7. El espinazo de la noche.

Summary

The episode traces humanity’s changing understanding of the stars, from mythological explanations to scientific inquiry. It presents the exploration of the cosmos as both a study of nature and a search for humanity’s place in the universe.

Stars, Planets, and Scale

Stars are distant suns; planets do not generate their own visible light but reflect starlight. The Sun is one star among many in the Milky Way, and the Solar System lies away from the galaxy’s center. The possibility that other stars have planets—and perhaps life—is raised as a question, not presented as a confirmed discovery.

Gravity and Planetary Shape

Gravity pulls matter toward the center of a large body, helping explain why planets are roughly spherical. Smaller bodies with weaker gravity can remain irregular; Mars’s moon Phobos is used as an example.

Myth and the “Backbone of the Night”

The Milky Way has inspired different cultural explanations. The episode describes Kalahari hunter-gatherers’ image of it as a backbone supporting the sky, and the ancient Greek myth that it was milk scattered across the heavens.

The Rise of Natural Explanations

In ancient Ionia, thinkers began asking whether nature could be understood through consistent rules rather than the direct actions of gods. The episode links this shift to diverse, connected communities and a tradition combining practical experience with abstract thought.

Early Scientific Ideas and Methods

• Thales proposed a natural explanation for how dry land formed, drawing on observations such as sediment in the Nile Delta.
• Anaximander used the shadow of a vertical stick to investigate the year and seasons, illustrating measurement through observation.
• Empedocles used a water-ladle demonstration to argue that invisible air is matter and can exert pressure.
• Democritus proposed that matter consists of atoms moving through empty space. He also argued that the Milky Way is made of the light of many stars and imagined many worlds.
• Anaximander’s proposal that life began in water and that humans evolved from simpler forms is presented as an early anticipation of evolutionary thinking.

Mathematics, Observation, and Dogma

Pythagoras and his followers emphasized mathematical order and helped popularize the idea of the cosmos as an ordered, knowable universe. The episode contrasts this with secrecy about some mathematical discoveries and with Plato’s preference for abstract reasoning over observation. It also recounts the Pythagoreans’ discovery that √2 cannot be expressed as a ratio of whole numbers.

The Sun-Centered Solar System

Aristarchus argued that Earth rotates and orbits the Sun, reasoning that the Sun’s much greater size made it unlikely to orbit Earth. The episode notes that this proposal was later associated with Copernicus.

Measuring Stellar Distance

The episode describes an experiment attributed to Christiaan Huygens: comparing the apparent brightness of Sirius with a small opening that reduced sunlight. It illustrates how apparent size and brightness can help estimate distance, while noting that the inferred distance was inaccurate.

Searching for Planets Around Other Stars

Two detection approaches are demonstrated:

• Blocking starlight: Suppress a star’s glare to reveal a nearby, dim planet.
• Observing a star’s motion: Detect a planet indirectly through the small movement it induces in its star.

The Vastness of the Universe

The Milky Way is presented as a disk-shaped galaxy with the Sun in its outer regions, and the universe as containing many galaxies. The episode closes by emphasizing how small Earth’s place is on cosmic scales and humanity’s continuing impulse to explore.

Researchers and Sources Featured

• Carl Sagan — presenter and narrator.
• Ancient thinkers: Thales, Anaximander, Empedocles, Democritus, Anaxagoras, Pythagoras, Aristarchus, Plato, and Aristotle.
• Other scientists and historical figures: Theodore (the Ionian engineer), Charles Darwin, Johannes Kepler, Leonardo da Vinci, Nicolaus Copernicus, Christopher Columbus, Christiaan Huygens, and Harlow Shapley.
• Other sources or groups mentioned: Kalahari hunter-gatherers; Babylonian and ancient Greek traditions; the Voyager spacecraft.