Viewers of last week’s solar eclipse were mesmerized by stunning celestial phenomena along the path of the event during totality. But those watching from Montreal saw a couple of additional surprises in the half-hour before and after the eclipse, when the moon blocked the sun on April 8.
The first was an unusually sharp shadow caused by a passing plane shortly after the total eclipse concluded. The second came in the form of a spectacular halo around the partially eclipsed sun.
The plane that flew over Montreal during the partial phase of the eclipse left a typical contrail in its wake. When this happens in bright sunlight, the shadows cast by trails on clouds are usually too diffuse to see. In this case the sharpness of the shadows was explained by the eclipse.
“Shortly after totality, the sun is a thin crescent that tends to make shadows much sharper, thus accentuating the shadow of the jet’s contrail,” said Fred Espenak, a retired NASA astrophysicist nicknamed “Mr. Eclipse” for his knowledge of the events.
Although the shadows of the contrails appeared to be on a cloud layer above the aircraft, as if cast by light reflected from the Earth, this was an illusion. The shadows were created by sunlight, cast downward onto the clouds below.
The halo around the sun, however, was not related to the eclipse. This type of circular ring of light, which appeared both before and after last Monday’s total eclipse, is called a 22-degree solar halo.
Such halo displays occur very frequently – more than 100 days a year, according to Walter Tape, a professor emeritus of mathematics at the University of Alaska Fairbanks who has studied the phenomenon extensively.
Solar halos are optical illusions created when ice crystals form into the correct shape in the upper atmosphere, usually accompanying thin, thin cirrus clouds. At the right shape and angle (note the “22 degrees” in the halo’s name), the crystals reflect and refract incoming sunlight, sometimes resulting in multiple and even multicolored rings around the sun or moon.
Although they are most commonly seen in the polar regions, these displays can be seen from anywhere in the world. You may be more likely to spot bright halos of the sun during an event like an eclipse while you’re already looking skyward, but if you keep your eyes open you may witness another one. Just make sure you don’t look directly at the sun while looking for it.
