Eclipse!: The What, Where, When, Why, and How Guide to Watching Solar and Lunar Eclipses

By Philip S. Harrington

The most complete guide to viewing eclipses-including details on every solar and lunar eclipse through 2017

Want to observe the most fleeting eclipse phenomena, take dramatic photos, and keep a detailed record of the experience? Now you can be prepared. This comprehensive one-stop resource covers everything you need to know about solar and lunar eclipses-why they happen, how to view them, how to photograph them, even when and where they will occur through the year 2017. Here's where to turn for:
* Detailed explanations of eclipse mechanics and dynamics, viewing techniques, and what to look for, both in the sky and all around you
* Extended discussions of eclipse photography and videography-film selection and developing, filter requirements, special care of equipment, and more
* Intriguing individual and group activities you can carry out during an eclipse to heighten your enjoyment and deepen your understanding of the event
* Detailed maps and discussions on how and where to best view each eclipse, plus travel considerations, likely weather conditions, and equipment recommendations

Whether you're a backyard astronomer, a dedicated eclipse chaser, or a teacher guiding students through their first eclipse experience, Eclipse! provides the in-depth, detailed, practical information you need to make the most of these thrilling celestial marvels of nature.

• Language: English
• Publisher: Turner Publishing Company
• Release date: Apr 21, 2008
• ISBN: 9780470302453

Book preview

Gossamer clouds of glowing hydrogen and cluster of colorful stars. Alien planets enveloped in noxious atmospheres and airless worlds pitted with innumerable craters. Frigid comets with long, graceful tails, and infinitely distant galaxies. These are just some of the wondrous sights that the sky holds in store for stargazers. All are unique, all are special. But of all there is to enjoy in our universe, none has drawn more attention, struck greater fear, or captured the hearts and souls of stargazers more than eclipses of the sun and moon.

Those who have witnessed their beauty firsthand describe total solar eclipses as the most awe-inspiring events that nature has to offer. People travel great distances just to witness the stark beauty of the solar corona, chromosphere, and prominences—all features of the Sun that are normally hidden by the intensity of the Sun’s brilliant surface, the photosphere.

Lunar eclipses can also be powerfully moving events. As the Moon slips into the shadow of the Earth, it will frequently take on a colorful, ruddy tint that many observers have compared to the view of the red planet Mars through telescopes. The view can be especially memorable if the Moon is nestled within a star-filled backdrop. And unlike a solar eclipse, whose maximum phase is visible over a comparatively small portion of the Earth’s surface, a lunar eclipse appears exactly the same from anywhere on the night side of the Earth. This convenience affords many more people the opportunity to enjoy an eclipse’s beauty without leaving their homes.

Our fascination with these captivating celestial events is nothing new. Solar and lunar eclipses have attracted a great deal of attention for as long as humans have looked at the heavens.

A BRIEF HISTORY LESSON

Throughout human history, the night sky was for the most part considered to be serene and never-changing. Our ancestors took great solace in that feeling of permanence. Although every once in a while an unexplainable event would pass over their heads, such as a shooting star, or meteor, flashing briefly across the heavens, or perhaps simply a wandering star (which is the literal meaning of the word planet) moving slowly against the backdrop of fixed stars, they usually trusted that these were benign acts of the gods.

But occasionally sky events took place that struck unbridled fear in people’s souls. Perhaps a hairy star, or comet, would grace the skies. Until relatively recent times, these were believed to be omens of evil. But for many, the most feared events occurred when either of the two greatest sky gods, the Sun and Moon, were in peril—during an eclipse. Most ancient cultures saw eclipses as portents of evil. Some interpreted eclipses as signs that something or someone was trying to consume or steal these vital sky entities; others viewed them (or at least saw solar eclipses) as the Sun and Moon battling each other. Entire civilizations would drop everything when an eclipse occurred, doing everything in their power to scare away the awful creature bent on destroying their gods.

Perhaps as far back as 2800 B.C., the ancient Chinese saw a rhythm in the occurrence of solar eclipses, although they believed a ferocious dragon was devouring the Sun! One frequently recounted tale comes from the Hsia Dynasty. Fearing that the wrath of the cosmos would influence earthly events, the dynasty’s fourth emperor, Chung K’ang, relied heavily on two court astrologers, named Hsi and Ho, for celestial forecasts. The story goes that on one fateful day Hsi and Ho failed to predict the occurrence of a solar eclipse. When the emperor confronted the astrologers with this grievous error, he found them both drunk on wine. He became so infuriated by their irresponsibility that he ordered them executed!

