The crescent moon was the first thing to catch sky watchers’ attention last night in the western twilight sky. What a beautiful thing. It lingered from sunset until after nightfall. I last saw it on the horizon at 11 o’clock, glowing orange like a horseshoe fresh from the forge.
Around the time of moonset the lower half of the northern sky was filled with low curtains and streaky rays of aurora. NOAA space weather forecasters predicted the display, but it was brief-lived. Aurora storm conditions started earlier that afternoon and maxed out around the time of sunset for the Midwestern U.S. Shortly after 11 p.m., the aurora began to subside, and by midnight, it had “settled” into a quiet, low arc in the northern sky.
For a few minutes, there was a fine display of rays, including the purple one pictured above. Purple is a mixture of red and blue emissions from nitrogen molecules in the atmosphere.
The most common color of aurora is green, caused by oxygen atoms “spitting out” green photons as they “relax” or return to their ground states after being energized by electrons streaming in from the sun through Earth’s magnetic field. Oxygen at higher altitudes is also responsible for the red tops you’ll sometimes see on very bright and active auroras.
Forecasters call for a continuation of aurora tonight for northern latitudes (and far southern ones), so maybe we’ll get a repeat performance.
As long as we seem to be spending a lot of time looking north lately, you might be interested to know that the Little Bear, otherwise known as Ursa Minor, balances on the tip of his tail during the month of June. Polaris the North Star represents the tail, while Kochab (KO-cab), the Little Dipper’s second brightest star, forms part of the bear’s back. Go out and have a look the next time you can. You won’t find a better act under any circus tent.
Kochab and Pherkad (FUR-cad) are sometimes referred to as the “Guardians of the Pole” as their location near Polaris confers them a sort of gatekeeper status. While not in the exalted position of North Star, Kochab’s an interesting star of its own. Look closely and you’ll notice that it appears reddish in color compared to white Polaris. That’s because it’s an orange giant star similar to the much brighter Arcturus. Only its greater distance of 126 light years vs. Arcturus’ 37 prevents it from shining as one of the brightest stars in the sky. Were we able to move in for a closer look, we’d see that Kochab is 50 times larger than our sun and 500 times more luminous.