You can hardly turn around with running into hoopla about the upcoming solar eclipse in the US but in case you missed it – the big day is Monday, August 21, 2017. The good news is that in 7 years time there will be another on April 8, 2024, but why wait?
The following is everything you need to understand and watch a solar eclipse. Yes it’s long – but it’s worth reading!
Eclipses are all about shadows. It turns out that shadows are a bit more complicated than you may have thought.
Grab a flashlight, turn it on and hold your hand between the flashlight and a wall. Check out the shadow. You can see a dark shadow of your hand on the wall (called the umbra) but you can also see a lighter shadow around the edge of your hand (called the penumbra). When the sun shines on the moon and the earth, they also have an umbra and penumbra, just like your hand.
Now move the flashlight further from and closer to your hand. Notice how the size of the shadow changes. When the flashlight is further from your hand, the shadow is smaller. The sun is like a humongous flashlight that is very far away from the earth and moon. When the moon is between the earth and sun, it’s shadow is relatively small. That is why only a small area of the earth is in the umbra – where the total eclipse occurs.
There is a third shadow as well called the antumbra, which is a bit trickier to see with a flashlight. Get a friend to help. Keep your hand a couple feet from the wall and have your friend shine the flashlight from the other side of the room. You will see that the light from the flashlight is too big to be completely blocked by your hand so that you see the shadow of your hand surrounded by the bright light of the flashlight.
So what does all of this have to do with eclipses?
An eclipse occurs when the earth and moon is lined up just right so that the shadow of one covers the other. In a lunar eclipse, the shadow of the earth covers the moon. In a solar eclipse, the shadow of the moon covers the earth. In both cases we are stuck watching them from the earth.
In a lunar eclipse, the full moon appears to go dark or turn a dark red. A lunar eclipse only happens during a full moon, when the earth is between the sun and the moon. Sometimes lunar eclipses are full – the whole moon is covered by earth’s umbra – or partial when only part of the earth’s shadow covers the moon. Since the earth is larger than the moon and relatively close, a lunar eclipse can be seen by a large part of the earth.
In a solar eclipse, the sun appears to go dark as the moon lines up just right between the sun and earth. However, because the moon is so small and we are so far away from the sun, only a small part of the earth can see a solar eclipse. Parts of the earth that fall in the umbra will see a full eclipse while other parts that fall in the penumbra will only see a partial eclipse.
Occasionally, the earth, moon and sun line up for a solar eclipse but the moon is in a part of its orbit where it is further way from the earth. In this case, the earth falls in the antumbra and we have an annular eclipse. When this happens, the moon goes across the front of the sun but doesn’t completely cover it, leaving a ring of sun around the moon when it is in the center.
In the case of the upcoming 2017 eclipse, the full shadow or umbra of the moon will move across the United States starting in Oregon and across the country to South Carolina. If you aren’t in this strip, which is only about 70 miles wide, you will be in the penumbra and only see a partial eclipse.
The coolest thing about a solar eclipse is that the moon is JUST the right size and JUST the right distance to block out the sun JUST completely. This is because the sun is 400 times binger than the moon AND 400 times further away from the moon than the earth. The result is that the moon and the sun appear the exact same size in the sky.
This really is a marvel not to be missed!
How to Watch
Rule #1: NEVER look at the sun! Never, ever – unless you want to damage your retina and go blind. Just don’t do it.
And while I’m at it – leave the camera and binoculars at home unless you have the proper filter. Even looking through binoculars or a camera with your glasses on can focus the small amount of sunlight enough to damage your eyes. Again, just don’t do it.
If you must take a photo, here are some good guidelines
The best way to watch the eclipse is with protective glasses that block out 99.99% of the sun’s light Sunglasses will not work for this.
However, if you are like a lot of people who didn’t plan this out several months ago, you may be out of luck as glasses are quickly selling out. Don’t worry; there are other ways to see the moon travel between the earth and sun, like a pinhole viewer. Grab some poster board (or even just a flattened empty cereal box) and a pin. Make a hole in the middle of the board with the pin. Stand with your back to the sun and hold the board so the sun can shine through the pinhole and make a shadow on the ground. As the moon slides across the sun, you still see the shape of your pinhole change in the same way!
