On 21 August 2017, a total eclipse could be seen in the contiguous United States for the first time in nearly four decades. The path of totality stretched from Oregon to South Carolina, and an estimated 1.85 million to 7.4 million people were expected to travel to experience totality.
In the following excerpt from Totality: The Great American Eclipses of 2017 and 2024, Mark Littmann and Fred Espenak explain what causes a solar eclipse—and totality.
How big is the Moon in the sky? What is its angular size? Extend your arm upward and as far from your body as possible. Using your index finger and thumb, imagine that you are trying to pluck the Moon out of the sky ever so carefully, squeezing down until you are just barely touching the top and bottom of the Moon, trapping it between your fingers. How big is it? The size of a grape? A plum? An orange?
It is the size of a pea. (You can win bets at cocktail parties with this question.) The Moon has an angular size of only half a degree.
Now, how large is the Sun in the sky? Your friends will almost all immediately guess that it is bigger. Before they damage their eyes by trying the Moon pinch on the Sun, just remind them that a total eclipse is caused by the Moon completely covering the Sun, so the Sun must appear no bigger than a pea in the sky as well. It is the brightness of the Moon and especially the Sun that deceives people into overestimating their angular size.
Now that you have collected on your bets and can lead a life of leisure, think about the remarkable coincidence that allows us to have total eclipses of the Sun. The Sun is 400 times the diameter of the Moon, yet it is about 400 times farther from the Earth, so the two appear almost exactly the same size in the sky. It is this geometry that provides us with the unique total eclipses seen on Earth when our Moon just barely covers the face of the Sun. If the Moon, 2,160 miles (3,476 km) in diameter, were 169 miles (273 km) smaller than it is, or if it were farther away so that it appeared smaller, people on Earth would never see a total eclipse.
It is amazing that there are total eclipses of the Sun at all. As it is, total eclipses can just barely happen. The Sun is not always exactly the same angular size in the sky. The reason is that the Earth’s orbit is not circular but elliptical, so the Earth’s distance from the Sun varies. When the Earth is closest to the Sun (early January), the Sun’s disk is slightly larger in angular diameter, and it is harder for the Moon to cover the Sun to create a total eclipse.
An even more powerful factor is the Moon’s elliptical orbit around the Earth. When the Moon is its average distance from the Earth or farther, its disk is too small to occult the Sun completely. In the midst of such an eclipse, a circle of brilliant sunlight surrounds the Moon, giving the event a ring-like appearance—hence the name annular eclipse.
Because the angular diameter of the Moon is smaller than the angular diameter of the Sun on the average, annular eclipses are more frequent than total eclipses.
But the Moon does not just dangle motionless in front of the Sun. It is in orbit around the Earth. It catches up with and passes the Sun’s position in the sky about once a month.
The Moon gives off no light of its own. It shines only by reflected sunlight. So half the Moon is always lighted by the Sun. But as the Moon orbits the Earth, sometimes we see the Earth-facing side fully illuminated and sometimes we see only a thin crescent. As the days pass, the Moon changes phase—crescent, gibbous, full. . .
In 29.53 days, the Moon goes from new moon through full moon and back to new moon again. Solar eclipses can take place only at new moon, and lunar eclipses may occur only at full moon.
So why don’t we have an eclipse of the Sun every 29.53 days—every time the Moon passes the Sun’s position? The reason is that the Moon’s orbit around the Earth is tilted to the Earth’s orbit around the Sun by about 5°, so that the Moon usually passes above or below the Sun’s position in the sky and cannot block the Sun from our view.
The Moon’s tilted orbit crosses the plane of the Earth’s orbit at two places. Those intersections are called nodes. The point at which the Moon crosses the plane of the Earth’s orbit going northward is the ascending node. Going south, the Moon crosses the plane of the Earth’s orbit at the descending node.
A solar eclipse can occur only when the Sun is near one of the nodes as the Moon passes. If the Sun stood motionless in a part of the sky away from the nodes, there would be no eclipses, and you would not be agonizing over this. But the Earth is moving around the Sun, and, as it does so, the Sun appears to shift slowly eastward around the sky, through all the constellations of the zodiac, completing that journey in one year. In that yearly circuit, the Sun must cross the two nodes of the Moon. Think of it as a street intersection at which the Sun does not pause and runs the stop sign every time. It is an accident waiting to happen. When the Sun nears a node, there is the “danger” that the Moon will be coming and—crash!
No. The Moon is 400 times closer to the Earth than the Sun, so the worst—the best—that can happen is that the Moon will pass harmlessly but stunningly right in front of the Sun. The Sun’s apparent pathway in the sky is called the ecliptic because it is only when the Moon is crossing the ecliptic that eclipses can happen. Thus, twice a year roughly, there is a “danger period,” called an eclipse season, when the Sun is crossing the region of the nodes and an eclipse is possible.
Featured image credit: “sunset-evening-romantic-sun” by Alexas_Fotos. CC0 via Pixabay.