Jonathan Kemp is telescope and scientific computing specialist at Middlebury.

MIDDLEBURY, Vt. – A solar eclipse will be visible across the United States on Monday, August 21, 2017, starting on the west coast at 12:05 p.m. ET and concluding on the east coast about four hours later at 4:09 p.m. ET.

The path of totality – a 70-mile-wide band across the country where viewers will experience a total solar eclipse – will stretch from Oregon to Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, and the Carolinas. The majority of the nation will experience a partial eclipse of the Sun, and for viewers in Vermont the Sun will be about 60 percent obscured by the Moon.

Jonathan Kemp, the telescope specialist at Middlebury’s Mittelman Observatory, said viewers in the path of totality “can have an awe-inspiring experience, weather permitting of course. The skies can dim and change color, the atmospheric temperature can cool slightly, animal behavior and sounds may change, and planets and bright stars may become visible for a few minutes.”

Astronomer Kemp, who spent 12 years operating the telescopes at observatories on Mauna Kea in Hawaii before coming to Middlebury, answered questions about the upcoming August 21 solar eclipse.

How does a solar eclipse occur?

A solar eclipse happens when the Earth passes through the Moon’s shadow. Effectively, the Moon is passing between the Sun and the Earth, and the Moon is blocking the Sun’s light from reaching the Earth.

How rare is a total solar eclipse?

Total solar eclipses are not that rare. On average, they can happen every year and a half somewhere on Earth. However, for any specific location on Earth, a total solar eclipse may happen, on average, every four hundred years or so. The last coast-to-coast transcontinental total solar eclipse in the U.S. occurred in 1918, and the next coast-to-coast one after this year’s will be in 2045. However, a prominent solar eclipse will also happen in 2024, and the path of totality will stretch across parts of the eastern U.S., including northern Vermont.

What about partial solar eclipses?

They are far more common. We in Vermont experience partial solar eclipses, like the one we will have later this month, every few years or so.

What are the safety considerations when viewing a solar eclipse?

Solar eclipses are not safe to view without proper training and equipment. One should never look at the Sun with the naked, unaided eye, or with sunglasses, or with binoculars, or with a telescope without a filter specifically designed to block almost all of the Sun’s light. Looking at the Sun even momentarily can cause severe and permanent eye injury. With that in mind, following safety procedures can result in a memorable viewing experience. Solar eclipses are a unique spectacle not to be missed if you can identify a safe observing method and employ it accordingly.

Okay, so how should people experience the eclipse if they can’t look directly at it?

While experienced astronomers may use specialized solar filters on optical telescopes, casual eclipse viewers can consider a variety of more accessible methods, as long as they are done properly. So-called “eclipse glasses” are a popular direct-viewing method for individuals. The safest method, however, is indirect projection, such as projecting the eclipse onto the ground through a pinhole in a piece of cardboard. NASA has an excellent web page on eclipse safety.

This image of a total eclipse, courtesy of NASA, was taken in Southeast Asia in 2016. (Click to enlarge.)

If a solar eclipse occurs when the Earth passes through the Moon’s shadow, then why don’t we see one every month?

The answer has to do with the inclination of the Moon’s orbit around Earth and the resulting geometry. Consequently, the Earth does not pass through the Moon’s shadow every month.

So, geometry is important?

Most definitely. In fact, because of the unique details of our solar system’s geometry, we are able to enjoy a stunning variety of eclipses here on Earth. The Sun is roughly 400 times larger than the Moon, and the Sun is roughly 400 times farther from Earth, which means that the Sun and the Moon appear to be approximately the same angular size in the sky.

What can we expect to see in Vermont on August 21, and when will it occur?

As the eclipse develops, the Moon will appear to gradually take a bite out of the Sun, so to speak. And since Vermont is north of the path of totality, the Moon will obscure the lower portion of the Sun. In Middlebury, this partial eclipse will begin at 1:23 p.m. and end at 3:54 p.m. The maximum partial eclipse here will be at 2:41 p.m., and the most pronounced views will be right around this time.

And what about the weather?

Cloudy or overcast skies can obscure one’s view of an eclipse. Based on historical data, the cloud cover in the western continental U.S. is likely to be less than it will be in the eastern continental U.S. As we get closer to August 21, meteorologists will be in a better position to forecast the weather.

Any final thoughts about the eclipse?

Yes. My greatest concern for the public is safety. Some things that people might think are safe to use when looking at an eclipse are, in fact, not safe. It is not safe to use sunglasses. It is not safe to use eclipse glasses in combination with a telescope. And it is never safe to look directly at the Sun. However, viewing a solar eclipse, and especially from within the path of totality, can be an amazing and transformative experience, and provide a memory that can last a lifetime. No matter the appropriate method, always remember to be safe and to know the correct equipment for the job, how to use that equipment, and how to inspect the equipment for safety. Each viewing method has its own best safety practices. That said, proper preparation and good weather can potentially result in an amazing eclipse viewing experience!

For readers seeking more information about the eclipse, Jonathan Kemp recommends the following websites:

NASA publication about the coming eclipse

NASA on eye safety

The American Astronomical Society on eye safety

Animation showing the expected path of the August 21 eclipse

Total solar eclipses in the U.S. – 2001-2050

Solar eclipses of historical interest

Photo of Jonathan Kemp by Todd Balfour.