by Kim Malville

This is the month of the Great American Solar Eclipse on August 21. First, let’s see what is in the sky before that event.

August 2: Saturn will be below and to the left of the moon.

August 7/8:  Full moon. This is the eclipse season when often there are connected eclipses of both moon and sun, separated by two weeks. In anticipation of the approaching solar eclipse, there is a partial lunar eclipse in practically every part of the world but the Americas: eastern Europe, Africa, Asia, and Australia.

August 11-13: Perseid meteor shower. The moon rises before midnight which will cut into the visibility of fainter meteors.

August 21: New moon and total eclipse. I hope you get to view it!

August 25: Soon after sunset, the crescent moon forms a triangle with Jupiter and Spica.

The Great American Total Eclipse

The Great American Eclipse will be on August 21. Here is a superb picture of the solar corona taken during the total eclipse of 2008. See story in Skies Over Crestone page B-2.

The total solar eclipse begins August 21 near Lincoln City, Oregon, at 10:15am PDT and ends on the American continent at 2:48pm EDT near Charleston, South Carolina. The total eclipse itself will take about one hour and 40 minutes to cross the country, the first total solar eclipse in the continental U.S. in 38 years. This eclipse will be the most-viewed ever. If you plan on driving to the center line, try to arrange getting there at least a day ahead of time. Highways will be packed with cars on the morning of the eclipse.

A solar eclipse occurs because the moon casts a shadow on the earth, and can only occur at new moon. Total eclipses occur frequently on Mars when one of its moons casts its shadow. Eclipses are also common on the cloud tops of Jupiter due to its four large inner moons.

Everyone in the continental U.S. will see at least a partial eclipse. The least coverage will be 48% at the northern tip of Maine. In Crestone, the moon will cover 87.7% of the sun. Good, but no cigar. You should head north. In Breckenridge, it will be 91.1%; in Boulder, the moon will cover 93.4%. Moving further north, coverage will be 97.6% in Cheyenne. On the center line at Casper and Douglas it will reach 101%. Further to the east in Alliance Nebraska, coverage will also be 101%.

Don’t be fooled by these high numbers in the 80 and 90 percentages. The difference between 100% and 99% is like standing outside an opera house while the performance is going on inside, or it is almost dying vs dying.  Only the total eclipse is the ultimate experience. Only then can you observe the pearly white whips of the mysterious corona, still mysterious after millennia of humans watching eclipses, all the more mysterious because it appears as if in a dream, briefly seen, but not easily forgotten.

Total solar eclipse 2008.

Total solar eclipse 2008.

In the 19th century it was thought to be a cloud of dust surrounding the sun, perhaps due to falling meteors and asteroids. Then, in the early 1900s, because its appearance changed with the cycle of sunspots, its connection with the sun seemed obvious. Some astronomers thought the loop-like structures of the corona was produced by volcanos on the sun. One great puzzle was why its structure looked so much like a magnetic field. (see the figure). A cloud of dust or neutral gas shouldn’t be influenced by a magnetic field. Finally, around 1940, the emissions lines of the corona were found to be produce by highly ionized particles of iron, stripped of many electrons by a very high temperature, some two million degrees. One mystery was replaced by another. The surface of the sun is 6000°. How could its outer atmosphere be so much hotter? The answer lies in the magnetic field that surrounds the sun. The magnetic field flutters in the gas flowing outward, and it gets tangled. Electric currents flowing in the magnetic field get short circuited producing heat. Fires get started in houses with short circuited wires; the corona somehow gets heated in a similar manner. But exactly how is still a puzzle.

Try very hard to find eclipse glasses. In Boulder, you can find them in McGucken’s Hardward for two dollars. Or get a welding glass filter. If all else fails, make a pin hole projection device with a carboard box and a piece of aluminum foil. As you view the beginning through a safe solar filter, the first hint that an eclipse is coming will be when the moon takes a small bite out of the edge of the sun, known as “first contact”. When more than three-quarters of the sun is covered, you’ll notice that shadows are getting sharper. The reason is that the Sun’s disk is shrinking and the smaller light source produces better-defined shadows. At about 85% coverage, you might see Venus appearing 34° west-northwest of the Sun. (A clenched fist at arm’s length is approximately 10°.) If trees are nearby, you should see their leaves act like pinhole cameras as hundreds of crescent Suns appear in their shadows. This is a truly magical time. Look on the ground for shadow bands, which are produced by waves in the earth’s atmosphere. The experience will be like watching waves in the bottom of a swimming pool. The chance of seeing shadow bands will be increased if you lay out a large piece of white cardboard or sheet on the ground ahead of time.


The sun’s magnetic field.

The sun’s magnetic field.

During the time the moon’s disk covers that of the Sun, totality, it’s safe to look at the eclipse. It will be as bright as a full moon. In fact, to experience the glory of the event, you must look at the sun without a filter during totality. But be very careful, and put your eclipse glasses back on as soon as the edge of the sun appears (third contact). During totality see how far you can trace a streamer out from the sun. Look for the very fine streamers at the north and south poles of the sun. (see the photographs). Watch for the diamond ring, red prominences at the edge, and any possible disturbances in the corona.

The next total solar eclipse over the continental U.S. occurs April 8, 2024. It stretches from Texas to Maine, and the duration will be more than 4 minutes in southwestern Texas. After that eclipse, it’s a 20-year wait until August 23, 2044, visible only in Montana and North Dakota. Don’t miss this opportunity!