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Is tonight’s moon a supermoon?

The first of 2017’s 12 full moons falls on today’s date. It’s the 3rd-closest full moon in 2017, but is it a supermoon? Even experts disagree. Learn why here.

Apparent size of a full supermoon, or close moon, contrasted with a full micro-moon, or far moon. Image by Peter Lowenstein.

The first of 2017’s 12 full moons falls on January 12. Despite the fact that this full moon ranks as the third-closest full moon of 2017, it’s less than clear whether or not it should be dubbed a supermoon. Some say yes. Others say no. Follow the links below to learn why experts disagree on what constitutes a supermoon.

Who are the experts, and what do they say?

Original definition of supermoon

Nolle’s 90% based on 2017’s closest perigee and farthest apogee

Espenak’s 90% based on perigee and apogee of each month’s orbit

January full moon’s distance relative to 2017’s closest perigee and farthest apogee

Is the January 12 full moon a supermoon? Depends on which perigee/apogee distances you choose

Astrophysicist Fred Espenak, aka Mr. Eclipse – a 30-year NASA veteran and world-renowned expert on eclipses – says the January 12, 2017 full moon is a supermoon in his post Moon in 2017.

Who are the experts, and what do they say? The International Astronmical Union (IAU) is the group that has taken it upon itself to name and define things in astronomy. But the IAU has been, so far, silent on the subject of supermoons, which, professional astronomers tend to call perigean full moons.

Meanwhile, Fred Espenak, the go-to astronomer on all things related to lunar and solar eclipses (Mr. Eclipse!) and an EarthSky partner, lists the full moons of January, November and December 2017 as full moon supermoons in his great post, Moon in 2017.

We also need to consider astrologer Richard Nolle. Whatever your thoughts or feelings are on astrology, Nolle is, after all, the person who coined the term supermoon. He lists only the December full moon as a supermoon in 2017. The supermoon definition, as originally defined by Nolle, comes with ambiguity. That’s why there are different answers to the question of the number of supermoons in 2017.

We refer you to two different supermoon tables for the 21st century (2001 to 2100). Here is Richard Nolle’s table, and here is Fred Espenak’s table.

Richard Nolle lists only one full moon supermoon for 2017:

2017 December 3

Meanwhile, Fred Espenak lists three full moon supermoons in 2017:

2017 January 12

2017 November 4

2017 December 3

Why are their lists different?

Click here to learn more about Richard Nolle

Click here to learn more about Fred Espenak

Image credit: NASA. The moon's orbit is closer to being a circle than the diagram suggests. The moon is closest to Earth in its orbit at perigee and farthest away at apogee.

Image via NASA

Original definition of supermoon. Supermoons are based on lunar perigee and apogee. Each month, the moon comes closest to Earth at perigee and swings farthest away at apogee.

In his original definition, Richard Nolle defined 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.

If a new or full moon aligns with apogee, then it’s at 0% of its closest approach to Earth. On the other hand, if a new or full moon aligns with perigee, then it’s at 100% of its closest approach to Earth. That’s something we can all agree on.

But the phrase 90% of perigee is ambiguous. Read on.

Anthony Lynch in Dublin, Ireland, wrote on May 24,

A 2013 supermoon, as captured by EarthSky Facebook Anthony Lynch in Dublin, Ireland.

Nolle’s 90% is based on 2017’s closest perigee and farthest apogee. Looking at Richard Nolle’s list for all the supermoons in the 21st century, it appears that Richard Nolle bases his 90% figure on the year’s closest perigee and farthest apogee. Take the year 2017, for instance, whereby any new or full moon coming closer than 362,146.6 km qualifies as a supermoon.

This year, in 2017, the moon comes closest to Earth on May 26 (357,207 kilometers) and swings farthest away toward the end of the year, on December 19 (406,603 kilometers). That’s a difference of 49,396 km (406,603 – 357,207 = 49,396 km). Ninety percent of this 49,396-figure equals 44,456.4 kilometers (0.9 x 49,396 = 44,456.4). Presumably, any new or full moon coming closer than 362,146.6 kilometers (406,603 – 44,456.4 = 362,146.6) would be “at or near (within 90% of) its closest approach to Earth.”

