Well, I’m a tad late on this one if my goal was to be timely. But since my goal is to cover scientific topics that appear in current news, I’d be remiss if I didn’t use the recent solar eclipse across North America to talk about eclipses. Fortunately, there are a few things worth talking about (questions I had as a child that weren’t adequately explained to me) and it’ll give me an excuse to talk about some orbital mechanics.
Why Eclipses Happen (Or Don’t)
I’ll assume you all know the basics about the Moon’s movement around the Earth; one orbit per month, full moons happen when the Moon is on the far side of the Earth from the Sun so the illuminated side is facing Earth, new moons happen when the Moon is on the near side of the Earth from the Sun so the illuminated side is facing away from Earth. Solar eclipses happen when the Moon is directly between the Sun and Earth, so the Moon blocks the Sun and casts a shadow on Earth. Likewise, a lunar eclipse is when the Earth is directly between the Sun and Moon, so the Earth casts a shadow on the Moon. Now, if you’re like me as a child, your first question upon hearing this was if the new moon occurs whenever the Moon is between the Earth and Sun, and a solar eclipse occurs whenever the Moon is between the Earth and Sun, then why isn’t there a solar eclipse every month along with the new moon?
The reason for this has to do with orbital inclination. You see, a satellite’s orbit around a planet does not have to be around a planet’s equator, just around its circumference. A satellite’s orbit can be tilted relative to the planet’s equator, the orbit being the same size and dimensions as an equatorial orbit, but at an angle. Such an orbit, regardless of its angle, would spend half its time above the equator, half its time below the equator, and would cross over the equator twice in a single cycle.
And an orbit’s angle need not only be measured from the equator, it can also be measured relative to the ecliptic plane. To understand this, picture the Earth’s orbit around the Sun as a giant ring in space. Now imagine a perfectly flat plane emanating to this ring, as if it were a circle drawn on a sheet of paper. The Earth and the Sun would always have this plane running through their centers.* In order for any object to be directly between the Sun and Earth, it will inevitably be in this plane as well. The Moon has an orbital inclination of about 5° relative to the ecliptic plane, so it is almost always slightly above or slightly below the ecliptic. This is why every new moon isn’t a solar eclipse; to be directly between the Earth and Sun, the Moon has to be on the ecliptic plane. If the Moon is on the same side of the sky as the Sun, but it’s slightly above or below it, you get the effect of the new moon without the Sun getting blocked.
Please note: this image depicts how the Sun appears to move across the sky for the sake of simplicity. It is not saying the Sun orbits the Earth.
The two points on the Moon’s orbit where it crosses the ecliptic plane are called nodes. For a solar eclipse to happen, a node has to pass between the Earth and Sun. More often than not, these nodes are elsewhere in the Moon’s orbit, meaning the Moon passes through the ecliptic during a crescent or quarter moon and nobody notices. But these nodes are not fixed in space; the Moon’s orbit wobbles over time in a process called precession. The angle of this orbit is always roughly 5°, but the Moon doesn’t pass through the ecliptic in quite the same spot each time (animation by AstroTubo). With each pass, the location of the nodes moves slightly, with one node passing between the Sun and Earth every six months. This period every six months is called an eclipse season and it lasts for roughly 35 days, long enough to guarantee a solar and lunar eclipse.
Other Factors
So, if we get an eclipse season every six months, why is it always years between solar eclipses? Well, each season there is at least one eclipse visible from somewhere on Earth, but only about 12.5% of the Earth’s surface area has humans on it. The majority of solar eclipses are only visible from the ocean, poles, or other uninhabited regions. This is all to do with the fact that the Moon is less than a quarter the diameter of the Earth, so its shadow only covers an area roughly 270 kilometers (167 miles) wide at a time. This is the part in the path of totality, i.e., the region where all of the Sun’s light is being blocked. A partial eclipse, where some of the sun’s light is blocked but other parts of the Sun’s surface are still visible, can be seen in a region up to 6,400 kilometers (4,000 miles) wide. But a given location will only experience a total solar eclipse every 360 to 410 years.
