If you missed the supermoon on July 12, you’ll get second and third chances to see one on Aug. 10 and Sept. 9. The term “supermoon” applies to a new or full moon that occurs when the moon is within 90 percent of its closest approach to Earth in a given orbit.
Although it is commonly used, “supermoon” — coined in 1979 by a astrologer Richard Nolle — is not an astronomical term. Astronomers instead use the terms perigee-syzygy or perigee-full/new moon.
The distance between Earth and its moon varies each month as the moon orbits our planet. Perigee is the point when the moon is closest, about 222,000 miles, as opposed to apogee, when the moon is farthest from Earth, about 252,000 miles. Because the moon is about 30,000 miles closer at perigee, it appears to be about 14 percent larger.
Syzygy is when the Earth, the moon, and the sun are aligned. As the moon orbits Earth each month, there are two points when syzygy occurs.
One occurrence is when the moon is between the Earth and the sun. The plane of the moon’s orbit around Earth is tipped 5 degrees from the plane of Earth’s orbit around the sun, so in this alignment — the “new moon” phase — the moon will appear above or below the sun.
The other alignment is when Earth is between the sun and the moon. In this alignment, the sun and moon are opposite each other in the sky as seen from Earth. This means that the moon rises as the sun sets. Because the moon is in full sunlight, this is the “full moon” phase.
Full and new moons can occur when the moon is at any distance from Earth, but every 13 to 14 months full moons occur near perigee. Thus, there will be at least one supermoon about every fourteen months.
On July 12 the moon becomes full on the same day as perigee, and will do so again on Sept. 9. On Aug. 10 it becomes full in the same hour as perigee.
The August full moon will be just 221,765 miles away, making it the closest supermoon of the year. Because of the closeness, it will appear to be about 30 percent brighter than the normal full moon. But dust, haze and clouds can easily mask this difference.
The difference in size will also be hard to determine when the moon is high overhead. With no reference points to provide a sense of scale, one full moon looks about the same size as any other.
The “moon illusion” is probably what will help people see this full moon as a supermoon. The illusion occurs when the moon is near the horizon. For reasons neither astronomers nor psychologists understand, the moon looks much larger when it is viewed near the ground, in relation to buildings or other foreground objects.
Look at any full moon just as it rises and again an hour or two later, and the moon will appear to have shrunk. But if you hold up a small card and mark the edges of the rising moon on the card, and later hold the card up to the higher moon, you will see that the size remains the same.
Because ocean tides are caused by the gravitational pull of the sun and moon, new and full moons create higher-than-usual tides. These are called spring tides and they occur every month. Spring tides this month and next, as they were in July, will be further accentuated by the extra-close full moons.
Will these high tides cause flooding? No, the closeness of the moon will only raise the tides by an inch or two.
So check it out on Aug. 10 and see if you can spot any differences in the full moon.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.