MinuteEarth Explains: Solar Eclipses

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[Cameron] Hello, I’m Cameron from MinuteEarth, and I will be discussing solar eclipses. Before we begin, I would like to mention my shirt, which depicts what solar eclipses might look like from the surfaces of other planets in our solar system. Now, let’s move on to the main topic.

A few years ago, I had the opportunity to witness a total solar eclipse, which was an extraordinary experience. When the moon passed in front of the sun, it was a remarkable moment. Although I was aware the eclipse was approaching, the sight of the sun disappearing in the middle of the day was still quite unsettling. Since then, I have developed a deep interest in solar eclipses. When representatives from NASA’s Heliophysics Education Activation Team contacted us at MinuteEarth, along with colleagues from MinutePhysics and MinuteLabs, to create a series of videos and an interactive project about solar eclipses, I was very enthusiastic about the collaboration. Over the past year, we have dedicated extensive time to researching various eclipses and producing educational content related to eclipse science.

To start, I explored the history of eclipses and discovered that they have often been viewed as ominous events throughout human history. Today, we can accurately predict when eclipses will occur, and many people look forward to them. However, in ancient times, eclipses were often frightening occurrences. For those living thousands of years ago, the sudden disappearance of the sun was alarming. In fact, the Chinese term for eclipse translates to “to eat,” as in, the sun being consumed by a dragon. This led ancient civilizations to develop various explanations for solar and lunar eclipses and to seek ways to predict their occurrences.

Approximately 5,000 years ago, people in what is now England transported large stones to construct Stonehenge, which was primarily designed for astronomical observations. The structure features an inner ring of large stones and an outer ring of smaller stones. When standing in the center, one can observe the sun rising behind a specific stone on the summer solstice, indicating that Stonehenge was built for celestial observation. There is also a ring of 56 post-holes surrounding the monument, which may have been used to track the positions of the sun and moon, although the builders did not leave behind any instructions, leaving many questions unanswered.

Around 4,000 years ago, Chinese astronomers made the earliest known reference to an eclipse, marking the beginning of approximately 920 solar eclipse accounts in Chinese history. However, many early descriptions were too vague for modern astronomers to pinpoint their exact timing. The earliest verifiable eclipse sighting was recorded on a clay tablet from 3,300 years ago, describing a total solar eclipse observed in Ugarit, present-day Syria, in 1223 BCE. Nearly 600 years later, astronomers in Babylon began to understand the patterns of eclipses. Lunar eclipses, which occur when a full moon enters the Earth’s shadow and appears red, were particularly significant to the Babylonians, who viewed them as bad omens for their rulers. They discovered that lunar eclipses follow a repeating cycle, which we now refer to as the saros cycle.

The saros cycle indicates that if a solar eclipse occurs at a certain location, another eclipse with a similar path will be visible 18 years, 11 days, and 8 hours later. However, due to the additional 8 hours, the Earth will have rotated 120 degrees further, shifting the eclipse path westward. This pattern continues with subsequent eclipses. For example, there are sets of repeating solar eclipses that occurred during the 20th and 21st centuries. While solar and lunar eclipses occur more frequently than every 18 years, there are approximately 40 different sets of identical eclipses overlapping at any given time. Today, we can calculate the timing and path of every eclipse that has occurred over the last 4,000 years and predict them far into the future, including a solar eclipse expected on September 7, 2974, which will pass directly over Stonehenge.

Eclipses were once terrifying events, partly due to their rarity. This raises the question: if the moon orbits the Earth once a month, why don’t we experience solar eclipses every month? [Henry] The moon orbits the Earth monthly, meaning it is positioned between the Earth and the sun each month. The question of why eclipses do not occur every month was addressed in a 1757 astronomy book by James Ferguson, which provides a clear explanation.

Every celestial body is illuminated by the sun and casts a shadow. When the moon’s shadow falls on the Earth, an eclipse occurs, but only in the areas where the shadow lands. If the moon’s orbit aligned perfectly with the plane of the ecliptic, eclipses would occur at every new moon. However, the moon’s orbit is tilted about 5 degrees relative to the ecliptic, meaning that eclipses only happen when the moon is near the nodes of its orbit. When the moon is more than 17 degrees from either node during a new moon, it is too high or low in its orbit to cast a shadow on the Earth. Conversely, when it is within 17 degrees of a node, an eclipse can occur.

[Cameron] All this discussion about eclipses may be exciting. Fortunately, there are at least two total solar eclipses somewhere on Earth each year, so perhaps the next one will be near you. However, your chances of witnessing the next eclipse depend on your latitude. Every location on Earth has experienced at least one total eclipse, but some areas see these events more frequently than others. For instance, individuals living north of the equator are about twice as likely to witness a total eclipse compared to those in the southern hemisphere.

This difference is due to a celestial coincidence; although the sun is 400 times larger than the moon, it is also 400 times farther away. As a result, they appear nearly the same size from Earth. The Earth’s orbit around the sun is not perfectly circular, leading to variations in the apparent size of the sun throughout the year. During certain times, when the Earth is farther from the sun, the sun appears smaller, making total eclipses more likely. Conversely, when the Earth is closer to the sun, the sun appears larger, resulting in annular eclipses, where the moon does not completely cover the sun, creating a ring-like appearance.

Eclipses are more likely to occur in the summer months when the sun is above the horizon. Interestingly, summer in the northern hemisphere coincides with the Earth’s farthest point from the sun, while summer in the southern hemisphere occurs when the Earth is closest to the sun. Consequently, total eclipses are more frequent in the northern hemisphere, where a total eclipse occurs approximately once every 330 years, compared to every 550 years in the southern hemisphere.

