by On April 8, 2024, millions across the US will have a once-in-a-lifetime opportunity to view a total solar eclipse. Cities including Austin, Texas; Buffalo, New York; and Cleveland, Ohio, will have a direct view of this rare cosmic event that will last a few hours.
Although you can see many astronomical events, like comets and meteor showers, from anywhere on Earth, the eclipses are different. You must travel to what is called the path of totality to experience the total eclipse. Only certain places get a full eclipse show, and that’s because of scale.
The relatively small size of the moon and its shadow creates truly once-in-a-lifetime opportunities. On average, total solar eclipses are visible somewhere on Earth once every few years. But from one location on Earth, it’s about 375 years between solar eclipses.
I’m an astronomer, but I’ve never seen a total solar eclipse, so I plan to drive to Erie, Pennsylvania, in the path of totality, for this one. This is one of my few chances to see a total eclipse without making a much more expensive trip to somewhere more remote. Many people have asked me why nearby eclipses are so rare, and the answer has to do with the size of the moon and its distance from the Sun.
Size and scale
You can see a solar eclipse when the Moon passes in front of the Sun, blocking part or all of the Sun from view. For people on Earth to be able to see an eclipse, the Moon, while orbiting the Earth, must lie directly along the observer’s line of sight to the Sun. However, only some observers will see an eclipse because the Moon will not block everyone’s view of the Sun on the day of an eclipse.
The fact that solar eclipses occur at all is a numerical coincidence. It turns out that the Sun is about 400 times larger than the Moon and also 400 times further from Earth.
So, although the Moon is much smaller than the Sun, it is close enough to Earth to appear the same size as the Sun when seen from Earth.
For example, your pinky finger is much, much smaller than the Sun, but if you hold it up at arm’s length, your eye sees it as big enough to block the Sun. The Moon can do the same – it can block out the Sun if it is perfectly aligned with the Sun from your perspective.
The path of wholeness
When the Earth, Moon and Sun are perfectly aligned, the Moon casts a shadow over the Earth. Since the Moon is round, its shadow is round when it lands on Earth. The only people who see the eclipse are those in the area on Earth where the shadow falls at a particular time.
The Moon is continuously orbiting the Earth, so as time goes on during the eclipse, the Moon’s shadow moves across the face of the Earth. Its shadow ends like a thick line that can cover hundreds of miles in length. Astrologers call that line the path of totality.
From any particular place along the path of totality, an observer can see the Sun completely eclipsed for a few minutes. Then, the shadow moves from that place and the Sun slowly becomes more visible.
Tilted orbit
A solar eclipse does not occur every time the Moon passes between the Earth and the Sun. If that were the case, there would be a solar eclipse every month.
If you could float above the Earth’s North Pole and see the moon’s orbit from above, you would see the Moon come up with the Sun once every time it orbits the Earth, which is about once a month. From this high vantage point, it looks like the moon’s shadow should land on Earth every orbit.
However, if you could change your perspective to look at the Moon’s orbit from the orbital plane, you would see that the Moon’s orbit is tilted by about 5 degrees compared to the Earth’s orbit around the Sun. This tilt means that sometimes the Moon is too high and its shadow goes above the Earth, and sometimes the Moon is too low and its shadow goes below the Earth. An eclipse only occurs when the Moon is positioned correctly and its shadow falls on the Earth.
As time goes on, the Earth and the Moon continue to spin, and eventually the Moon aligns with the Earth’s orbit around the Sun at the same moment that the Moon passes between the Sun and the Earth.
Although only certain cities are in the path of totality for this year’s April eclipse, the entire US is still close enough to this path that observers outside the path of totality will see a partial eclipse. In those locations, the Moon will appear to pass in front of part of the Sun, leaving the crescent shape of the Sun still visible at maximum ellipse.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by Christopher Palma, Penn State.
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Christopher Palma does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article, and does not disclose he has any relevant affiliations beyond his academic appointment.
