The neologism “supermoon” was coined in an effort to make a technical term easier for non-astronomers to understand. It seems, ironically, to have only increased confusion.

In 1979, astrologer Richard Nolle decided the scientific term “perigee-syzygy” — wherein the moon arrives at the closest point to earth on its elliptical orbit (perigee) around the same time as full or new moon, when the sun, earth, and moon are nearly lined up (syzygy) — was too technical for the average adult’s vocabulary, and subsequently published his own definition in “Horoscope” magazine, calling it a “supermoon.”

The term largely escaped attention until March 2011, with the closest full moon in 18 years. Since then it’s been trotted out whenever another supermoon (there can be up to four per year) is nigh. Nolle claims that supermoons increase the chances of earthquakes, enticing news outlets to gleefully ignore the total lack of evidence to support such a correlation.

By Nolle’s definition, a “supermoon” is a new or full moon that lies more than 90% of the way from the moon’s farthest point (apogee) to perigee. (Why 90%?) Also, since the moon’s apogee and perigee distances vary over time, one must ask which perigee and apogee distance is used to determine if a moon is “super” or not. Nolle is vague on this point.

Since “supermoon” now seems indelibly ensconced in our lexicon, at the risk of being presumptuous I’ll propose a simpler definition: A moon is “super” when perigee and syzygy fall within 24 hours of one another.

Admittedly, a one day cut-off is arbitrary but since most of us perceive the moon’s changes day-by-day, it’s easy to understand. And, since there’s only one perigee per lunar orbit, its distance variability becomes irrelevant.

Next column: A close rendezvous of the brightest and dimmest naked-eye planets.

Sky calendar through March 22nd:


One hour after sunset:

Venus: W, extremely low

Mercury: W, very low

One hour before sunrise:

Saturn: SSE, low.

Mars: SSE, low.

Jupiter: SSW, low.

Moon: Last quarter 4:20 a.m. 3/9. New moon 7:12 a.m. 3/17. Upper right of Mars 3/9. Upper right of Saturn 3/10. Left of Saturn 3/11.

Other data: Daylight Saving Time begins 2:00 a.m. 3/11 (set clocks 1 hour later). Vernal Equinox 10:15 a.m. 3/20.

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 208-732-6663 or