Solar Eclipse August 21, 2017

1. OUR SUN

The sun is the closest star to our beautiful planet, and its warmth sustains life on Earth. Without its enormous and constant emission of energy, life would not be possible. At a distance of 93 million miles, its light and heat takes about 8 minutes to reach us, traveling at the enormous speed of 300.000 km per second.

Our own galaxy, the Milky Way, contains billions and billions of stars. The distances are unfathomable: the closest star after our sun is Proxima Centauri, at 4.25 light years. This means that the light from this star, traveling at that incredible speed, needs well over four years to reach us. Our galaxy measures 100.000 light years in diameter. And there are billions of other galaxies, each with billions or even trillions of stars. The total amount of stars in the universe is a number we can hardly imagine and their distances are beyond staggering. Let’s keep things in perspective here and focus on our own star, the sun.

2. A SOLAR ECLIPSE

Annular Eclipse

A solar eclipse occurs when the moon moves right in between the earth and the sun. The sun is much further away than the moon (about 400 times as far) but since it is also so much larger they appear to be the same size in the sky. Consequently, the moon can cover the whole sun as seen from Earth. Since the orbit of the moon around the earth is elliptical, it will easily cover the sun at its closest point to earth, but not quite at its furthest point. In that case, we will see an annular solar eclipse: the outer part of the sun will still be visible as a ring of light.

The graphic below depicts the geometry of a solar eclipse showing the umbra and penumbra shadows cast onto the surface of the earth.

Total-Eclipse-Depiction
Image Credit: Wikipedia Commons

The upcoming total solar eclipse of August 21st will be visible from almost anywhere in the USA, however only a small 70 mile wide strip, moving from Oregon all the way to South Carolina, will be showing the full totality. This totality zone is by far the most spectacular place to watch the eclipse. The rest of the country will show a partial eclipse, and the further away you are from the totality zone, the smaller the part of the sun that will be covered by the moon. At the very center of the zone, totality will last well over two minutes. Here are some helpful links:

Nice website that shows the path of totality in Oregon

Cities and towns (down to the tiniest hamlets) in the totality zone

3. EQUIPMENT

Looking at the sun with your naked eye is dangerous and will cause permanent damage if you do so for any prolonged time. Therefore the first important tool to observe and photograph the sun is a solar filter. This filter blocks 99.999 % of the light – in other words, it passes only 1/100,000th of the total sunlight. You look through the filter at anything except for the sun and you’ll see exactly nothing.

For simple viewing, you can buy solar eclipse glasses: they are available at the larger camera stores or online (Ebay, etc) for only a few dollars each. Of course, since the sun is fairly small in the sky, you will not see any fine details like sun spots, but you will certainly be able to witness the progress of the eclipse. If you are lucky enough to be in the totality zone, there is a moment where it is safe to take the glasses off and look directly at the sun: that is when the sun is fully obscured by the moon. Not before!! And careful when the solar disc reappears from behind the moon, make sure to put them right back on.

Solar eclipse 1999In those magical one or two minutes of totality, you will see the corona – the atmosphere of the sun. It will be so dark that some of the brightest stars and planets will be visible as well. The corona still gives a lot of light right around the obscured disk of the sun, and with a long telephoto lens or telescope, it will be possible to take some amazing photographs.

Your eye is able to see a much wider range of contrast in the corona than your camera. Close to the sun, the corona is the brightest and with some good magnification you may even see the reddish solar prominences (or filaments): arcs of gas that erupt from the surface of the sun. Prominences can loop hundreds of thousands of miles into space, held above the sun’s surface by strong magnetic fields. They can last for months and at some time will erupt, spewing enormous amounts of solar material into space. Further away from the sun, the corona gets fainter and needs longer exposures to be photographed. Therefore, it is advisable to use bracketing when photographing the eclipse during the totality phase: this means that the camera will choose a few different exposures shot in rapid succession. These can later be combined in Photoshop to show the full extent of the corona.

 Binoculars are extremely helpful for observing an eclipse. Again, strong solar filters will be needed in front of them to protect your eyes. One of the least expensive ways to safely cover binoculars, telephoto lenses and telescopes is using Baader solar film. This is an entirely safe, very thin foil that you can tape in front of the lens or attach with rubber bands – it does not need to be 100% flat, so a few wrinkles in the foil will not affect the resolution. Don’t try to stretch it, just let it loosely sit in front of the lens. For quick and easy removal (crucial for the totality phase, where you don’t want the filter on) it is recommended to have a filter holder. Many online sites (Ebay for sure) sell Baader solar film in different sizes and filter holders, depending on the size of your lenses for very reasonable prices. Sturdier and more expensive are the glass solar filters, which will do the same job but are more durable. They are also easily available online.

As far as cameras are concerned, almost any camera with a long lens (at least 200mm) will be good for shooting the eclipse, provided you use a sturdy tripod. I recommend a good DSLR, Nikon, Canon, Sony – all good! Lenses in the 300 to 500mm range would be my first choice. Tele-converters will multiply your focal length by a factor 1.4x or 2x – they can be very helpful if you don’t own a long telephoto lens. With the solar filters attached, you will be looking at exposures of 1/500 to 1/2000 sec at ISO 100 at f/8, depending on how bright you want the sun to be and the transparency of the sky. I strongly urge anybody who wants to photograph the sun to do exposure tests with your system and decide what works best for you.

When it comes to totality, the exposures will vary hugely depending on the part of the solar atmosphere you want to capture. At ISO 400 at f/8 here are some basic guidelines (note: this is without the solar filter):

Solar prominences: 1/1000 sec
Inner corona (3 degree field): 1/250 sec
Outer corona (10 degree field): 1/4 sec

Note the large difference in exposure between the inner and outer corona. This is why bracketing is a very good idea. If you have any experience using Photoshop or shooting HDR, this will be putting your skills to the test. The very best images of a total eclipse invariably have been made with small telescopes or long telephoto lenses, combining different exposures into one final image.

No matter what your level is of photographic expertise, a total solar eclipse will offer a unique, once-in-a-lifetime experience. Even with a small point-and-shoot camera, you can beautifully capture the quickly changing light in the seconds before and during totality. A tripod is recommended because of the long exposures; also make sure to turn off your camera’s electronic flash. Another eclipse effect that is easily captured is the diffraction of the sunlight through foliage of the trees and other small holes, like a straw hat: they act like pinhole cameras and each one projects its own image of the eclipsed Sun. You will see hundreds of little crescents, depending on the stage of the eclipse, projected on the ground or a wall. It’s a very cool effect that can only be seen during an eclipse.

Wishing you a great solar eclipse experience in August!

Martin Cohen, May 2017

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