The Chinese were not alone in viewing solar eclipses with fear. Diverse cultures in Europe, Africa, Asia, and the Americas believed that a solar eclipse was caused by a terrible monster eating the Sun. The ancient Norse tribes thought that an enormous wolf, named Sköll, gobbled up the Sun during an eclipse, while the tribespeople living in Mongolia and eastern Siberia felt that they were caused by Alkha, another creature who had an insatiable appetite for the Sun. Legend had it that Alkha was beheaded by the gods, but even this did not stop the severed head from searching the heavens for the Sun!

Many civilizations decided that the best way to vanquish the demon that was consuming their Sun was to band together and make as much noise as possible to scare it away. At the first sign of an eclipse, everyone would immediately gather in the center of town to bang drums, and shout and scream as loudly as they could. It must have worked; the Sun returned every time!

Solar eclipses have even altered the course of human history. In 585 B.C. the Lydians and the Medes were doing battle in what is present-day Turkey. The Greek historian Herodotus recorded that, at the height of a particularly fierce battle, darkness fell upon the land. Apparently the two armies had the good fortune to wage war very near the path of a total solar eclipse! The armies took this as a sign, stopped fighting instantly, and came together to make peace.

Lunar eclipses are not without interesting tales, as well. Many ancient cultures, including the Greek, Chinese, Islamic, and Mayan, had legends that associated lunar eclipses with plagues, earthquakes, and other disasters.

Perhaps the most famous lunar-eclipse anecdote comes from the exploits of Christopher Columbus. Times had become desperate during his fourth voyage to the New World, as an epidemic of shipworms turned his fleet into a collection of sieves. Finally, conditions forced him to beach his sinking armada on Jamaica. The natives provided the castaways with food and shelter, but tension mounted among the shipwrecked crew as the passing weeks turned into months. Finally, some six months later, half of the crew mutinied, attacking the remaining crew, murdering the natives, and stealing their food. Not surprisingly, this put an immediate halt to Columbus’s bartering with the natives for additional food.

Columbus knew from an astronomical almanac he had brought along on the voyage that a total lunar eclipse would be seen from Jamaica in just a few days. He also knew that the Jamaicans were terrified by such events. Capitalizing on this, Columbus told the native chiefs that unless they immediately gave his crew food, the angry Christian God would turn the Moon blood red. Sure enough, that night the eclipse went off as predicted. The terrified natives quickly made amends with ample food offerings, and continued to keep the crew well fed until help from home arrived.

THE AGE OF UNDERSTANDING

While many peoples dreaded eclipses, others yearned to understand exactly what was happening. Perhaps the oldest testament to early humans’ attempt to understand the universe is Stonehenge, situated on Salisbury Plain in England. Stonehenge, constructed by several cultures between about 2800 B.C. and 1500 B.C., is believed to have been used to measure the motions of the Sun and Moon. Though most authorities agree that none of the cultures who constructed and used Stonehenge could predict exactly when an eclipse would occur, they may have been able to issue warnings of the likelihood of an eclipse on the order of days or even weeks before it happened.

The ancient Greek astronomer Hipparchus attempted to understand eclipses by using them to make scientific observations. Around 130 B.C., from observations of a solar eclipse seen from Hellespont and Alexandria, Hipparchus determined that the Moon was approximately 429,000 kilometers (268,000 miles) away, only about 11 percent more than today ’s accepted distance.

Just as Hipparchus was anxious to understand how and why eclipses of the Sun and Moon occur, so also were the Chinese. The first-century-B.C. astronomer Liu Hsiang showed that he was one of the first to understand basic eclipse mechanics when he wrote that the Sun is eclipsed because the Moon hides him as she moves on her way.

Although these early eclipse pioneers showed great genius in their conclusions, well over a millennium passed before much of the human race began to comprehend the workings of eclipses. One of the first western astronomers to record a scientific observation of a total solar eclipse was Johannes Kepler in 1605, although little attention was apparently paid by his contemporaries. More than a century later, Edmund Halley published his account of the 1715 April 22 (OS) total solar eclipse in the Philosophical Transactions of the Royal Society in London. He, too, described the sight, though he misinterpreted much of what he saw.

HOW DOES AN ECLIPSE WORK?