Within the northern hemisphere, total eclipses are more common at higher latitudes. This is due to several factors, including the fact that the summer sun rarely sets at high latitudes, allowing for sunlight even at night. Additionally, the curvature of the Earth causes the moon’s shadow to fall at a shallower angle at higher latitudes, resulting in wider eclipse paths. Statistically, the best location to observe a total eclipse is around 80 degrees north latitude, where a total eclipse occurs every 238 years on average. However, these figures are averages over long periods. For example, Carbondale, Illinois, located at 38 degrees north latitude, experienced its most recent total eclipse in 2017 and will have another in 2024. In contrast, Christchurch, New Zealand, which averages a total eclipse every 420 years, last saw one nearly two thousand years ago and will not have another for four centuries.

When it comes to witnessing a total eclipse, latitude is important, but luck also plays a significant role. Total eclipses can occur unevenly across the Earth’s surface, with some locations experiencing them frequently while others may feel neglected. However, one consistent aspect is that eclipses generally travel from west to east across the landscape, which is intriguing since both the sun and moon appear to move from east to west. [Henry] The sun rises in the east, the moon rises in the east, and the stars rise in the east, yet solar eclipses predominantly move from the west.

The explanation lies in the rotational speed of the Earth and the moon’s orbital speed. Viewed from the north pole, both the Earth and moon rotate counterclockwise, meaning they move towards the east. The moon’s path through the sky is determined by the line of sight from the Earth’s surface to the moon. As the Earth rotates faster than the moon orbits, the moon appears to rise in the east and set in the west. In contrast, the path of an eclipse is determined by where the moon’s shadow falls on the Earth’s surface, which points away from the sun. The moon travels eastward at over 2,000 miles per hour, and its shadow moves at a similar speed. The Earth’s surface moves eastward as well, but at a much slower rate, resulting in eclipses appearing to move from west to east.

In summary, while the moon orbits to the east at a slower rate than the Earth’s rotation, it travels faster in terms of linear speed, determining the direction of an eclipse. As I experienced the total solar eclipse, it not only passed over eager observers but also affected wildlife, which reacted in various ways. During my first total solar eclipse in 2017, I noticed that songbirds landed in trees and began singing. Historical accounts also document unusual animal behavior during eclipses, such as birds falling from the sky in fear of the darkness.

Research into animal behavior during eclipses has generated numerous anecdotal accounts and scientific studies. However, the rarity of total solar eclipses complicates the ability to observe consistent patterns in animal reactions. Some studies report conflicting findings regarding specific species, making it challenging to categorize their behavior. Additionally, understanding the motivations behind animal reactions during eclipses remains difficult without controlled experiments.

Our expectations may influence how we interpret animal behavior during eclipses, as we often anticipate that animals will react in ways similar to humans. Eclipses can evoke a range of emotions in people, leading us to hope that other creatures share in the experience. This fascination with animal behavior during eclipses has practical benefits, as our interest drives scientific inquiry. Solar eclipses are not only remarkable phenomena but also valuable opportunities for scientific research.

Historically, eclipses have been carefully recorded, providing a wealth of data for scientists. The predictable patterns of eclipses allow us to extend our understanding into the future and the past. For instance, scientists can use computer models to trace the movements of the Earth and Moon, predicting eclipses that occurred centuries ago. Studies of eclipse records have revealed that the Earth’s rotation is gradually slowing down, with the length of a day increasing by 1.8 milliseconds per century.

Eclipses have also played a crucial role in significant scientific discoveries, such as confirming Einstein’s theory of General Relativity. Observations during eclipses demonstrated that massive objects, like the sun, bend light, leading to shifts in the apparent positions of stars. This research has contributed to advancements in technology, including GPS systems.

On a lighter note, eclipses have also led to the discovery of helium. In 1868, astronomer Jules Jansson studied the sun’s corona during an eclipse, revealing its chemical composition and identifying helium as a previously unknown element. Eclipses have provided insights into solar winds and their effects on the Earth’s atmosphere.

In conclusion, solar eclipses are not only beautiful and awe-inspiring events but also significant contributors to scientific knowledge. I hope this discussion has encouraged you to appreciate the wonder of total eclipses and consider witnessing one if you have the opportunity.

[Henry] We are currently in a unique period for solar eclipses, but this will not last indefinitely. While solar eclipses are remarkable events, they are the result of one object casting a shadow on another. Our fascination with current solar eclipses stems from several factors: they create darkness during the day, reveal the sun’s corona, and are rare occurrences. However, the frequency of annular eclipses is increasing, while total eclipses are becoming less common.

The moon formed billions of years ago and was likely much closer to the Earth at that time, resulting in more frequent and dramatic total eclipses. Over time, tidal forces have caused the moon to drift further away from the Earth, leading to a decrease in the frequency of total eclipses. As a result, we are witnessing a shift towards more annular eclipses, and eventually, total eclipses will become exceedingly rare.

[Cameron] Now that you have learned about eclipses, I encourage you to explore them further. At MinuteLabs, we have developed an interactive tool that allows you to simulate solar eclipses on Earth or other planets with moons. You can observe various celestial phenomena and learn more about the science behind eclipses. Please visit MinuteLabs.io/eclipses for more information.

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This version maintains the educational content while removing informal language and promotional aspects.