History of Astronomy

Professor Dave wants to tell us about the solar system. We now have a good understanding of how stars and galaxies, including our own Milky Way, form. It's time to focus on our specific location within this cosmic structure.

If we zoom in on the Orion arm, which is fairly distant from the galactic center, we see a yellow main sequence star. This star is not particularly special - it's about the average size, with a mass equal to our sun. But this star is significant because it's the one our planet orbits, and we call it the sun.

The sun formed around 4.6 billion years ago from a cloud of gas and dust that was rich in heavy elements, produced by the deaths of older stars in supernovas. This cloud began to spin and flatten into a protoplanetary disk. The bulk of the cloud coalesced at the center due to gravity, igniting nuclear fusion and forming the sun. The remaining material in the disk then gradually accumulated into the planets and other objects we see in the solar system today.

Over hundreds of thousands of years, the dust and debris in the disk collided and stuck together, growing into larger and larger objects. The inner, rocky planets formed as these accumulating clumps became massive enough to take on a spherical shape under their own gravity. Meanwhile, the outer regions, containing more ice and gas, formed the gas giant planets.

The solar system has not remained static since its formation. Gravitational interactions, like the alignment of Jupiter and Saturn billions of years ago, have caused disruptions and bombardment events. But overall, the basic structure we observe today emerged from the gradual accretion of material in the protoplanetary disk.

The sun itself is a fairly typical G-type main sequence star, with a hot, dense core where nuclear fusion occurs, surrounded by radiative and convective zones. It has an atmosphere, including a relatively cool chromosphere and a very hot corona, whose high temperatures are not yet fully understood.

The solar system is minuscule compared to the Milky Way galaxy, but the sun is immense compared to the planets. It makes up over 99% of the mass of the solar system and dominates the orbits of the planets. These planets, from the innermost rocky worlds to the outer gas giants, along with moons, asteroids, and other objects, make up the diverse system we call our solar home.

The formation of this complex system can be understood through two key astronomical processes - the creation of heavy elements in stars, and the accretion of interstellar gas and dust into a protoplanetary disk. This helps explain how even a seemingly spontaneous system like our solar system could arise naturally from these fundamental mechanisms.

Throughout human history, civilizations have observed the night sky, but science involves more than just observation. It's about finding explanations, creating models and predictions, taking measurements, and refining our understanding when predictions don't match observations.

After centuries of pure observation, astronomy became more mathematical, with some of the earliest known scientific calculations happening in ancient Greece and other contemporary civilizations. One of the first realizations was that the Earth is round. This idea was initially based more on the aesthetic appeal of the sphere rather than logic, but Aristotle later provided a more scientific explanation - he noticed the curved edge of the Earth's shadow on the Moon during a lunar eclipse. Additionally, the visibility of different stars depending on one's location on Earth was easily explained by the planet's spherical shape.

Once the Earth's spherical nature was established, the next step was to measure its dimensions. Eratosthenes was the first to do this with impressive accuracy. He used the fact that the sun shone directly down a well in one part of Egypt, while casting a shadow on an obelisk in another location, to deduce the Earth's circumference. By simple geometry, he calculated the circumference to be around 250,000 stadia, or about 25,000 miles - an impressive feat given the limited tools available at the time.

The ancient Greeks also made progress in measuring distances and sizes of other celestial bodies. Aristarchus, for instance, deduced that the Moon's diameter is about one-third that of the Earth by analyzing the curvature of the Earth's shadow on the Moon during a lunar eclipse. He also proposed that the Sun is much larger than the Earth, and that the Earth orbits the Sun - an idea that was not widely accepted at the time, but eventually led to a major shift in our understanding of the cosmos.

Previously, Aristarchus had correctly proposed that the sun, rather than the earth, is at the center of the solar system. However, this heliocentric model was ahead of its time, and the geocentric model, perfected by Ptolemy in second century Egypt, remained dominant for centuries. Ptolemy's model could explain the puzzling retrograde motion of the planets by proposing that they move on smaller circles, or epicycles, that orbit the larger circular path around the earth. While this model was reasonably accurate, it grew increasingly complex over time, requiring different formulas for each planet.

