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On April 8, 2024, millions of people across North America will have the chance to witness a rare and spectacular phenomenon – a total solar eclipse. 

The Moon’s shadow will follow a curving path, starting over the Pacific Ocean, making landfall on the west coast of Mexico at around 11 am MST, it will travel northeast, entering Texas around 11:30 am MST (1:30 pm CDT), exiting the United States through Maine at 12:30 pm MST (3:30 pm EDT), and finally leaving Canada’s Maritime Provinces into the North Atlantic at 12:45 pm MST (4:45 pm ADT). 

Remote image Url

Solar Eclipses 101

Total solar eclipses occur thanks to a fortuitous coincidence – the vastly different sizes of the Sun and the Moon are almost perfectly canceled out by their vastly different distances, so they appear the same size in the sky. That means that when the Moon’s orbit takes it between the Earth and Sun, it blocks out the disk of the Sun perfectly. Due to variations in the Moon’s orbit, sometimes it is slightly farther from the Earth, and the result is an annular eclipse, where the Moon does not fully block out the sun, forming a bright “ring of fire” instead of a total eclipse.

A total eclipse begins as the Moon drifts in front of the sun, making our star’s bright disk look like a brilliant crescent. In a partial eclipse, the Moon’s path doesn’t cross the very center of the sun, so the sun just shrinks to a crescent, then grows again as the Moon continues on its way. In a total eclipse, the crescent gets narrower and narrower until it is so thin that it is no longer a crescent, but an arc of bright points called Baily’s Beads (more on this phenomenon later). These wink out one by one until finally the entire face of the sun is blocked. 

Satellite image showing the eclipse.
A total solar eclipse darkened the skies over the United States on Aug. 21, 2017. Oshkosh, a town in the Nebraska Sandhills, was a prime location for viewing the eclipse. This Landsat 7 image shows the town at 11:33 a.m. local time on Aug. 18, 2016, a year before the total eclipse.

For an observer on Earth, when this crossing happens, the sky goes dark, the temperature falls, and animals may even act strangely. And, most dramatically, blotting out the Sun’s brilliant disk reveals the wispy corona: tendrils of superheated gas that stretch in all directions from the sun. These are normally lost in the sun’s glare, but a total eclipse reveals them in all their glory.

At the end of the eclipse the process happens in reverse – first a single brilliant point of light appears, then Baily’s Beads, then a crescent sun emerges from behind the Moon. The strange half-light of a partial eclipse continues for some time until the Moon has fully moved on, but the period of totality only lasts for a few minutes.

Baily’s Beads and the Mountains on the Moon

A solar eclipse. Light from the sun shines from behind a black circular silhouette of the moon.
A solar eclipse. Light from the sun shines from behind a black circular silhouette of the moon.

An eclipse may seem like a purely astronomical phenomenon, but it has a surprising link to the work done by the USGS Astrogeology Science Center. Baily’s Beads, those points of light that linger just before the sun is fully eclipsed, are actually caused by the uneven topography of the lunar surface! Points of low elevation allow the disk of the sun to peek through just slightly longer than high points. If the Moon were perfectly smooth, there would be no “beads,” just an infinitesimal crescent that smoothly fades to nothing. 

Astronaut Training in Flagstaff
Astronaut training In Flagstaff, 1968.

The USGS Astrogeology Science Center is world-renowned for their expertise in extraterrestrial topography, using specialized photogrammetry software to derive detailed digital terrain models (DTM) of the Moon, Mars, and other bodies from satellite images. When NASA needs the most reliable possible image mosaics and topography, such as when landing a rover on Mars, they turn to USGS Astrogeology

Astrogeology has a long history of partnership with NASA missions. The center was founded in 1963 to map the Moon and train the Apollo astronauts so that when they set foot on the Moon’s surface, they could collect the most useful samples and observations possible. Over the years, the center has played a similar role on robotic missions: providing the scientific and technical expertise to process a wide variety of data and maximize the science return from every mission. Now, as NASA prepares to return humans to the Moon, the Astrogeology Center has returned to its roots and is involved in training the next cohort of moonwalkers and providing base maps for the Artemis missions. 

Viewing the Eclipse

If you are able to be in the path of totality on April 8, or even if you are in an area that will see a partial eclipse, it is well worth making time to view it. Just remember to use sun-safe eclipse glasses to view the sun when it is not fully eclipsed. Even a small sliver of un-eclipsed sun is bright enough to damage your eyes. To learn more about eclipse safety, refer to this NASA site.

One great way to safely view the sun while it is partially eclipsed is to use a pinhole projector. You can make one at home or take advantage of household objects that already have small holes in them, such as a colander. 

Eclipses are a vivid demonstration of the beauty of the natural world, as well as a testament to our ability to understand and predict the physical processes of the universe. They remind us that we are on a planet, whirling through space in a stately dance with the Moon and the Sun, and that our planet is one of many worlds, all of them as beautiful and fascinating as our own.

So, on April 8, step outside, put on your eclipse glasses, and look up!

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