Most of our scientific knowledge of solar and lunar eclipses has been gained within the last century and a half. Today we are well aware that a solar eclipse is the result of the Moon coming between the Earth and the Sun, and that a lunar eclipse is the result of the Earth coming between the Sun and the Moon. Why eclipses only happen at certain times is a bit more complicated.

The Moon orbits the Earth once every 27.3 days (what astronomers call the sidereal month), and the Earth orbits the Sun in 365.2 days. From these combined motions, it has been found that the Moon takes 29.5 days—about two days longer than the sidereal month—to go through a complete set of phases (New Moon, then on to First Quarter, Full Moon, Last Quarter, and back to New again). This period is called the synodic month (figure 1.1), or a lunation.

Figure 1.1 The Moon’s orbit around the Earth. Note that a solar eclipse, when the Moon passes between Earth and Sun, can only occur during the New Moon phase. A lunar eclipse, when the shadow of the Earth is cast upon the Moon, can only occur during Full Moon.

Why isn’t an eclipse seen every month? The answer appears in the edge-on view of the Earth-Moon-Sun system shown in figure 1.2. Notice how the Moon’s orbit about the Earth is inclined about 5° with respect to the Earth’s orbit of the Sun. As a result, the Moon only crosses the Earth’s orbital plane (the ecliptic) twice every orbit, at points called nodes. The Moon is usually above or below the Sun in our sky at New Moon, and misses the Earth’s shadow at Full. Only on the comparatively rare occasions when the Moon passes near a node at the New and Full phases can eclipses take place. If the Moon crosses the Earth’s orbit from south to north (i.e., from below the plane to above the plane as seen from the northern hemisphere), it is referred to as the ascending node (abbreviated by the symbol ). Descending node refers to the passage from north to south (above the plane to below), and is symbolized by Ω in the figure.

The nodes gradually shift location along the ecliptic as both the Earth and the Sun play a game of celestial tug-of-war called the regression of the nodes, with the Moon caught in the middle (figure 1.3). This drifting of the nodes will realign the New Moon and Full Moon phases with the Sun every 173.3 days, a period referred to as an eclipse season. Therefore, each calendar year sees at least two eclipse seasons when solar and lunar eclipses can occur. Two eclipse seasons make up an eclipse year, or 346.6 days.

Figure 1.2 Why don’t eclipses occur every month? An edge-on view shows how the Moon’s orbit is tilted with respect to the plane of Earth’s orbit, causing the shadows to pass out into space.

Figure 1.3 A diagram showing an eclipse year. Only when the ascending or descending nodes line up can eclipses occur. Notice how they line up twice during the cycle, each creating two (or more) eclipses.

Notice how an eclipse year is shorter than a calendar year by 18.6 days. This difference gives some calendar years not two, but three, four, or even five solar eclipses! There may also be up to five lunar eclipses, but the combined number of lunar and solar eclipses will never exceed seven.

Eclipses typically occur in pairs, with a solar eclipse immediately preceding and/or following (by about 15 days) a lunar eclipse. Imagine this busy scenario. If, at the January New Moon, the Moon just grazes part of the solar disk (producing a partial eclipse of the Sun), it is possible to have a second partial solar eclipse 29.5 days later, at the next New Moon, with the chance of a lunar eclipse in between. The same set of circumstances may occur when the New Moon and the nodes line up again in June and/or July. Finally, the calendar year will end with a fifth solar eclipse in either November or December, also possibly paired with a lunar eclipse. Seven-eclipse years are rare. The last such year, 1982, featured three total lunar and four partial solar eclipses, while the next, 2038, will bring with it four penumbral lunar, one total solar, and two annular solar eclipses.

SOLAR ECLIPSES

As shown in figure 1.1, a solar eclipse is only seen at New Moon, when the Moon moves between Earth and Sun. When all three bodies are aligned, the Moon casts its shadow across a portion of our world’s surface, blocking some or all of the sunlight from reaching the affected region. Just how much of the Sun will be hidden depends on where the observer stands, relative to the Moon’s shadow.