A total solar eclipse is what you probably think of when you think about eclipses, which is what occurred a couple of weeks ago. This is where all the sun’s light is blocked somewhere on Earth, darkening the sky and leaving a glowing halo around the Moon. This is the Sun’s corona, the outer edge of the Sun’s atmosphere which is normally only visible when the Sun’s glare is blocked by an eclipse. But total eclipses only occur every 18 months on average, or once for every three seasons. While solar eclipses do occur in these other seasons, they won’t necessarily be total eclipses. There are eclipses where the point of totality, where all the sunlight is blocked by the Moon, doesn’t hit Earth at all.** There can be partial eclipses across parts of the world, but nowhere will the Sun be completely blocked. There are also annular eclipses; eclipses where the entire Moon is in front of the Sun but where this isn’t enough to completely block the Sun. Understand, the vast majority of orbits aren’t perfect circles. The Moon’s orbit around the Earth is slightly oval-shaped, meaning it will be about 40,000 km (25,000 miles) closer to Earth at its closest point versus its furthest point. This oval precesses just like the nodes where it crosses the ecliptic, so the location of the nearest point and the farthest point change over time. Annular eclipses occur when the Moon is further from Earth during an eclipse, so it can’t cover all of the Sun (such as the eclipse in October 2023). A total eclipse only occurs when the Moon is closer to the Earth. While annular eclipses are cool-looking in their own right (the Sun turns into a ring), it does not cause the sky to darken as in a total eclipse.
But of course, solar eclipses are not the only type of eclipse. When one orbital node is directly between the Earth and Moon, the other node is in Earth’s shadow. As such, eclipse seasons have one solar eclipse and one lunar eclipse. Because the Earth is far larger than the Moon, the Earth’s shadow covers the Moon’s entire surface and the eclipse is visible from the entire night side of Earth. Like solar eclipses, lunar eclipses can be partial, where part of the Moon are still illuminated by the Sun, or total, where the Moon falls entirely behind the Earth. During a total lunar eclipse, the Moon can appear to take on a reddish hue despite no longer being in direct sunlight. This is because the Moon is still being illuminated by light that passes through Earth’s atmosphere. As light travels through air, its path bends and certain colors of light get filtered out. Light from the Sun enters the Earth’s atmosphere along the “sides” of the sphere, primarily red light comes out the other side***, and its path is now bent such that it hits the Moon.
All of this is why eclipses are as rare as they are, but are still able to occur at all. It’s why when they happen, they’re noteworthy events that bring people together to watch the sky change in unfamiliar ways. And there is one other way that solar eclipses are incredibly, absurdly rare and uncommon. Eclipses can only occur because the Sun and the Moon are roughly the same size in the sky due to the Sun being 400 times bigger and 400 times further away than the Moon. This is purely a coincidence, there is no law of physics making this arrangement more likely than any other. If the Moon were significantly further from Earth, the only eclipses we’d ever get would be annular eclipses. If the Moon were significantly closer to Earth, the Moon would completely block the Sun and its corona, making the sky near identical to normal night. This extremely precise placement of the Moon in relation to Earth makes the uniqueness of eclipses possible, and it is not a permanent state. When the Moon formed, it was seventeen-times closer to Earth, but has slowly moved away at about 1.5 inches per year due to tidal forces. It was about 1.6 billion years ago that the Moon became far enough from Earth for the Sun’s corona to become visible during a solar eclipse. In about 750 million years, the Moon will be too far away to cause a total eclipse ever again. This will cover the vast majority of the time where Earth is an inhabitable planet, but it goes to show that we did not have to live in a world with solar eclipses. We are profoundly lucky to have this phenomenon, even if it's only once every few years.
For More Details
*To be clear, this plane would not be running through the Earth’s equator. Earth is tilted relative to the Sun (which causes the seasons), so the ecliptic plane would be tilted relative to us as well. The ecliptic plane does always run through the tropics (defined by the region of Earth between the Tropics of Cancer and Capricorn) and this region is defined as being the part of the planet where the Sun is directly overhead at least once during the year.
** For the purposes of eclipses, think of the node not as a point, but as a three-dimensional zone about as wide as the Earth that the Moon passes through to cause eclipses. The Moon can pass directly through the zone and cause a total eclipse, or it can clip the side and cause a partial eclipse. Remember, the solar system isn’t a perfect clockwork mechanism where everything happens exactly the same way each time, there are a ton of small variations that lead to small differences in an object’s position over time.
***This is related to why the sky appears red at sunrise and sunset. Quickest explanation possible; all light from the Sun is a mix of different colors that appear white when mixed together. For metaphor’s sake, imagine the light as a shower of raindrops made of different colors of paint. Blue light contains the most energy (see here), so when these energetic drops hit atoms in the atmosphere, they break up into a fine mist that inundates the area. This is why the sky appears blue during the day, only the blue light is getting scattered like this and it’s blocking one’s view of the other colors. But when light/paint scatters like this, it can’t travel any further. When the Sun is directly overhead, light only has to travel ~10,000 km to get from the top of the atmosphere to one’s eye. But during sunrise/sunset, this light is coming in from the side and has to travel through much more atmosphere to reach your eye, roughly 250,000 km. After traveling through so much air, all the blue light is long gone and the red light is all that’s left to scatter. And this red light is the only light that can make it through the 500,000 km of atmosphere to come out the other side and illuminate the Moon.
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