U-M Astronomer: Get to the path of April's total solar eclipse

Listen to David Gerdes, Arthur F. Thurnau Professor of Physics, Chair of Physics Department, and Professor of Astronomy speak with WWJ's Mike Campbell about the upcoming eclipse on WWJAM: On Demand.

EXPERT Q&A - David Gerdes recently talked with Morgan Sherburne, U-M Michigan News:

The sun and moon will trace a path across North America April 8, bringing a total solar eclipse to a large swath of the United States.

The path of totality—the line across Earth along which the sun and moon will be in lockstep, and the moon will completely block out the sun—will fall across a tiny sliver of southeast Michigan. The last time North America experienced a total solar eclipse was 2017, and the last time Michigan experienced a total eclipse was 1954.

University of Michigan astronomer and physicist David Gerdes says, “Get thee to the path of totality.” It will be another 75 years—2099—before Michiganders glimpse another total solar eclipse.

What’s the difference between an annular solar eclipse and a total solar eclipse?

Both types of eclipses happen when the moon passes directly in front of the sun. By an amazing cosmic coincidence, the moon’s diameter is 400 times smaller than the sun, and the sun is about 400 times farther away. So, as seen from Earth, they have almost the same apparent size, and the moon can almost exactly cover the sun. But the moon’s orbit isn’t perfectly circular. Its apparent diameter varies by about 12% as it moves around its elliptical orbit. This is enough to make a difference between fully covering the sun (a total eclipse) and not-quite-covering the sun (an annular, or “ring of fire,” eclipse). Because the sun isn’t completely covered in an annular eclipse, the spectacular sights of a total eclipse—the delicate solar corona, darkness in the middle of the day, 360-degree twilight around the horizon—are not visible.

Fortunately for us, the April 8, 2024, total eclipse will occur when the moon’s apparent size is near its maximum, so people in the path of totality will be treated to a total eclipse that lasts over four minutes, nearly twice as long as the total eclipse of 2017.

Is there any pattern to the occurrence of total eclipses?

One of the fascinating things about total eclipses is that their time and location can be predicted with great accuracy years in advance. Today, we use computers for this, but it’s been possible to predict eclipses ever since people understood the periodic motions of the Earth and moon. Our upcoming eclipse appears in a catalog compiled in the 1880s. But eclipse predictions go much further back. Around the 7th century BC, Babylonian astronomers discovered the Saros cycle, a repeating pattern of solar eclipses that take place every 18 years, 11 days and eight hours. This cycle results in groups of “eclipse siblings” that span several centuries. The next Saros-sibling of our upcoming eclipse that will be visible from the U.S. will happen in May 2078. Book your AirBnB now!

It looks like Ann Arbor will be just outside the path of totality on April 8. What will the eclipse look like to us in southeast Michigan, or otherwise outside of the path of totality?

The path of totality will pass just about an hour south of Ann Arbor, through northern Ohio. People in the Ann Arbor area will experience a very deep partial eclipse of about 98.5%. The surroundings will become noticeably dimmer, colors will appear flat and shadows will be sharper than usual. You may observe changes in nature, with animals, birds and insects displaying evening behavior. The temperature may drop 10 degrees or so. You may be able to spot Venus in the daytime sky a bit west of the sun.

But it’s very important to note that a 98.5% partial eclipse is a zero-percent total eclipse. The remaining 1.5% of the sun is still very bright. At no point will it be safe to look directly at the sun without eye protection. You will not be able to see the solar corona or other amazing sights of totality like the Diamond Ring Effect or Baily’s beads. If it’s at all possible for you, I encourage you to make your way into the path of totality to see one of nature’s most awe-inspiring sights. Most of us will not have the opportunity to view another total eclipse this close to home in our lifetime—the next total eclipse visible from Ann Arbor isn’t until 2099.

How can people safely view the eclipse and what are some alternate ways of viewing the eclipse?

Except during the total eclipse itself, which will last about four minutes, you must wear eye protection when looking directly at the sun. Eclipse glasses are made with very dark film and are an inexpensive way to keep your eyes safe (they are so dark that you pretty much can’t see anything except the sun). You can also use a homemade pinhole projector—just a piece of cardboard with a small round hole punched in it—to safely project an image of the sun onto the ground. A kitchen colander is also great for this. Or you can use natural pinhole projectors, such as the spaces between the leaves of a tree that provided a lovely view of the annular eclipse last October.

Do you have tips for people who want to take photos of the eclipse, either with traditional cameras or phones? Are there safety considerations people should keep in mind?

If this will be your first total eclipse, my best advice is to simply put your camera down and allow yourself to be fully present for the awesome sights and sounds of totality. There will be many outstanding photographs of this eclipse, but your experience and memories will be your own.

If you decide to photograph it, though, almost any camera or phone will show something. To photograph the partial phases, you must place a solar filter in front of your camera lens. The sun is actually really small, so detailed photos of the corona require a long zoom or telephoto lens. The corona is also very high-contrast, so use manual settings and choose a range of exposure times.

You might want to set up your phone or video camera on a stable mount and just leave it to record the full event. The moment of totality can set off a profound reaction among the participants, and often the best part of a video is the audio. If you are going to do anything more complicated than point-and-shoot, practice your procedure ahead of time so that you don’t waste precious seconds of totality messing with equipment. Finally, beware of streetlights and other exterior lights that may suddenly turn on when it gets dark and spoil your view. Whatever you decide to do, I wish you clear skies and an unforgettable experience on April 8!