What is a Solar Eclipse?
Many people are excited about the upcoming Solar Eclipse that is passing through the US from Portland, OR to Charleston, SC. What is a solar eclipse? How do you stay safe while viewing one? How should you plan your day to have a good experience? Grab some eclipse glasses and Ask an Astronomer!
An eclipse is an example of syzygy. A syzygy occurs when three gravitationally bound objects form a straight line. The dominant syzygies for we Earthlings involve the three celestial objects that have the largest impact in our lives: The Sun, The Earth, and The Moon. When the Earth ends up between the Sun and Moon, it blocks sunlight from hitting the Moon, causing a Lunar eclipse. When the Moon ends up between the Sun and Earth, it blocks some sunlight from hitting the Earth, causing a Solar Eclipse.
Why isn't there a solar eclipse every new moon, and a lunar eclipse every full moon?
The orbits of the Earth around the Sun and the Moon around the Earth aren't perfectly coplanar (aligned). The orbit of the Moon around the Earth has a small tilt relative to the orbit of the Earth around the Sun. For half of each month (or "moonth", one lunar orbit) the moon is above the plane of the Earth's orbit, and for the other half it is below that plane. Eclipses occur only when the Moon is new or full while it passes through the plane of the Earth's orbit.
Why doesn't the Moon block out the Sun everywhere on Earth?
The Earth is a big place, and only some parts of the Earth will line up perfectly for any given eclipse. This means that there are three types of solar eclipses: Total, Partial, and Annular. To understand their differences, we must first understand shadows.
Umbra vs Penumbra:
The Earth, Sun, and Moon are three-dimensional objects, and therefore cast light and shadow in three dimensions. In physics, shadows are broken down into two categories: an Umbra and a Penumbra. The Umbra is the part of an object's shadow where it blocks all light from a given source. The Penumbra is the part of an objects shadow where only some of the light is blocked from a given source. This is drawn out in more detail in the following images.
Because the Moon orbits around the Earth in an ellipse, sometimes it is close enough that parts of the surface of the Earth lie within its Umbra. Places within the Moon's Umbra experience a Total Solar Eclipse. Sometimes the moon is too far away, and its Umbra does not reach the Earth. Places on the Earth's surface that pass directly behind the Moon's Umbra experience an Annular Solar Eclipse. Places that pass through any other part of the Moon's Penumbra experience a Partial Solar Eclipse.
Wherever you are, the time when you first enter the Moon’s Penumbra is called "C1", short for "Contact 1". The time when you first enter the Moon's Umbra is called "C2", the time when you exit the Umbra is called "C3", and the time when you exit the Penumbra is called "C4". This terminology is used in many eclipse timing apps and tools. Although it is straightforward once you understand it, it can be confusing if you don't.
Just before C2 and just after C3, only a tiny sliver of the Sun is peeking out from around the Moon. This can lead to some interesting diffraction effects, so be sure to keep an eye on your surroundings. Between C2 and C3 is the time period known as "Totality". During this time you can see the Sun's corona, and even stars! The temperature drops by a few degrees, and it feels like night has fallen. This is the truly spectacular part of the show, and it is only viewable in the narrow band between Portland, OR and Charleston, SC.
How should I view the Solar Eclipse?
The Sun is very bright, and emits a lot of light. Some of that light is very high energy UV light that can damage your eyes. Even the best sunglasses don't block enough UV light to make it safe to look directly at the sun. To look at the sun safely, you have to look through a special filter. Welding hoods with a #14 (or darker) filter are sufficient, as are the Solar Filters used in telescopes. Many places also have "eclipse glasses" for sale, which are made of cardboard and have lenses made of Solar Filters. Protect your eyes and use the proper eyewear!
If you cannot find a proper filter to safely view the eclipse directly, you can also view it indirectly using a pinhole camera. Pinhole cameras are very easy to make: poke a small hole in a paper plate or piece of cardboard, line it up with the sun, and an image of the sun will be projected behind the pinhole. DO NOT LOOK THROUGH THE PINHOLE WITH YOUR NAKED EYE! Instead, have the image of the sun project onto something like a white bedsheet, piece of paper, or sidewalk. You can look up more detailed instructions for fancier pinhole cameras (sometimes called pinhole projectors) online.
What else should I do to prepare for the eclipse?
If you want the best possible view of the eclipse, try and get as close to the center of the track (the so called 'zone of totality') as possible. The closer to the center you are, the longer the eclipse will last and the darker your surroundings will become.
If you are near the zone of totality, there will probably be a lot of people wherever you go, and local businesses may be overwhelmed by all of the tourism. If you are traveling, we advise that everyone bring your own food and water, and maybe some extra to share with people who came less prepared.
Wherever you may be, be sure to check weather reports and avoid clouds. Also, if you're in a more rural area, stay away from wild animals. Animals don't usually understand what's going on. They see everything get dark, and sometimes get scared. Be careful and stay safe.
Try to make sure that everyone has a good time, and enjoy the darkness!