Again referring to figure 1.1, you will see that there are two parts to the Moon’s shadow, just as there are with all shadows. The dark, central, cone-shaped region shown on the figure is called the umbra, while the lighter gray, fan-shaped area is the penumbra. If an observer is located outside of the penumbra, then no eclipse will be seen. Those who are positioned within the penumbra will see a portion of the Sun covered by the Moon. The farther an observer is within the penumbra, the greater the percentage of Sun covered by the Moon. Those situated within the umbra will see a total solar eclipse (figure 1.4). Since both Earth and Moon are moving, the umbra will trace a line along the Earth’s surface during an eclipse, creating a central path of totality. The region affected by a partial eclipse is usually bow-shaped, owing to the Earth and Moon’s movement, as well as the Earth’s curvature.

Figure 1.4 A total eclipse of the Sun, the heavens’ most glorious celestial sight. Photo by Ernie Piini. (Total eclipse of 1983 June 11 from Jogjakarta, Java; 88-mm f/6.8 refractor, 1-second exposure on Ektachrome 200 slide film.)

The Total Solar Eclipse

For a few precious moments the Moon will completely cover the Sun entirely along the central path of a total solar eclipse. During totality, the photosphere, the blindingly bright surface of the Sun that is visible on any sunny day, is hidden from view, allowing other, normally invisible features of the Sun to be seen. Surrounding the photosphere is a thin, deep-red layer of the Sun called the chromosphere. Measuring only a few thousand kilometers thick, the chromosphere can usually be seen for only scant seconds at the beginning and end of totality. Protruding from behind the Moon’s silhouette are the glorious, flamelike prominences, stretching for thousands of kilometers into space. Finally, encircling the eclipsed Sun and extending for several times the Sun’s diameter, is the pearly white corona. As we will explore in chapter 3, the appearances of both the prominences and corona vary from eclipse to eclipse.

Timing and location are everything when viewing a total solar eclipse. By the time it reaches Earth, the Moon’s umbra is only 270 kilometers (170 miles) across at its widest, even though it can travel a third of the way around the Earth in a matter of a few hours (figure 1.5). Therefore, the chance that the umbral shadow will pass over any one particular spot on the Earth is slim. Oddsmakers say that any one given point on the Earth can expect to see a total solar eclipse on average only once in 360 years.

The Sun measures 1,392,000 kilometers (864,900 miles) in diameter, while the Moon is a comparatively puny 3,476 kilometers (2,160 miles) across. That works out to be a ratio of approximately 400 to 1; that is, the Sun is about 400 times larger in diameter than the Moon. At the same time, the Sun is about 149,600,000 kilometers (93,000,000 miles) from Earth, while the Moon is right next door at 384,500 kilometers (240,000 miles) away. Call it divine intervention or just dumb luck, but that ratio also works out to be about 400 to 1. As a result, the Moon and Sun each appear the same size in our sky—about half a degree.

Figure 1.5 The shadow of the Moon projected onto the Earth during the 1991 July 11 total solar eclipse. This montage of weather-satellite images shows the shadow at several discrete points along the path of totality. Courtesy of Dr. William Emery and Timothy D. Kelley, Colorado Center for Astrodynamic Research, University of Colorado at Boulder.

This near-perfect fit, with the Moon just covering the Sun’s brilliant surface while still exposing our star’s normally invisible chromosphere and corona, is critical to the majesty of a total solar eclipse. If the Moon appeared noticeably larger than the Sun in our sky, these characteristics would be blocked from view; if it were smaller, then the bright surface of the Sun would never be fully covered, leaving them lost in the glare. (It turns out that of all the planets and satellites in our Solar System, Earth is the only world that enjoys this situation. On all of the other planets, their satellites are either too small or too large to cover the Sun so perfectly.)

The Moon’s orbit around the Earth is not circular, but rather oval or elliptical. At its closest point (called perigee), the Moon is 356,000 kilometers (221,000 miles) away, while at its farthest (apogee), the Moon is 407,000 kilometers (253,000 miles) distant. Likewise, Earth’s elliptical orbit of the Sun brings it as close as 147,100,000 kilometers (91,452,000 miles) at perihelion (the point where Earth is closest to the Sun), and as far as 152,102,000 kilometers (94,562,000 miles) at aphelion (the point where Earth is farthest from the Sun). As a result, the apparent sizes of the Moon and Sun vary slightly in our sky (see figure 1.6 for an example of the Moon’s apparent size change). The greater the Moon-to-Sun size ratio, the longer an eclipse’s duration. At its longest, totality can last 7

Reviews for Eclipse!

[flemmingv-1 · Rating: 4 out of 5 stars]

Good for photographers. Get more from the eclipses, good tips for what to look after when observing an eclipse.