Great American Eclipse: Frequently Asked Questions
Rebecca Harbison, Assistant Professor of Practice, Astronomy
- Q. What is a solar eclipse?
- A. An eclipse is when the Sun, Moon and Earth line up so that the shadow of the Earth falls on the Moon (or vice versa). For a solar eclipse, that means the Moon is between the Earth and Sun, so the Moon’s shadow falls on the Earth. Another way to say that is that the Moon appears to block the Sun in the sky.
- Q. If the Moon orbits the Earth once a month, why doesn’t an eclipse happen every month?
- A. The Moon orbits the Earth every 27.3 days, but a solar eclipse happens only about two or three times a year. Most months, the Moon appears to pass above or below the Sun in the sky, rather than lining up. This is because the Moon’s orbit around the Earth is slightly tipped compared to the Earth’s orbit around the Sun. So for an eclipse, the Moon has to be crossing from above to below (or vice versa) the Earth’s orbit—which we astronomers call the orbital node—and doing so when it is lined up with the Earth and the Sun (which happens at Full Moon and New Moon).
- Q. If solar eclipses happen multiple times a year, why haven’t I see one?
- A. When the Earth gets a solar eclipse, only a subset of Earth can see it. The people on the night side of the Earth can’t see the Sun, so will miss the eclipse. Furthermore, the Moon’s shadow is smaller than the Earth, so people in Chile and Argentina will be unable to see the eclipse: the Moon will look to be slightly north of the Sun in the sky.
Even more so, the Moon is large and close enough to cover the larger, more distant Sun, but only just barely. The spot on Earth that will see this perfect alignment, a total solar eclipse, is only some tens of miles wide, and it moves across the surface of the Earth (as the Earth turns and everything orbits) so that any one place is only in full shadow, the umbra, for a few minutes.
A far wider area is covered by the penumbra, the partial shadow created by the Moon blocking some of the Sun. As a result, most people may see a partial solar eclipse once every few years without much effort, by observing from the penumbra.
The last total solar eclipse visible from the contiguous (48-state) US was in 1979. The next eclipse visible from the contiguous US will be in 2024.
- Q. What’s the big deal for me?
- A. Eclipses can be awe-inspiring natural phenomena, and total solar eclipses can be quite troublesome to travel to. This one, however, will not be. The summer 2017 eclipse is particularly well-positioned by being over a populated area, with plenty of good roads, and Nebraska in August tends towards particularly clear skies. While partial solar eclipses can be safely viewed, they rarely have the effect that a total eclipse has on the environment, as it gets dark during midday.
- Q. What if it’s cloudy?
- A. Sadly, astronomers cannot control the weather. As the total eclipse will be visible in a band across the United States, if the clouds look patchy on satellite, you can try to take to the road and find a clear spot. Or you can stay in one place and hope it clears.
- Q. Are solar eclipses dangerous?
- A. No more so than observing the Sun on any other day. Which is to say that you should never look at the Sun directly, regardless of what is happening in the sky, but the light from an eclipse is no worse than normal sunlight.
- Q. So how can I observe the eclipse safely?
- A. There are two methods to do so. The first is to not look at the sun directly. If you take a piece of cardboard, stick a pinhole in it, and look at the shadow, the pinhole will act like a little lens and form an image of the Sun.
The other method is to get 'eclipse glasses,' which have special filters to block a large fraction of the Sun's light, far more than ordinary sunglasses. These will let you safely observe the Sun directly. Do not substitute anything not designed to observe the Sun: in addition to blocking the visible light, these eclipse glasses block invisible ultraviolet and infrared light that can harm your eyes without you realizing it.
During the total solar eclipse, while the Sun is totally covered, you can remove your eclipse glasses. As the total eclipse will last for only a minute or two, if you are not at an event, have a timer set and ready to go so you can look away or put your glasses back on before the Sun returns.
- Q. Where can I buy eclipse glasses?
- A. In Lincoln, eclipse glasses are for sale at Morrill Hall and Pioneers Park Nature Center. NASA has a Safety page which lists several manufacturers of safe solar viewers.
- Q. Why do astronomers care?
- A. A few reasons. The first is that eclipses are beautiful, and accessible to everyone, and we do like showing off the universe to other people. It doesn’t take a PhD to enjoy a solar eclipse.
As for the science, nearly one century ago, science teams used the May 1919 solar eclipse to test that the Sun’s mass would bend light from stars behind it, a prediction of Albert Einstein’s Theory of General Relativity. For nearly 50 years, this was a gold standard for testing that relativity was our best picture of how gravity and motion work. (Now, we use radio waves, because the Sun is less bright in radio waves than in visible light, and we have spacecraft with radio transmitters and receivers off of Earth.)
Even today, solar astronomers use the chance of an eclipse to look at the Sun’s outer atmosphere, the corona. The corona is constantly changing and creates what we call 'space weather' as the Sun sheds hot plasma through the corona in a solar wind, which can affect our power grids and satellites (and produce the Northern Lights when a particularly strong gust hits Earth). Space observatories can do some work all the time watching the corona, but the glimpses we get during an eclipse are closer in to the Sun than those satellites can see.