Astro Bob: Full Buck Moon biggest of year
Find a scenic spot to watch an oversized full moon rise Wednesday night, July 13.
The moon is always only one size. It measures 2,159 miles (3,475 kilometers) across, about the distance between New York City and Albuquerque. It appears to shrink and expand slightly each month because its distance from Earth changes. We like to say the moon "circles" the Earth, but its orbit is actually an ellipse.
At one end of the ellipse it's closest to us, called "perigee," while at the other end, called "apogee," it's farthest. The moon completes one orbit every 27.3 days. After arriving at perigee it reaches apogee about two weeks later. Round and round it goes with perigee and apogee passages every month.
If the full moon occurs at or near perigee, it's known popularly as a "supermoon." "Super" means a larger and brighter moon than normal. Perigee and full moon coincide (or nearly so) several times a year, so supermoons aren't uncommon, though some are closer, or "more super," than others.
Of this year's several supermoons, July's comes closest. At 4:09 a.m. CDT on July 13, the moon will be just 221,992 miles (357,263 kilometers) away, or about 17,000 miles closer than an average full moon.
Naturally, an apogean moon — dubbed a "micromoon" — appears neither as large nor as bright. The most distant micromoon of 2022 occurred Jan. 17. When the Full Wolf Moon rose that evening, it shone from 245,560 miles (400,000 kilometers) away, nearly 10,000 miles farther than average. This month's Full Buck Moon will appear 17% larger and 1.3 times (30%) brighter than that January moon.
While those numbers are significant, you really have to train your eye to see the difference because we have no way to compare one full moon to another, since we can only see one moon at a time. If you could view them together (panel of photos above) the difference would be immediately obvious.
When the moon rises this Wednesday evening compare it to a natural or human-made feature in your environment. During the next distant micromoon on Jan. 6, 2023, make the same comparison. Can you see a difference?
For U.S. observers the full supermoon will rise shortly after sunset July 13 . Look for it well to the south of east. The moon's orbit is inclined 5.1° relative to Earth's orbit, causing it to weave above and below the ecliptic during the month. What's the ecliptic? Simple really. Just the plane of Earth's orbit writ large in the sky.
When the moon hovers above Earth's orbital plane, it's above or north of the ecliptic. When it shines south of the ecliptic, it's below it. Right now, the moon is near its southernmost point. That's why it rises so far to the south and keeps low in the sky for northern hemisphere observers.
I'm always game for a moonrise, and summer is the most comfortable time of the year to see one. I suspect we share that passion. Find a spot with a fabulous view to the southeast, check your local rising time and then sit back. Let the moon do the rest.
"Astro" Bob King is a freelance writer for the Duluth News Tribune.