And if you forget to make a pinhole viewer, you can even just find a tree or colander – anything that makes small holes will work the same way!
What to Look For
An eclipse is actually much more than the moon sliding between the earth and sun. There are a whole lot of phenomena to look for during the 3 hours it takes the moon to travel across the sun.
At the start of the eclipse, not much will change. It takes about 90 minutes from when the moon first touches the edge of the sun until it is covered totally. While you wait, look around. You will notice that the sky looks like sunset all the way around. A solar eclipse is like a 360 degree sunset! Depending on the clouds and humidity you may see vibrant oranges and reds all around.
Once the sun is over halfway covered find a light colored surface (or bring a white sheet or piece of poster board). You may be lucky enough to witness shadow bands, even if you are not in the path of totality.
Shadow bands are pale shimmery parallel shadows that seem to wiggle and vibrate on the ground. They are best viewed on a pale surface but they are not seen at every eclipse. Your best chance to see them is about 5 minutes before and 5 minutes after the sun is completely covered. Scientists believe the lines are caused by the narrow last rays of light from the sun traveling through the Earth’s atmosphere. This is the same effect that causes stars to twinkle.
As the sky gets darker you will be able to see stars and planets near the sun. Mercury is only visible during an eclipse since it is so much closer to the sun than Earth. It will be very faint and to the lower left of the sun. Venus, Mars and star Regulus can also be seen.
Take a look at your shadow. The change in light makes shadows so sharp you can see individual hairs on your shadow head.
Just seconds before totality you will see bits of sunlight shine between the craters and mountains of the moon, giving the appearance of a ring with sparkling diamonds, called Bailey’s Beads. And the, before you know it, the sun is covered. The temperature will drop – as much as 10 degrees. Listen. The birds will fall silent and even start to roost for the night.
If you are in the area of totality, take off your glasses (or turn around when your pinhole turns dark) for just a minute and witness the milky-white corona of the sun, streaming out around the moon. Then put them back on and dance as the light of the sun returns.
Eclipse Related Activities
NASA has a good collection of activities to do at home or in school from measuring the mass of the earth or lunar shadow speed. If you have a thermometer or light sensor (like those made by Vernier) you can measure the change in light and temperature during the eclipse. The American Astronomical Society also has a collection of activities here.
There are several Citizen Science projects related to the eclipse that allow you to share your observations with scientists. Check out this list from the AAS https://eclipse.aas.org/resources/citizen-science and NASA has an app that lets you collect cloud and temperature data and send it straight to NASA. Some of my other favorites include
Eclipse Soundscapes (Harvard-Smithsonian Center for Astrophysics)
During a total solar eclipse, we gaze in amazement as day becomes night. But, along with the striking visual effects, the soundscape of natural environments changes dramatically. These changes, to be recorded by a special mobile app, are not only of interest to sociologists, birders, and naturalists, but will also give the blind and visually impaired an opportunity to experience this rare celestial event.
Solar Eclipse 2017: Life Responds (California Academy of Sciences)
How does life respond to a total solar eclipse? There is evidence that plants and animals react to the dramatic changes in the environment, but many such reports are anecdotal. CalAcademy invites eclipse-watchers across the USA to use the iNaturalist app (available for both iOS and Android mobile devices) to record behavior changes you observe during the August 2017 eclipse.
Modern Eddington Experiment (Bradley E. Schaefer, Louisiana State University)
The MEE will attempt to confirm Einstein’s general theory of relativity with higher precision than that achieved by Arthur Eddington at the 1919 eclipse. Can you photograph the deflection of starlight by the Sun’s mass with your own telescope and camera? Give it a try!
Other Earth/Moon/Sun Activities
- Scale model of the sun, moon and earth using playdough
- How big is the moon? from an earlier post about the Super Moon.
- Phases of the moon demonstration using a styrofoam ball on a pencil.