Sunday, March 19, 2017

The Big Eclipse - Part 5: Totality

As we approach second contact (the start of totality), things start happening pretty quickly.  One phenomenon to look for a minute or 2 before totality is the shadow bands.  These are very subtle wavy bands of alternating light and shadow similar to ripples seen on the bottom of a swimming pool on a sunny day.  The best way to look for them is to put a white sheet or large (several square feet) piece of white paper on the ground.  It's unclear exactly what causes this, but it's thought to be an effect of the very thin sliver of the sun's surface causing a diffraction effect.  Photos and a good explanation can be found here.

Baily's Beads
As totality finally approaches, you may start to see the corona appear on the side of the sun away from the last spot of bright light.  The corona is part of the sun's outer atmosphere and is always present but is much too faint to see in the glare of the sun's surface.  Only when the surface is covered as during an eclipse can you see this faint wispy part of the sun.  On the side where the sun's surface is still visible, you will begin to see alternating dark and bright areas instead of a smooth bright crescent.  This is caused by the mountains and valleys on the edge of the moon blocking parts of the sun (mountains), while other parts are still visible through the valleys.  These are known as "Baily's Beads".  I should remind you that you should still be using your eye protection at this point since some small part of the sun's surface is still visible.  However, you can take the filter off of your camera if you want to photograph Baily's Beads and the Diamond Ring.  Just don't look directly through the view finder of an SLR camera yet.

The Diamond Ring
The corona, combined with the last piece of the sun's bright surface comprises the so-called "diamond ring" effect, which occurs immediately before totality.  The last glimpse of the surface is the diamond, and the corona forms the ring.  Again, it's OK to quickly glance up to see the corona and diamond ring, but don't look directly at the diamond.

Finally, the last bit of the sun's surface disappears behind the moon and you will see the corona completely surrounding the dark moon.  It is now perfectly safe to stare at the eclipse without eye protection.  Now is also a great time to use binoculars to study the eclipse without the need for filters.  However, you must keep track of the time so you aren't looking through binoculars when totality ends.  For safety, I generally stop using binoculars at least 30 seconds before the predicted end of totality for my location.

Now is the time to enjoy the beauty of the corona.  Is it round or elongated?  Does it seem evenly illuminated or are there streaks and streamers?  While using binoculars, look for red prominences along the very edge of the moon.  These are streams of hot glowing gas that rise off the surface of the sun.  During the 1991 eclipse, the prominences were so large that it was possible to see them even without binoculars.  Also take time to look around you during totality.  It's twilight that doesn't seem quite right.  The horizon in every direction looks like sunset.  Also look for planets in the sky.  Venus and Jupiter should be quite easy to see, and you may catch Mars and Mercury too, if you know where to look.

All too quickly the moon will move off the surface of the sun, resulting in a second diamond ring.  The whole thing will then play out in reverse, starting with the diamond ring, then Baily's Beads, the shadow bands, and the moon slowly leaving the sun.  Be sure to put the solar filter back on your camera at this point, and of course use appropriate eye protection for the remaining parts of the eclipse.

My next blog entry will discuss some pointers if you want to take pictures during the eclipse.  However, my best piece of advice for photographing your first total eclipse is this: don't do it.  There will be many, many pictures available on the internet after the eclipse, but only your eyes can register the grandeur that will stay with you for the rest of your life.  This is a relatively short eclipse (at most 2.5 minutes, and considerably less depending on where you are), so you should spend most of your time just using your eyes and binoculars.