Tuesday, February 14, 2017

Get Ready for the August Solar Eclipse!

On August 21, 2017, in some parts of the United States around mid-day, the Sun is going to disappear. Then, about 2 minutes later, it is going to come back. How is the Sun going to accomplish this “vanishing trick?” On that day, the Moon will move into just the right position to completely block the disk of the Sun seen from parts of the United States. We call this a total solar eclipse.

What is a Solar Eclipse?

Solar eclipses happen when the Moon passes between the Sun and the Earth. The Moon doesn’t sit perfectly on a line drawn between the Sun and the Earth. Instead, it orbits at an angle of about 5 degrees with respect to that line.

Image Credit: SMU Physics
Usually when the Moon passes between the Sun and the Earth it will pass above or below the Sun as seen from the Earth because of that angle. However, when the Moon is at just the right spot in its orbit at just the right time, it will actually block the Sun as seen from the Earth. The Earth passes into the Moon’s shadow. The Moon is a lot smaller than the Earth, so the shadow the Moon casts is also a lot smaller. In fact, the Moon is so far away, and so small, that the darkest part of its shadow, called the umbra, just barely reaches the Earth. If an observer is standing on the Earth in the path of the Moon’s umbra, they will see the Moon totally cover the disk of the Sun. Astronomers call the line traced by the Moon’s umbra the “path of totality.”

Image Credit: The New Student's Reference Work, 1914

But how can that be? Isn’t the Sun really, really big? And the Moon, it’s even tinier than the Earth! How could it cover up something as big as the whole Sun? It is all about being at the right place at the right time. The Sun is about 400 times bigger than the Moon, but the Sun is also about 400 times further away from Earth than the Moon is from the Earth. From the surface of the Earth, the Sun and Moon look nearly the same size in our sky, even though if we could place them side by side it wouldn’t even be close.

Why is this eclipse special?

Total solar eclipses are not incredibly rare. They occur every few years. However, the path of a total solar eclipse is tiny compared the surface of the Earth, so an eclipse being visible from any particular point on the Earth is quite rare. The last total solar eclipse to happen in the United States was in 1991 (and then only in Hawaii), and the next one won’t occur until 2024.

What makes this solar eclipse special is that the path of totality stretches across all of North America, from Oregon to South Carolina. This means that nearly every person in the United States will be within a day’s drive from seeing a total solar eclipse.


Anyone standing in the thick yellow line in the middle of the map will see a total solar eclipse. Those outside the yellow line will not be able to see Moon completely cover the Sun, and will only see a partial solar eclipse.

Where can I see the eclipse?

The closest place to Bay City to see the total eclipse will be in southern Illinois, about 600 miles away. That is pretty far away, but if you do make the journey you will be treated to a special sight that not many people get to see.

Maybe southern Illinois is too far away, is it even worth trying to see the eclipse from Bay City? Yes! While you won’t be able to see the Moon completely cover the Sun from Michigan, you will be able to see the Moon cover nearly the entire disk of the sun. This is a partial solar eclipse. The Moon will cover over 80% of the Sun at the height of the eclipse from Bay City. For those that want to see the total event but can’t get to the path of totality, or those that find themselves looking up into clouds, the eclipse will also be streamed live on the internet.

How do I view the eclipse safely?

The first rule of viewing an eclipse is never look directly at the Sun. Looking directly at the Sun can permanently damage your eyes.

The second rule is NEVER EVER look at the Sun through any kind of magnifying device (such as binoculars or a telescope) that does not have a proper solar filter securely installed on it. Looking at the Sun through an unfiltered telescope WILL damage your eyes, quickly and permanently. If you are not sure if something is a proper solar filter, do not use it.

Eye protection when viewing the Sun is very important. Although invisible, sunlight contains dangerous infrared and ultraviolet rays. These rays of light are damaging to the human eye, so observers need to take special precautions. Solar eclipse glasses are an inexpensive option. These glasses are made with a thin plastic filter that blocks out nearly all the of the Sun’s light, including all of the dangerous rays, and allows a person to look directly at the Sun safely. We sell them at the Delta College Planetarium Gift Store for $1 each.

Do not use sunglasses! They will not block enough of the harmful rays of the Sun to protect your eyes. No, two pairs isn’t good enough either. There is no number of sunglasses you could wear to protect your eyes from the harmful rays of the Sun.

Image Credit: flickr user Mr.TinDC, CC-BY-ND 2.0
Special solar filters for telescopes and binoculars can be used to view magnified images of the Sun. Only use filters that cover the entire aperture of the telescope. Never use a solar filter that attaches to an eyepiece, as they are prone to failure and can quickly damage your eyes. Not sure if your filter is a solar filter? Ask an astronomer. Always inspect your filter for cracks and pinholes before using. Never look through a telescope with a filter you are not 100% sure will protect your eyes.


But what if you don’t have solar eclipse glasses or a proper solar filter for your telescope or binoculars? There many indirect viewing techniques you can use. You can build a pinhole viewer out of a box and some tin foil. The viewer will project an image of the Sun on to another surface that is safe for viewing. You can also use a telescope to project an image of the Sun onto another surface, but there are a couple of things to consider before using this method. All that energy from the Sun’s light is passing through the lenses or reflecting off the mirrors of the telescope. This could cause them to heat up and crack, damaging the telescope. There will also be a dangerous beam of light from the telescope’s eyepiece to the projection surface where all of the harmful rays of the Sun will be concentrated. Do not stick your eye or skin in that beam, as it could injure you easily. With those considerations in mind, telescopic projection can be a safe technique for viewing a magnified image of an eclipse.

What will I see during the eclipse?


Image Credit: Tom Ruen, CC BY-SA 4.0
The Moon will slowly creep in front of the Sun. It will take the Moon nearly 90 minutes to cover the Sun. For most of the process, there will be no noticeable darkening of the surrounding area.




Image Credit: Arief R. Sandan (Ezagren), CC BY 1.0
About 15 minutes before totality, it will begin to get noticeably darker outside. As the Moon moves to cover the entire Sun, the remaining portion of the Sun will look as though it were a string of beads. These are called “Baily’s beads,” and are caused by portions of the Sun still being visible between mountains on the Moon. When only one Baily’s bead remains, the eclipse may resemble a diamond ring.




Image Credit: Luc Viatour / www.Lucnix.be, CC BY-SA 3.0
When the moon finally covers the sun, it will get very dark out. The stars will be visible, and the surrounding horizon will look like sunset. Around the edge of the Moon, you will be able to see the Sun’s corona, the outer layers of the Sun’s atmosphere. Usually the Sun is so bright that it washes out any view of the corona, but because of the eclipse you will be able to see this feature with your own eyes. During totality, the eclipse is safe to look at without eye protection. But still take care, because as soon as any part of the Sun reappears from behind the Moon, so do those dangerous infrared and ultraviolet rays. The exact length of the totality will depend on your location, but for most places in the path of totality it will be longer than 2 minutes.


Then, as the Moon moves along its orbit, the Sun will peak out, spilling its light all around. Baily’s beads will be present again, and it will take about 90 minutes for the Moon to exit the disk of the Sun.

Where can I learn more about the eclipse?

Stop by the Delta College Planetarium. We’re always ready to answer your questions. Our new show Eclipse: The Sun Revealed, opening Saturday, May 13, is all about the science and history behind solar eclipses.



Another fantastic resource is GreatAmericanEclipse.com. There you will tons of great maps and information leading up to the August 21, 2017 solar eclipse.