Full Thunder Moon And A Spotted Sun

A charcoal sketch of the full moon as viewed in binoculars or a small telescope by Frank McCabe

Full moon tonight! The July moon had a couple traditional names: Full Buck Moon, after buck deers’ antlers that are now nearly fully grown, and the Thunder Moon, for the thunderstorms common in the summer months. Both have been regular sights and sounds in northern Minnesota this year.

I’m looking forward to tonight’s moonrise, hoping to catch sight of the hot, airless globe rising from the cool waters of Lake Superior, a delicious contrast. If you’d like to know the time of moonup for your town, go to the Moonrise and Moonset Calculator and type in your location. Works like a charm. Moonrises define the word beautiful but also demonstrate better than anything else how Earth’s atmosphere affects a celestial object.

Tonight’s Full Thunder Moon rises in eastern Sagittarius. Saturn lies about 2 1/2 fists to the moon’s upper right. Created with Stellarium

Air thins with altitude, so the higher up you look, the less air you look through. The bottom 10 miles of atmosphere, called the troposphere, contains 80% of the total atmosphere. The closer to the ground you go, the thicker or denser the air. So you can easily imagine that when you’re looking at something near the horizon, you’re seeing it through hundreds of miles of the thickest air possible. It’s also some of the “dirtiest” air, littered with natural materials like dust, sand, salt particles, volcanic ash, aerosols released by forests and fields and manmade products such as soot, car exhaust and sulfur dioxide.

This spectacular sequence illustrates how the thick layers of the lower atmosphere not only scatter the blue and green light from the moon, making it appear orange, but how air layers of different densities (thickness) can further warp the moon’s appearance. Credit: Randy Scholten

The particulates scatter away the blue and green light from the rising moon, the reason it appears such a lovely orange.  The dense air also acts like a prism and bends the light that passes through it the same way (but to a lesser degree) that water bends light. Ever try to grab a fish in water? You might miss on your first try, since the fish is deeper down than it appears. That’s because the light from the fish gets bent or refracted when it leaves the water and enters the air, a different medium, throwing you off. Spearfishers take note!

When light passes from one medium to another (water to air / air to water) or from denser air to thinner air, it gets bent or refracted. Refraction causes this “split” pencil. Credit: Bob King

Air near the horizon is like that, too. Denser air closer to the horizon refracts or bends the bottom portion of the moon more than the thinner air at the top. This makes the bottom of the moon appear to “squeeze” into the top. You know what you get when you squeeze a circular object, right? An oval! The moon looks less squished the higher and higher it climbs in the sky because the air along your line of sight thins rapidly and refraction effects lessen.

Shifting winds and air density changes along your line of sight to the moon can add their own distortions and ripples. These and other atmospheric effects make watching each moonrise fun and educational.

The difference in the moon’s shape between horizon and zenith is dramatic! Credit: Bob King

The moment of Full Thunder Moon occurs at 11:06 p.m. CDT. Because this happens before sunset for most of the Americas, the moon will rise a little before sunset. Tomorrow, one day past full moon, moonrise occurs shortly after sunset.

Sunspot group 2665 is the only sunspot on the sun’s face today, but it’s a big one. Credit: NASA/SDO

We also have some solar news. First, there’s a rapidly growing sunspot group that’s just come into view. It’s a beauty. With my #14 welder’s glass, I checked the sun this morning and with a little effort could see region 2665 as a very small dark fleck without optical aid. Although it’s not producing flares yet, its explosive growth could change that.

The other solar-related news is the aurora forecast. We have a minor G1 storm forecast to begin at nightfall and continue to dawn sparked by a high-speed solar wind blowing our way from a coronal hole in the sun’s atmosphere. But with a full moon painting the sky with light all night, this might be hard or impossible to see. If the northern sky appears to be glowing, you can double check if it’s from moonlight or true aurora by taking a time exposure photo. If the image shows a deep blue sky, it’s moonlight. If there’s green in there, the aurora’s underway.