On August 21, 2017, a total solar eclipse moved from the Pacific to the Atlantic, throwing a narrow corridor through 14 U.S. states under the moon's shadow in the first coast-to-coast totality for 99 years. On that day, the shadow moved from Oregon across the U.S. to South Carolina, from roughly northwest to southeast.
It's about to happen again, with the total solar eclipse on April 8 again beginning in the Pacific and ending in the Atlantic, but this time, the path appears to track from southwest to northeast.
There are other differences, too, including the maximum length of totality (2 minutes 40 seconds in 2017 and 4 minutes 28 seconds in 2024) and the width of the path of totality (about 70 miles in 2017 and around 115 miles in 2024). This time around, many millions more people will experience totality — and at a time when the sun is close to solar maximum.
Related: Total solar eclipse April 8, 2024: Live updates
Here's how the 2024 total eclipse differs from the 2017 total eclipse:
Eclipse circumstances
"The eclipse circumstances are just completely different," said Dan McGlaun, an eclipse expert at Eclipse 2024, which has an interactive map and eclipse simulator for the upcoming eclipse, in an interview with Space.com.
"It's all about the distance to the sun and the moon — just simple geometry." Although the distance from Earth to the sun does change — it's closest at perihelion in January and farthest at aphelion in July — that doesn't affect these two eclipses because the sun is roughly the same distance away.
What does differ hugely is the distance from Earth to the moon, which has a slightly elliptical orbit.
The distance to the moon
During the peak of the 2017 eclipse, the moon was 231,155 miles (372,008 km) from Earth, while on April 8, it will be 223,392 miles (359,515 km) away. On April 8, the moon will be a couple thousand miles closer, so its conical shadow that falls on Earth will have a larger diameter. In 2017, the path of totality was 62 to 71 miles wide, according to NASA, while on April 8, it will range between 108 and 122 miles wide, covering much more of Earth.
The path of totality — a projection of the moon's shadow — moves from west to east because that's the direction the moon orbits Earth (and the moon travels much faster than Earth is rotating, visibly overtaking it during the eclipse). That partly cancels out the moon's speed, but its shadow will still sweep across Earth from west to east at more than 1,500 miles (2,400 km) per hour on April 8, according to NASA.
The position of the moon
The moon's orbital path around Earth is tilted by 5.1 degrees with respect to the orbit of the Earth around the sun. That means its orbit crosses the ecliptic — the apparent path of the sun through our daytime sky — at two points, which astronomers call nodes (ascending and descending). When it does so at a new moon, it causes a solar eclipse, but what node it occurs at makes a difference.
"An ascending node eclipse has the moon going upward while a descending node eclipse it's going downward," said McGlaun. However, since both the 2017 and 2024 are ascending node eclipses, there's something else that's much more important when looking at the different paths — the angle of Earth.
The rotation of Earth
Our how to read and understand a solar eclipse map will help you get the most out of your eclipse viewing venture!
The sun and moon combine to produce a shadow in space that's always there. It's only when a planet rotating on an axis titled by 23.4 degrees gets in the way that a path of totality is created. "As the year progresses, the Earth changes its orientation and rotates in place, and this, more than anything else, is why the shadow in 2017 is different to 2024," said McGlaun. In short, Earth's axis is leaning different ways during both eclipses.
"In 2017, it's the Earth rotating during totality that's causing that path to look like it's going down, but it's really going up," said McGlaun. As seen from the sun, North America appears to move up the globe as Earth rotates during the eclipse, as you can see on McGlaun's solar eclipse simulator. "For the 2024 eclipse, the moon's shadow is doing the same thing, but the Earth is leaning the other way." During the eclipse, North America appears to move down the globe as Earth rotates.
Change the angle of the Earth's axis, and you can make the path of totality go anywhere you want.
The 'Eclipse Crossroads'
Another effect of all this is that the 2017 and 2024 paths cross, creating an "Eclipse Crossroads" area of about 9,000 square miles (14,500 square km) in parts of southeastern Missouri, southern Illinois and western Kentucky that will soon experience a second totality in less than seven years.
The centrelines cross in Makanda, Illinois, close to Carbondale. Also in the lucky quadrant are Paducah, Kentucky, Cape Girardeau and Farmington, Missouri. Texas Hill Country, meanwhile, has a 14,000 square miles (36,000 square km) "Eclipse Crossroads" region of its own — centered on Vanderpool — where a "ring of fire" annular solar eclipse on October 14 was merely a warm-up for the totality to come.
The solar cycle
Another big change between 2017 and 2024 is the sun's magnetic activity. The sun has its own cycle, the solar cycle, which lasts about 11 years. During that time, its magnetic activity waxes and wanes. The 2017 eclipse occurred close to the solar minimum when magnetic activity was low. Solar maximum is predicted to occur in 2024, meaning more to see on the sun during the eclipse.
Use your solar eclipse glasses during the partial phases of the eclipse, and you'll see sunspots — dark areas of magnetic activity — on the surface, close to its equator. If you're in the path of totality, totality will likely reveal a larger corona, streamers throughout the corona and prominences, which appear as bright, pink curls or loops coming off the sun.
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