New moon image via U.S. Naval Observatory
The new moon coming on June 13, 2018, is a supermoon. But you won’t see it. That’s because at new moon, the moon more or less rises and sets with the sun and is lost in the sun’s glare all day. Moreover, the dark side of a new moon faces Earth, while the lit side faces the sun. However, you might – if you’re really lucky – see the young moon returning to visibility in the western sky for a brief while after sunset June 14.
The term supermoon, which has entered the general lexicon during the last decade or two, was coined by the astrologer Richard Nolle in 1979. He defines a supermoon as “a new or full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit.” By that somewhat vague definition, we can say any new moon or full moon coming to within 224,000 miles (361,000 km) of our planet, as measured from the centers of the moon and Earth, counts as a supermoon.
This is the first in a series of three new moon supermoons, which are to fall on June 13, July 13 and August 11, 2018. The closest supermoon of the trio will come in July, to stage a partial eclipse of the sun in the far southern regions of the globe on July 13, 2018.
As a general rule, the year’s closest new moon or full moon is about 14 percent (30,000 miles or 50,000 km) closer than the farthest new moon or full moon. Therefore, the angular diameter of the closest new/full moon versus the farthest new/full moon is 14 percent greater as well. The proportion is akin to that of a U.S. quarter to a U.S. nickel.
For instance, the year’s farthest full moon is sometimes called a micro-moon or mini-moon. The micro-moon on July 27, 2018, will be 252,334 miles (406,092 km) distant. That’s in contrast to the year’s closest full moon that took place on January 2, 2018, which swept to within 221,583 miles (356,604 km) of Earth.
Perhaps, the farthest new moon (like the farthest full moon) could be called a micro-moon, too.
Although the diameter of the largest new/full moon is about 14 percent larger than that of the smallest new/full moon, the square area of the moon’s disk is actually 30 percent greater. In the case of the full moon, that means the closest full moon is 30 percent brighter than the farthest full moon, or 15 percent brighter than the full moon at its average distance of 238,885 miles or 384,400 km.
Some people assert that a new moon supermoon has no actual relevance in sky gazing because you can’t see the new moon. That may be so, but people living along the ocean shorelines may well notice the wide-ranging spring tides in the few days following the new supermoon, during which the variation in high and low tide is especially profound.
Strictly speaking, it’s not always true that you can’t see the moon at new moon. At favorable times, you can view the new moon silhouette during a solar eclipse. When the new moon goes directly in between the Earth and sun, the result is either a total solar eclipse or an annular eclipse – whereby a ring of sunshine surrounds the new moon silhouette. The new moon is closer to Earth at a total solar eclipse and farther away from Earth during an annular eclipse.
It’s no accident that the new moon supermoon on July 22, 2009, brought about the longest total solar eclipse of the 21st century (2001 to 2100), and that the new moon “micro-moon” on January 15, 2010, presented the longest annular eclipse of the 21st century.
At greatest eclipse during the total solar eclipse on July 22, 2009, the new moon supermoon was only 222,161 miles (357,534 km) distant. Another contributing factor was the Earth being rather close to aphelion – its farthest point from the sun in its orbit. The lunar diameter was 1.08 times the solar diameter.
At greatest eclipse during the annular eclipse of January 15, 2010, the new moon “micro-moon” was a whopping 251,897 miles (405,389 km) away. This time around, the Earth was near perihelion – its closest point to the sun. The lunar diameter was only 0.92 times the solar diameter.
One way or another, any new moon supermoon makes its impact, whether we directly see it or not.
Bottom line: The June 13 new moon is the 1st in a series of 3 new moon supermoons. The other 2 will fall on July 13 and August 11, 2018.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.