Make Way For The Thunder Moon

A double rainbow appeared in the wake of yesterday evening’s storm. The primary bow forms when sunlight enters raindrops, reflects off their backsides and then exits in the direction of our eyes. The fainter secondary bow forms when light is refracted (bent) twice inside a raindrop before exiting. Bob King

We had thunderstorms move through the area yesterday evening. As often happens during summer showers, the sun peeked out at the end, and shot light across the rain still falling at my house, producing a spectacular rainbow. The rainbow was fainter than most because the sun was nearly level with the western horizon and already deeply reddened. To my amazement parts of the bow were still visible as late as 9:05 p.m. — a full 5 minutes after sundown.

Perhaps you live where thunderstorms are common in the summer months. Today we’ll use them to introduce the Thunder Moon, one of the traditional names for July’s full moon. Watch for it to rise in the southeastern sky in Sagittarius Tuesday night (July 16) right around the time of sunset. To pinpoint the  time of moonrise for your location use this handy calculator. Full moons are always fun to watch. I look forward to their rising each month and try to figure out new ways to photograph them.

The stepped appearance of the bottom quarter of the rising full moon reveals horizontal layering in the lower atmosphere. Bob King

Watching a moonrise in binoculars or a small telescope reveals much about the otherwise invisible air layers in the lower atmosphere. Turbulence will make the magnified image flutter and ripple especially when viewed near the horizon. If you’re lucky, the air will be stratified in layers according to temperature and pressure and temperature like a nice, flaky baklava. As the rising moon pushes through them you can watch individual “steps” appear and disappear along the lunar circumference.

The view from Buenos Aires, Argentina in South America Tuesday evening at the end of twilight. Click here for more eclipse details for your location. Stellarium

Observers in India, Africa, Europe and much of South America will witness a partial lunar eclipse Tuesday night. The eclipse will be over by the time the moon rises for observers in North America. Too bad. But like the solar eclipse earlier this month, you can still see it via live feed. Italian astrophysicist Gianluca Masi will offer a live stream Tuesday starting at 3:30 p.m. CDT (20:30 UT) with a view of the chomped moon climbing over a Roman skyline.

This map shows where Tuesday night’s partial eclipse is visible or partially visible — white or pale gray areas — and where it is not — the dark gray zone. NASA

65 percent of the moon will soak in Earth’s shadow at maximum so skywatchers should definitely be able to see the red color of the moon’s darkened half. As well, the sky will darken enough to see the brighter parts of the Milky Way. Even though this is only a partial eclipse, it’s special because it happens exactly 50 years to the day the Apollo 11 mission blasted off for the moon.

Saturn and the almost-full moon tonight (Monday) in the southeastern sky in late dusk. Stellarium

Before you get excited about the Thunder Moon, take a look at tonight’s 99.5-percent-full moon (July 15). It will be in close conjunction with the planet Saturn, shining just a degree away. The duo will be closest around 3 a.m. CDT Tuesday morning with a separation of just ¾°. Fun!

Back to the rainbow! This photo was taken at sunset yesterday when orange and deep yellow suffused the entire sky. Bob King

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