In the 1500s, the geocentric model finally became untenable, leading to a paradigm shift in astronomy. Copernicus was the first to revive the heliocentric model since Aristarchus. He demonstrated that the problems with the geocentric model disappear if the sun is placed at the center, with objects closer to the sun orbiting faster. Copernicus even calculated the distances of the planets from the sun with great precision.

However, the heliocentric model faced some lingering criticism. One valid concern was that if the earth orbits the sun, the apparent positions of the stars should shift, which was not initially observed. It would take several centuries before telescopes could detect this "parallax" effect, which is indeed extremely small due to the vast distances to the stars. By measuring this parallax, astronomers could use trigonometry to calculate the distances to stars.

The Copernican revolution had profound philosophical implications, as it challenged the notion of the earth and humanity as the center of the universe. This threatened the power and supremacy of the Catholic Church, leading to the persecution of astronomers like Giordano Bruno. The story of our evolving understanding of the solar system serves as a reminder of the importance of defending the freedom to pursue knowledge, even in the face of opposition and tyranny.

The Copernican revolution, which placed the sun at the center of the solar system instead of the Earth, was a major event during the European Renaissance, a time when scientific thought truly began to flourish. In this climate, the Danish astronomer Tycho Brahe used his status and wealth to build the most sophisticated instruments ever constructed for studying the heavens, and he collected the best astronomical data of the time. Through meticulous observation of the planets and other objects, the Copernican model was further corroborated, but the stage was set for its refinement as well.

Copernicus believed the planets traveled in perfectly circular orbits around the sun, but this changed when Brahe's young assistant, Johannes Kepler, analyzed the new data. Kepler determined that the planets do not trace circles around the sun, but rather ellipses. These shapes are discussed in depth in the algebra portion of my mathematics playlist, but the key point is that unlike a circle with one center, an ellipse has two foci, and the sum of the distances from the foci to any point on the ellipse is constant. When the foci are very close together, the ellipse starts to resemble a circle, which is why the planetary orbits appeared circular for so long. Kepler's brilliance was recognizing that the planets actually follow elliptical paths, with the sun at one focus of each ellipse. The closest and farthest points in a planet's orbit are called perihelion and aphelion, respectively, and while the distances to these points are very close for most planets due to the low eccentricity of the ellipses, they are nevertheless not circular.

This realization was the first of Kepler's three laws, which essentially marked the birth of celestial mechanics. Kepler's second law states that a planet's orbital speed varies with its distance from the sun - it slows down when farther away and speeds up when closer in, such that the planet sweeps out equal areas in equal time intervals. Kepler's third law relates a planet's orbital period to the length of the semi-major axis of its elliptical path, with the precise relationship depending on the planet's mass. These laws, derived from observation, are remarkably precise in predicting the positions of the planets, demonstrating that the universe obeys mathematical principles that can be deciphered by humanity. This was a revolutionary development in scientific thought, as we were no longer helpless victims of the natural world but could now understand our surroundings.

Simultaneously, the Italian scientist Galileo Galilei was making important observations through the best telescopes of the time. He saw that the moon had distinct features like mountains and craters, transforming it from a mysterious glowing disk into a world of its own. He observed sunspots, deducing the sun's rotation, and Jupiter's moons, proving that not everything in the solar system orbits the Earth or sun. He saw Saturn's rings and the phases of Venus, further undermining the geocentric model. Galileo did as much as anyone to rewrite our perception of the cosmos, though his contentious status with the Catholic Church led to his house arrest in later life.

The year Galileo died was the same year Sir Isaac Newton was born, and Newton's contributions to science were immeasurable. Beyond his laws of motion, he provided the first accurate description of gravity, which was then used to demonstrate that Kepler's laws were consequences of Newton's own principles. Our understanding of the solar system changed dramatically from Ptolemy to Newton, with the discovery of new planets and small objects, but the heliocentric model with elliptical planetary orbits became firmly established.