Everything You Want to Know about the Solar Eclipse

On August 21st of this year, occupants of the United States will have a once in a lifetime opportunity to witness an incredible event – a total or partial solar eclipse.  Depending on where you reside on the globe, a total eclipse may only be visible from your hometown somewhere between every 1 and every 15 centuries.  The path traced by the August 2017 eclipse won’t occur again for nearly 600 more years.

What is a Solar Eclipse?

As you may remember from your school years, a solar eclipse occurs when the moon passes between the earth and the sun, blocking the sun’s rays.  Although the sun’s diameter is approximately 400 times greater than that of the moon, the sun is also around 400 times further from the earth than the moon is – giving us just the right conditions for the moon to obscure the sun, if it happens to be passing directly between the two.

An image showing the moon passing between the earth and sun, blocking the sun's light.
Despite the moon being much, much smaller than the sun, the sun is so much further from the earth that the moon can completely hide the sun. This is a bit like holding your thumb out in front of you at night and hiding the moon.

For more on this topic, check out this article at nasa.gov.

What’s so Special about the 2017 Solar Eclipse?

In comparison, consider a lunar eclipse.  Lunar eclipses occur 2-4 times each year, and when one does occur, everyone on the night side of the earth will be able to view the event.  That is because the earth is significantly larger than the moon; its shadow can cover the moon completely.

In a solar eclipse, the darkest part of the moon’s shadow – the umbra – traces a path over the surface of the earth, but the moon is not large enough to cover the entire earth with its shadow.

A colored map showing the projected path of the 2017 eclipse across the United States.
The projected path of the 2017 eclipse across the United States.

As a result, there is only a comparatively small portion of the earth’s surface on which a total solar eclipse can be observed.  The area of the earth that will experience a total solar eclipse is referred to as the “path of totality.”  Cool name.  Even though a solar eclipse occurs somewhere on Earth every 18 months or so, only areas in the path of totality will experience a total solar eclipse; unlike lunar eclipses, the moon is not large enough to block the sun’s light across all of Earth.

To see whether you’re in the path of totality for the 2017 eclipse, check out this list of cities by eclipse2017.org.

How Much of the Eclipse Will I Be Able to See?

Even if you’re not in the path of totality, you may still be able to witness a partial solar eclipse.  In my home town of Louisville, KY, for instance, we are about a 2 hour drive from the path of totality… but we will still be able to experience a partial eclipse that will obscure more than 90% of the sun’s surface.

A map showing the projected path of totality and the bands of earth alongside the path on which partial eclipses will be visible.
Even if you’re not in the path of totality, you may still be in for a once-in-a-lifetime experience!

Cities that fall within the penumbra will see the sun as partially obscured, with more of the sun blocked the closer that you live to the path of totality.  Check out this NASA video on YouTube for an idea of where the shadows fall, or to see what the eclipse will look like from your home town, check out this awesome Eclipse Calculator from timeanddate.com.

What Do I Need to Enjoy the Eclipse Safely?

You need solar eclipse glasses!  Or you could bring your favorite welding helmet, if you have one.  During and shortly before and after an eclipse, it may be tempting to stare at the sun with your naked eye, and it may even be painless to do so – but unfortunately, damage to your retina may also be painless.  Don’t risk your or your family’s vision; use some appropriate means to ensure that you can enjoy the eclipse safely.

Sadly, a rash of counterfeit solar eclipse glasses have hit the market.  Glasses that will protect your eyes need to be rated in accordance with ISO 12312-2 – “Filters for direct observation of the sun.”  Some glasses have hit the market, however, that claim to be ISO 12312-2 rated but are not.  If you have a pair about which you’re unsure, you can follow these guidelines from the American Astronomical Society:

You shouldn’t be able to see anything through a safe solar filter except the Sun itself or something comparably bright, such as the Sun reflected in a mirror, a sunglint off shiny metal, the hot filament of an unfrosted incandescent light bulb, a bright halogen light bulb, a bright-white LED flashlight (including the one on your smartphone), or an arc-welder’s torch. All such sources should appear quite dim through a solar viewer. If you can see lights of more ordinary brightness through your eclipse glasses or handheld viewer, and you’re not sure the product came from a reputable vendor, it’s no good.

Looking for more information on solar filter safety?  Check out the full article from the American Astronomical Society.

Is It Really That Rare?

It all depends on where you live and how far you’re willing to travel, but for many of us, this may be the only opportunity in our lifetimes to witness a total solar eclipse in our home town.

An image of the earth with overlaid yellow bands, each band depicting the path of an eclipse that will occur in the 21st century.
An awesome map of the solar eclipses that will occur over North America this century. Credit Michael Zeiler, www.GreatAmericanEclipse.com.

So, yes, it really is that rare.  Would you like to know more about the frequency of solar eclipses?  Check out this space.com article, which features a very cool chart of how frequently solar eclipses occurs in various parts of North America, and also provides some backstory on the calculation of eclipse frequency – which has been shown to be around 400 years, on average, for any given point on Earth.

Living in Louisville, I may have another chance to see a solar eclipse in 2024… but I don’t want to waste this upcoming opportunity.  I picked up some solar glasses for myself, my wife, and my 3-year-old, and now I just have to figure out how to take off a few hours of work on the 21st.

Do you have any big plans for the Great American Eclipse?  Let me know in the comments below!

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