Chapter 2

Don't Look Up - But Wait, You Can! Solar Eclipses

IN THIS CHAPTER

Digging into the varieties of solar eclipses

Timing your viewing

Running (or driving, or sailing) into the path of totality

One of the most stunning phenomena is a total solar eclipse. Partial solar eclipses occur when the Moon passes in front of the Sun but doesn't completely block out its disk. Total solar eclipses happen when the Moon passes completely in front of the Sun, obscuring any sunlight from reaching us on Earth. An annular solar eclipse, on the other hand, occurs when the Moon will be completely in front of the Sun but is too far away to completely block it, leaving a bright visible ring.

Cosmic Coincidence?

Although eclipses are a sight to see, they are not created via any sort of divine intervention or mystical influence. The fact that eclipses exist at all can be thought of as a planetary coincidence. The Moon just happens to be the right size and distance from the Earth that it appears to be exactly the same size as the Sun in the sky. The Sun is much bigger than the Moon (its diameter is 400 times greater than the Moon's), but it's also much farther away (400 times farther from Earth than the Moon), and these factors conspire to create eclipses.

If the Moon was a bit smaller or farther away from Earth, it wouldn't block out all the Sun's disk in an eclipse. If the Moon was a bit bigger or closer to Earth, then it would more than cover the Sun's disk, and you wouldn't see the corona nearly as well. There's no inherent physical reason why the size of the Sun and the Moon appears to be the same to us, but that doesn't stop us from enjoying the end result. We are lucky to be able to observe solar eclipses, and our goal with this book is to make sure that you can too.

Eclipses are very rare from individual places on Earth, but a total solar eclipse occurs somewhere on Earth at least every few years. In addition to total and annular solar eclipses, you can also see partial eclipses, when the Moon blocks out only part of the Sun's disk. These aren't quite as impressive as total eclipses, but still well worth observing.

Solar eclipses occur when the Moon passes between the Sun and Earth, blocking the Sun's rays from reaching us on Earth as we pass into the Moon's shadow. Figure 2-1 shows the geometry of a solar eclipse.

DO ECLIPSES HAVE A FUTURE?

Will we always be able to view total solar eclipses from the Earth? The short answer is no. The Moon is slowly moving away from the Earth at a rate of about 1.5 inches (3.8 centimeters) per year. 600 million years in the future, the Moon will have moved far enough away that it can no longer completely cover the Sun during an eclipse. At this point in the far-off future, there will still be solar eclipses, but only annular and partial ones.

© John Wiley and Sons, Inc.

FIGURE 2-1: Diagram of a solar eclipse.

One Size Does Not Fit All

There are two main types of solar eclipses, partial and total. Another category consists of annular eclipses, a sub-type of partial eclipses. Here's the scoop on each, and how they relate to each other:

Total solar eclipse

A total solar eclipse occurs when there's a direct alignment of the Earth, Moon, and Sun. This type of eclipse is also called a blackout eclipse, one where the Sun is completely covered by the Moon for several minutes.

A total eclipse takes place in multiple stages:

• A partial eclipse appears first, as the Moon comes closer and closer to covering the Sun. The start of this phase is known as "first contact," and you're in the penumbra (outer shadow) of the Moon.
• The phase known as totality appears when the Sun is completely covered by the Moon. The start of this phase is known as second contact, and you're in the umbra (inner shadow) of the Moon at this point. Totality can last up to several minutes.
• Finally, the Moon continues moving until its motion removes it from the face of the Sun into another partial eclipse (third contact) and then finally back to normal (fourth contact). The moment when the Sun first reappears can produce what's known as the diamond ring effect (also seen at the moment the Sun disappears before totality). See Figure 2-2 to better understand the visual impact of this effect.

peterralph/Adobe Stock

FIGURE 2-2: Diamond ring phase of a total solar eclipse.

Annular solar eclipse

An annular solar eclipse occurs when the Moon passes directly in front of the Sun but doesn't completely block it out. This type of solar eclipse occurs because the Moon's distance from the Earth varies over the course of its orbit. If a total solar eclipse happens to occur when the Moon is at its farthest distance from the Earth, the apparent size of the Moon will actually be less than the apparent size of the Sun. The implication? The Moon can't completely cover the Sun, and the result is what's called an annular solar eclipse.

An infrequent scenario can even arise in the form of a hybrid eclipse, one in which a total solar eclipse converts to annular (or an annular solar eclipse converts to total) at some point along the path of the eclipse.

During an annular solar eclipse, you can see a thin bright ring that runs the full circumference of the Sun, as well as a dark interior where the Moon covers most of the Sun. While an annular eclipse can be spectacular, even the tiny amount of Sun left exposed means that the sky still remains bright - the amazing effects of a total solar eclipse won't be visible since the Sun is still shining through. As seen in Figure 2-3, which shows the stages of an annular eclipse, this kind of eclipse is also nicknamed a ring of fire.

Alameen R/Adobe Stock

FIGURE 2-3: Stages of an annular solar eclipse.

Annular eclipses can be viewed by us Earthlings, but they have also been captured from space! The crew of the International Space Station was able to capture an image of an annular eclipse on June 21, 2020, as shown in Figure 2-4. The path of the eclipse is visible as the Moon's shadow passes over the surface of the Earth (here, over China).

Courtesy of NASA ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center

FIGURE 2-4: An astronaut's view of an annular eclipse.

Partial solar eclipse

Partial solar eclipses occur when the alignment between the Sun, Earth, and Moon isn't quite right to produce a total solar eclipse. In a partial eclipse, the Moon passes in front of the Sun but only partially obscures it, because it passes either too high or too low to fully cover it.

During a partial eclipse, the Moon covers a portion of the Sun. Using eclipse glasses, the sun may look like it has a bite taken out of it, oddly (or not so oddly) in the shape of a crescent moon. Partial solar eclipses also take place before and after the totality phase of a total solar eclipse, and if you aren't directly in the path of totality, you'll also see a partial eclipse. Figure 2-5 shows the stages of a partial solar eclipse.

manuel805/123 RF

FIGURE 2-5: Stages of a partial solar eclipse.

Partial solar eclipses are more common than total eclipses, but they also don't have quite the drama of total solar eclipses. In fact, you might not even notice that a partial solar eclipse was taking place if you weren't in the know. Total solar eclipses, on the other hand, are impossible to miss because, quite literally, they turn day into night for a short period of time.

Eclipse Timing and Geometry

Solar eclipses occur about every 18 months somewhere on Earth, and are fast - on average, only 2 to 4 minutes will pass before totality is over. Depending on the geometry of the eclipse and your viewing location, totality can be as short as a few seconds or more than 7 minutes long. If you look at all the eclipses over a 12,000-year period that starts at 4000 BCE and ends at 8000 CE, eclipses range in length from a brief 9-second eclipse back in the year 919 CE up to a mammoth 7-minute-29-second eclipse in 2186! Your great grandchildren might be in for a huge treat if they head to South America in 2186, when the path of this super-long eclipse will pass through Guyana, Venezuela, and Columbia.

There's a new moon every month, so why isn't there an eclipse every month? It turns out that it is because the line of the Moon's orbit is tilted by about 6 degrees when compared to the plane in which the Earth orbits the Sun. Sometimes the shadow of the Moon is above the Earth, sometimes it's below the Earth, and only occasionally is it exactly crossing the Earth to produce an eclipse. Figure 2-6 shows the tilted geometry of the Moon's orbit and the Earth's orbit.

© John Wiley and Sons, Inc.

FIGURE 2-6: Geometry showing when conditions are favorable for a solar eclipse.

Although there isn't an eclipse every month, eclipses do occur in...