On April 8, 2024, millions of people in the United States will have the chance to watch a once-in-a-lifetime total solar eclipse. Cities including Austin, Texas; Buffalo, New York; and Cleveland, Ohio will be able to directly see this rare cosmic event, which will last only a few hours.
Although you can see many astronomical events, such as comets and meteor showers, from anywhere in the world, eclipses are different. To experience a total eclipse, you need to travel to a place called the path of totality. Only certain locations can see the entire eclipse, and this is due to scale.
The Moon’s relatively small size and shadow make eclipses truly once-in-a-lifetime opportunities. On average, total solar eclipses can be seen anywhere on Earth every few years. But the time between solar eclipses from any location on Earth is roughly 375 years.
I’m an astronomer, but I’ve never seen a total solar eclipse, so I’m planning to head to Erie, Pennsylvania on the way to totality for that. This is one of the few opportunities I have to see the total eclipse without making a much more expensive trip to a more remote location. Many people have asked me why close 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
When the Moon passes in front of the Sun, you can observe a solar eclipse by blocking part or all of the Sun from view. For people on Earth to see an eclipse, the Moon must be exactly in the observer’s line of sight with the Sun as it orbits the Earth. However, only some observers will be able to see the eclipse, because on the day of the eclipse, not everyone’s view of the Sun will be blocked by the Moon.
The occurrence of solar eclipses is a bit of a numerical coincidence. So much so that the Sun is approximately 400 times larger than the Moon and 400 times farther from the Earth.
So, although the Moon is much smaller than the Sun, it is close enough to the Earth that it appears the same size as the Sun when viewed from Earth.
For example, your little finger is much, much smaller than the Sun, but when you extend it at arm’s length, it appears large enough to block the Sun. The moon can do the same thing; If it is perfectly aligned with the Sun from your perspective it can block the Sun.
path of integrity
When the Earth, Moon, and Sun are perfectly aligned, the Moon casts a shadow on the Earth. Since the Moon is round, its shadow is also round when it falls to the Earth. Only people who are in the area on Earth where the shadow falls at a particular moment see the eclipse.
The Moon constantly revolves around the Earth, so as time progresses during an eclipse, the Moon’s shadow moves over the Earth. Its shadow turns into a thick line covering hundreds of kilometers. Astronomers call this line the path of totality.
From anywhere along the path of totality, an observer can see the Sun being completely eclipsed for several minutes. Then the shadow moves away from that place and the Sun gradually becomes more visible.
a 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 that the Moon aligns with the Sun every time it orbits the Earth, which is approximately once a month. Viewed from this high vantage point, it looks like the Moon’s shadow must fall on Earth with each orbit.
However, if you could shift your perspective to look at the Moon’s orbit from its 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 very high and its shadow passes above the Earth, and sometimes the Moon is very low and its shadow passes below the Earth. An eclipse only occurs when the Moon is in just the right position and its shadow falls on the Earth.
As time passes, the Earth and Moon continue to rotate, and eventually when the Moon comes between the Sun and the Earth, at the same time the Moon aligns with the Earth’s orbit around the Sun.
Although only select cities are on the path of totality for this April’s eclipse, the entire United States is still so close to that path that observers outside the path of totality will see a partial eclipse. In these regions, the Moon will appear to pass in front of part of the Sun, leaving the Sun’s crescent shape still visible at the time of maximum eclipse.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by: Christopher Palma, Penn State
Read more:
Christopher Palma does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic duties.