On Aug. 21, for the first time since 1918, a total solar eclipse will travel coast-to-coast across the entire United States, from Oregon to South Carolina. Here UNLV astrophysicist Jason Steffen talks about why this solar eclipse is such a big deal, what we will see from Las Vegas, and how to view it safely (No, sunglasses won't cut it).

What is a total solar eclipse?

A total solar eclipse happens when the moon blocks the whole sun, and casts its shadow onto the Earth. Though a total solar eclipse is the rarest kind, there are other types of eclipses where the moon only blocks part of the sun (a partial eclipse), or when the moon is not able to block the whole sun (an annular eclipse). This last type of eclipse happens because the moon’s orbit isn’t a perfect circle — sometimes it is closer to the Earth and sometimes farther away. When it is farther away it appears smaller in the sky than the sun and the sub appears as a ring or “annulus”. When the moon is closest to the Earth, and passes between the Earth and sun, then you will get a total solar eclipse like the one we will see.

Why is a total solar eclipse such a big deal?

They are a big deal for a few reasons. First, they only occur over populated areas once every few years. Before the days of air travel, it would be a once-in-a-lifetime event. Second, they are something that everyone can both see and appreciate. And, third, they are quite stunning to look at.

What time is the Aug. 21 solar eclipse?

In the Las Vegas region, the eclipse will happen mid-morning, mostly during the 10 a.m. hour. The full eclipse process will occur over about two hours from the beginning (ingress) to the end (egress). The totality lasts only a few minutes, and you would need to be in Oregon or Idaho to see it at that time. From Las Vegas, we should be able to see an eclipse of about 75 percent.

What are we going to see that day?

During a total eclipse, the moon comes between the Earth and the sun. With the sun’s light blocked, you are able to see the diffuse, upper layers of the sun’s atmosphere (called the corona). The corona is enormous in size and is quite picturesque. We just can’t see it because it is less bright than the sun’s visible surface. On the day of, it will get dark (similar to an overcast sky) directly under the shadow. The sky will be dimmer during the eclipse, even if it is just a partial eclipse, as though the sun went behind a thick cloud. The sky will not turn completely dark because we can see the sky over parts of the country where the eclipse occurs at a different time. So, the sky will still be relatively bright — there will just be less direct sunlight.

Will the entire country be able to see at least part of the eclipse?

Yes, the entire country will be able to see at least a partial eclipse. There is a good eclipse map that shows the percentages and rough times across North America.

What eyewear is recommended to view the eclipse?

Sunglasses are not OK. There are eclipse glasses that you can find online, but make sure you check your source. Alternatively, with relative ease, you can make a pinhole camera using a shoebox and some aluminum foil (again, look online). You can set up binoculars on a tripod or a small telescope to project the image onto a screen — just don’t look through them. NASA has an eclipse website with more information.

Are solar eclipses easy to predict?

Yes. Predicting eclipses was one of the main tasks that astronomers did for the last 2,000 years. With our current technology, predicting eclipses is quite easy to do. Solar eclipses happen a couple of times each year, but total solar eclipses happen only every few years, and since the Earth is mostly ocean, they are not always easy to see.

When will the next solar eclipses be over North America?

It looks like there will be eclipses over some parts of North America in 2024 (Texas to New York), 2044 (Montana and Canada), and a really good one in 2045 (California to Florida).

Why do these things capture people’s imaginations?

Because the sun, the moon, and the Earth are amazing. You have a blazing hot ball of plasma, one million times the volume of the Earth and 10,000 degrees Fahrenheit. It is briefly blocked by a gigantic, self-gravitating rock the diameter of North America. The shadow is cast 250,000 miles away onto another ball of rock, this one with a slice of habitability less than 10 miles thick and upon which there are 7.4 billion members of a species able to both appreciate how beautiful the situation is and understand how it all works.

I think that being humbled by the universe inspires gratitude for life—and gratitude is good medicine for whatever ails you.

Is this as big a deal to scientist like you as it seems to be to amateur stargazers?

No, not really. We have instruments on the ground and in space that allow us to study the sun without needing the eclipse. But, solar eclipses have been used in the past for many important discoveries. For example, Einstein’s theory of gravity was confirmed using star positions during a solar eclipse.

Was there an astronomical event that piqued your interest in this field?

There was an eclipse when I was in elementary school. We all made pinhole cameras and went outside. But it was overcast that day and we didn’t get to see anything. I have seen two transits of Mercury and one transit of Venus. But, what really got me going in astronomy was my introduction to astronomy class my first year of college.