The final “super moon” of the year will appear tonight above the eastern horizon as a large orange globe after 7:16 p.m.
The orange tint is an actual physical effect. The larger-than-usual size of a moon seen near the horizon, however, is mostly the result of a trick your eyes play on you, said Jim Todd of the Oregon Museum of Science and Industry.
It’s called “the moon illusion,” said Todd, OMSI’s director of space science education, and it has puzzled sky watchers for thousands of years.
According to NASA’s website, “when you look at the Moon, rays of moonlight converge and form an image about 0.15 mm wide on the retina in the back of your eye. High moons and low moons make the same-sized spot, yet the brain insists one is bigger than the other. Go figure.”
The moon’s orange color has a scientific explanation, Todd said. When looking toward the horizon, we are actually looking through a greater thickness of Earth’s atmosphere than when looking directly overhead. The atmosphere scatters blue light (the reason the sky looks blue). The thickness of the atmosphere in the direction of a horizon scatters blue light most effectively, but it lets red light pass through. So a moon near the horizon takes on a yellow, orange or reddish hue.
The moon may actually appear be a darker shade of reddish-orange because of particles in the upper atmosphere from the summer fires around the Pacific Northwest.
A so-called “super moon” occurs when the full moon reaches its closest point to Earth in its orbit. At 10:30 p.m. today, the full moon will be just 222,692 miles away from Earth. The moon was 222,612 miles at perigee for the July super moon and 221,208 miles in August. The moon will not be so close again until the full moon of Sept. 28, 2015, at 221,197 miles.
The closest point — perigee — is about 31,068 miles closer to Earth than the point of apogee, or farthest distance.