Farthest apogee (2017): 406,603 km
Closest perigee (2017): 357,207 km
Difference: 49,396 km

90% x 49,396 = 44,456.4 km

406,603 – 44,456.4 = 362,146.6 km = 90% of moon’s closest distance to Earth

Thus, figuring out “90% of the moon’s closest approach to Earth” by the year’s closest perigee and farthest apogee, any new or full moon swinging closer than 362,146.6 km to Earth in 2017 counts as a supermoon.

Since the full moon on January 12, 2017 only comes within 366,880 km of Earth, it doesn’t count as a supermoon on Richard Nolle’s list.

Full moon via Evgeny Yorobe Photography

July 2014 supermoon via Evgeny Yorobe Photography

Espenak’s 90% based on perigee and apogee of each month’s orbit. Ironically, Fred Espenak’s full supermoon list might more strictly adhere to Richard Nolle’s definition (at least as it is written) 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.

If given orbit can be taken to mean current monthly orbit, then the January 2017 full moon comes to within 91.3% of its closest approach to Earth relative to the January perigee and apogee.

January 2017 apogee: 404,914 km
January 2017 perigee: 363,238 km
Difference: 41,676 km

January 2017 apogee: 404,914 km
January 2017 full moon: 366,880 km
Difference: 38,034 km

38,034/41,676 = 0.9126 (91.26%) = distance of the January 2017 full moon relative to the January 2017 perigee and apogee

Depending on what meaning we give to the words in a given orbit, we could say the January apogee = 0% of the moon’s closest approach to Earth for January 2017, and the January perigee = 100% of the moon’s closest approach to Earth for January 2017.

That being the case, then the January full moon comes to within 91.26% of its closest approach to Earth for the month of January: 38,034/41,676 = 0.9126 = 91.26%

Super cool super-moonrise composite from Fiona M. Donnelly in Ontario.

Super cool super-moonrise composite from Fiona M. Donnelly in Ontario. This photo is from the August 2014 supermoon.

January full moon’s distance relative to 2017’s closest perigee / farthest apogee. However, if we compute the percentage distance of the January full moon relative to the year’s farthest apogee and closest perigee, then the January full moon only comes to within 80.42% of its closest approach to Earth:

Farthest apogee (2017): 406,603 km
Closest perigee (2017): 357,207 km
Difference: 49,396 km

Farthest apogee (2017): 406,603 km
January full moon (2017): 366,880 km
Difference: 39,723 km

39,723/49,396 = 0.80417 (80.42%) = distance of the January full moon relative to the year’s farthest apogee and closest perigee

Contrasting a full supermoon (full moon at perigee) with a micro-moon (full moon at apogee). Image credit: Stefano Sciarpetti

Another contrast of a full supermoon (full moon at perigee) with a micro-moon (full moon at apogee). Image credit: Stefano Sciarpetti

Is the January full moon a supermoon? Depends on which perigee/apogee distances you choose. The moon’s monthly perigee and apogee distances vary throughout the year, so it appears that the limiting distance for the supermoon depends on which perigee and apogee distances are being used to compute 90% of the moon’s closest approach to Earth.

If we choose the year’s closest perigee and farthest apogee, as Nolle did, we narrow the definition of supermoon.

If we choose the perigee and apogee for a given monthly orbit, as Espenak did, then we broaden the definition of supermoon.

Given the narrower definition, the full moon on January 12, 2017 is not a supermoon, but given the broader one, it is.

Take your choice!

The moon's apparent size in our sky depends on its distance from Earth.  The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left).  Image by Marco Langbroek of the Netherlands via Wikimedia Commons.

The moon’s apparent size in our sky depends on its distance from Earth. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image by Marco Langbroek of the Netherlands via Wikimedia Commons.

Bottom line: At least two commentators – Richard Nolle and Fred Espenak – disagree on whether the January 12, 2017 full moon should be called a supermoon. Is it? If you define a supermoon based on the year’s closest perigee and farthest apogee, then the January 2017 full moon is not a supermoon. If you define a supermoon based on the perigee and apogee for a given monthly orbit, then it is a supermoon.

Bruce McClure

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