April Brings America’s First Total Eclipse Since 2017: What You Need to Know

In just one month the skies of North America, including Mexico, the United States, and Canada, will be graced with a total solar eclipse. This is the first total eclipse to be visible from America since 2017, leading to increasing excitement as the big day approaches. This astronomical event is gearing up to be one of the largest social gatherings you’ll ever attend, so there are a couple of things to keep in mind as you make plans for April. Keep reading to learn more about eclipses, how to see one, and how to get the most out of this spectacular event.

A composite image of the phases of the 2017 total solar eclipse in the United States. Credit: Bryan Goff on Unsplash.

What is a Total Solar Eclipse?

An eclipse happens when the Sun, Earth, and Moon are in perfect alignment with each other. When the Moon travels directly in front of the Sun, it blocks sunlight and causes a solar eclipse. When the Moon is on the opposite side of Earth and the Earth travels directly in front of the Sun, it casts its shadow on the Moon and causes a lunar eclipse. These events, especially solar eclipses, can only take place because the Moon and Sun both have the same apparent size from our perspective on Earth.

Solar and lunar eclipses are much more common than you think. In fact, “eclipse seasons” occur approximately every six months. But if the Sun and Moon are the same apparent size, why don’t solar eclipses happen every new moon and lunar eclipses happen every full moon? And why only in specific locations? The answer: geometry.

The orientation of the Moon’s elliptical orbit is not fixed, shifting over time. This process is known as apsidal precession. Credit: Rfassbind on Wikimedia Commons.

Most orbital paths are not perfectly circular. Kepler’s First Law of Planetary Motion uses geometry to show that most orbits are actually elliptical. When two bodies interact gravitationally, one doesn’t orbit the other like you may have been taught. Instead, they both orbit a common center of mass. In the Earth-Sun system the center of mass is inside the Sun, just not directly in the middle. This makes Earth’s orbit have relatively low eccentricity. The Moon’s orbit is not highly eccentric either, but its distance from Earth changes as it orbits.