


Apogee: 404,897

Perigee: 362,476

Difference: 42,421


39,235/42,421 = 0.9249 (92.49%) = relative distance for the October 2014 full Moon


SuperMoon Definition To Be Revisited & Refined?


Who knows? One of these days some refinement for the term SuperMoon (or super moon) may unshackle the definition from its present-day ambiguity.


copyright 2014 by Bruce McClure


August 2014 Feature * October 2014 Feature