Lunar Eclipses

Podcast Transcript

For as long as humans have been watching the night sky, they noticed that every so often, during a full moon, the moon would briefly go dark, or at least change color.

They often created mythical explanations for the event.

Eventually, this ancient people began to record its occurrences and were able to calculate when it would happen.

As we learned more about the universe, the explanation for the moon changing became less mythical and more scientific, but it was still a sight to behold.

Learn more about lunar eclipses and how they work on this episode of Everything Everywhere Daily.

What a lunar eclipse is is pretty straightforward and I’m sure that most of you intuitively understand it.

A lunar eclipse occurs when the Earth comes directly between the Sun and the Moon, blocking sunlight from reaching the Moon. This can only happen during a full moon when the three celestial bodies are in a straight line, known as syzygy.

Just as an aside, syzygy is one of the greatest words in the English language. If you ever get a chance to use it, it’s worth 25 points in Scrabble.

A lunar eclipse is, in many ways, just the opposite of a solar eclipse, except instead of the moon being between the Earth and the Sun, in this case it is Earth between the Moon and the Sun.

While the two phenomena are conceptually the same, there are differences between the two.

For starters, solar eclipses occur relatively infrequently. Total solar eclipses are relatively rare events, occur somewhere on Earth about once every 18 months on average.

However, when solar eclipses do occur, they are only seen on a small stretch of Earth. For any specific location on the planet, a total solar eclipse happens only about once every 375 years.

Lunar eclipses, on the other hand, will occur about 2 to 5 times per year. When they take place, everyone on the planet that can see the moon can observe the eclipse.

So why do these similar phenomena behave so differently?

It all has to do with the relative sizes of the Earth and the Moon.

The Earth is much bigger than the Moon, which means that it casts a much larger shadow. Because the Earth is bigger and casts a bigger shadow, it is easier for the much smaller Moon to fit into its shadow.

This is why lunar eclipses are more frequent than solar eclipses and why the entirety of the moon can fit into the shadow of the Earth. It also explains why a solar eclipse only occurs over a very small area on the surface of Earth. It is because the Moon casts a very small shadow.

That being said, there are different types of lunar eclipses. There are three general classifications of lunar eclipses.

A total lunar eclipse is when the entire Moon passes through Earth’s umbra, which is the central, darkest part of Earth’s shadow.

A partial lunar eclipse is when only a part of the Moon enters the umbra, while the rest remains in the penumbra, the outer, lighter part of Earth’s shadow.

A penumbral lunar eclipse is when the Moon only passes through the penumbra, causing a subtle shading that is often difficult to notice. This is the least dramatic type of lunar eclipse and often isn’t even publicized.

If you’ve ever seen a total lunar eclipse, you might have noticed something about it. The moon isn’t totally dark.

Unlike a total solar eclipse where it becomes quite dark if you are in the path of totality, the moon doesn’t totally disappear as if it were like a new moon. Instead, it simply becomes less bright and develops a reddish hue.

Why does it do this? Why wouldn’t the moon totally disappear when it passes through the umbra of the Earth’s shadow, and perhaps, even a better question, why would the moon turn red?

Other than size, there is one big difference between the Earth and the Moon….the Earth has an atmosphere.

The Earth does completely block out the sun during a total lunar eclipse, however, light bends through the Earth’s atmosphere.

The Moon appears red due to what is called Rayleigh scattering, the same phenomenon that makes sunrises and sunsets red. Earth’s atmosphere bends sunlight and filters out shorter wavelengths, mainly blue and violet, leaving red and orange light to illuminate the Moon.

This effect is called the Blood Moon.

So, what would a total lunar eclipse look like if you were standing on the moon?

By sheer coincidence, the moon and the sun are approximately the same size as seen from the Earth. When the Moon passes in front of the Sun during a total solar eclipse, you can see a ring of light around the moon.

This is known as Baily’s Beads and it is literally sunlight shining between mountains on the moon.

If you look at the Earth from the Moon during a lunar eclipse, you’d see something similar, but for totally different reasons. You’d see a ring of light around the Earth.

That ring is coming from the light bending through the Earth’s atmosphere. That is where the light is coming from that provides the illumination and color of the Moon during a total lunar eclipse.

There is one photo taken between the Earth and Moon during the Apollo 12 mission which shows the Earth eclipsing the Sun.

Another thing to note is that because a lunar eclipse can only occur during a full moon, the Earth eclipsing the sun could only be seen from the side of the Moon facing the Earth, never on the far side of the Moon.

Why don’t Lunar eclipses occur every month during the full moon?

For the same reason solar eclipses don’t. Lunar eclipses do not occur every month because of the tilt of the Moon’s orbit and the need for precise alignment of the Sun, Earth, and Moon.

The Moon’s orbit around Earth is tilted about 5° relative to Earth’s orbit around the Sun (the ecliptic plane).

Because of this tilt, the Moon passes above or below Earth’s shadow during most full moons, avoiding an eclipse.

Everything I just described about lunar eclipses involves our modern understanding of astronomy. However, it wasn’t always that way.

Lunar eclipses have fascinated humanity since we first gazed at the night sky. These dramatic celestial events have shaped cultural beliefs, religious practices, and scientific understanding across civilizations.

