Moon shadow
A view of Earth captured during the March, 2016, eclipse by the Japanese weather satellite, Himawari. (JMA)

Eclipse 2017: The Celestial Shadow Show

What will cause the Sun to black out on August 21 and why doesn’t it happen everywhere?

The big day (August 21) is just around the corner and eclipse watchers everywhere are getting ready to enjoy the spectacle. But what’s really behind this wondrous celestial shadow show?

A total solar eclipse occurs when the Moon passes directly in front of the Sun. For a few precious moments, the Sun’s pearl-white corona and red prominences encircle the blackened solar disc. This magnificent spectacle is possible only because the Sun and Moon appear almost the same size. In other words, their angular diameters are nearly identical thanks to an amazing coincidence: The Moon is roughly 400 times smaller than the Sun, but it’s approximately 400 times closer to us. If the numbers for size and distance weren’t in balance, the Moon wouldn’t precisely cover the Sun to create a total eclipse.

Moon shadow
This view of Earth was captured during the March, 2016, eclipse by the Japanese weather satellite, Himawari. Although the Moon’s penumbral shadow appears large in this image, the region north of Australia experiencing totality (in the umbra) the instant this image was recorded, would be just a tiny dot at this scale. (JMA)

The highly anticipated total eclipse of August 21, 2017, will begin when the tip of the Moon’s conical shadow (the umbra) touches the Pacific Ocean, north of Hawaii. At that point, the oval umbra will be five dozen kilometres wide. Racing essentially southeastward at more than 3,000 kilometres per hour, the umbra will make landfall at Oregon, then traverse the contiguous United States, exiting at South Carolina. The dusky oval will almost double in size during this time, then slowly shrink before lifting away from Earth in the eastern part of the Atlantic Ocean, off the coast of Africa.

SkyNews-SO17-eclipse diagram
For a solar eclipse to happen, the Moon’s long conical shadow must reach Earth. The lunar umbra, which produces the brief period of totality, covers only a small patch of the Earth’s surface at any given moment. The lunar penumbra, where a partial eclipse is visible, spreads over a much wider area. (SkyNews)

Anyone located within that long, narrow path of totality will experience the rare sensation of a complete solar blackout. It will last mere seconds along either edge of that corridor but will persist for many dozens of seconds near its centre. The maximum possible length of totality—2 minutes 40.3 seconds—will unfold right on the centre line at the path’s halfway point, in southern Illinois. There, the crucial strip of darkness will be 115 kilometres wide.

An Important Word About Safety

Proper eye protection is mandatory for viewing any solar eclipse. (Gary Seronik)

You must use a solar filter made specifically for the purpose of viewing a partial solar eclipse. (It’s safe to look at the Sun without a filter only during totality—when the disc of the Moon completely covers the Sun.) Never use photographic filters, sunglasses, exposed film, X-ray film, household Mylar, CDs, etc. The smoked glass your dad made for the eclipse you remember seeing as a child? Unsafe. While all these make the Sun appear somewhat dimmer, they may pass ultraviolet and infrared light, which could permanently damage your eyes. Play it safe: Use an eclipse viewer.

If you plan to use a telescope or binoculars to view the event, you’ll need a specialized filter designed to fit over the front aperture of the instrument. Never use eclipse glasses intended for naked-eye viewing in conjunction with any optical device!

David Rodger, the founding director of Vancouver’s H.R. MacMillan Planetarium, has been explaining astronomy to the general public in person, in print, in planetariums and observatories and on television, for almost 60 years.