The earliest systematic observations of lunar eclipses come from ancient Mesopotamia. Babylonian astronomers created the first known astronomical diaries, recording celestial events on clay tablets in cuneiform script.

The Babylonians viewed lunar eclipses primarily as omens with political significance. Their records typically followed a pattern: first describing the astronomical event, then interpreting its meaning for the kingdom. For example, a tablet from around 700 BC states: “The Moon was eclipsed on the 14th day. The king will be overthrown by his son.”

By the 8th century BC, Babylonian astronomers had discovered the Saros cycle—an approximately 18-year period after which lunar eclipses repeat with similar characteristics. This discovery represented one of humanity’s first major achievements in predictive astronomy. The clay tablets known as the “Enuma Anu Enlil” contained records of about 7,000 celestial omens, with specific sections dedicated to lunar eclipses.

Chinese astronomers maintained perhaps the most comprehensive and continuous record of lunar eclipses in the ancient world. The earliest confirmed Chinese record of a lunar eclipse dates to 1136 BC during the Shang Dynasty, recorded on oracle bones.

In Chinese cosmology, lunar eclipses were explained as the celestial dragon or toad attempting to devour the Moon. Court astronomers were tasked with beating drums and shooting arrows into the sky to frighten away these creatures. The political significance was enormous—failing to predict an eclipse could result in an astronomer’s execution, as it suggested the emperor had lost the “Mandate of Heaven.”

While the Egyptians left fewer explicit records of individual eclipses than their Mesopotamian counterparts, lunar events were integrated into their religious calendar and mythology.

The Egyptian deity Thoth was associated with the Moon and measurement of time. During lunar eclipses, Egyptians believed the cosmic order was temporarily threatened. Some hieroglyphic texts suggest they interpreted the reddish color of a total lunar eclipse as the Moon being wounded.

The Vedic texts of ancient India, particularly the Rigveda, composed around 1500 to 1200 BC, contain references to lunar eclipses. Indian astronomers described the phenomenon as Rahu, a demon, temporarily swallowing the Moon.

Around the year 400 in the Surya Siddhanta, one of the earliest astronomical treatises, Indian mathematicians had developed sophisticated methods for predicting eclipses. In the 6th century, the astronomer Aryabhata correctly explained that lunar eclipses occurred when the Moon entered Earth’s shadow,

Mesoamerican astronomers achieved remarkable precision in predicting lunar eclipses. The Dresden Codex, one of the few surviving Maya books, contains tables that track the Moon’s position with enough accuracy to predict eclipses.

For the Maya, lunar eclipses represented moments when cosmic balance was threatened. They believed that during an eclipse, the Moon was being attacked by the Sun or was suffering from illness. Rituals would be performed to “aid” the Moon in its recovery.

The Aztecs similarly viewed eclipses as times of danger and possible cosmic disruption. Pregnant women would place metal objects on their abdomen during eclipses to protect their unborn children from disfigurement, showing how astronomical understanding influenced daily life and medical practices.

Greek philosophers made significant advances in understanding the true nature of lunar eclipses. Anaxagoras was reportedly the first Greek to understand that lunar eclipses resulted from Earth casting its shadow on the Moon.

Aristotle used the curved shape of Earth’s shadow during lunar eclipses as evidence that Earth was spherical—one of the earliest scientific deductions based on eclipse observations.

Astronomers during the Islamic Golden Age preserved and expanded upon Greek astronomical knowledge. In the 9th and 10th centuries, Al-Battani refined calculations for predicting lunar eclipses, while observatories in Baghdad, Cairo, and later Samarkand kept detailed records of these events.

The astronomer Al-Biruni, who lived in the 11th century, wrote extensively on lunar eclipses and calculated the distance to the Moon using eclipse observations. Islamic scientists generally understood the scientific explanation for eclipses while acknowledging their religious significance. Many viewed eclipses as reminders of God’s power rather than omens.

Monastic chroniclers often recorded lunar eclipses in medieval Europe. The Anglo-Saxon Chronicle, for instance, notes a lunar eclipse in 734: “In this year the moon was as if it had been sprinkled with blood, and Archbishop Tatwine and Bede died.”

While the scientific explanation for eclipses was known to educated Europeans through preserved classical texts, popular interpretations still tended toward the supernatural. Lunar eclipses were frequently seen as divine warnings or signs of impending disaster.

A famous historical example occurred in 1504, when Christopher Columbus, stranded in Jamaica with a mutinous crew and hostile locals refusing to provide food, consulted his almanac and discovered an upcoming lunar eclipse. He told the indigenous people that his god would show displeasure by making the Moon disappear. When the eclipse occurred as predicted, the frightened locals quickly provided supplies.

The history of lunar eclipse observations represents one of humanity’s longest continuous scientific endeavors, spanning cultures and millennia. From fear-inducing omens to calculable astronomical events, our changing relationship with lunar eclipses mirrors the evolution of human understanding about our place in the cosmos and the advancement of science.

The Executive Producer of Everything Everywhere Daily is Charles Daniel. The Associate Producers are Austin Oetken and Cameron Kieffer.

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