Begin with Earth itself. Our planet is about thirteen thousand kilometers wide. That number feels large but still understandable. Commercial aircraft circle Earth in less than two days. Orbiting satellites trace its curvature in less than two hours. From space, Earth appears as a smooth blue sphere. From the ground, it feels vast but still navigable.Earth orbits our star, the Sun. The average distance from Earth to the Sun is called one astronomical unit. That distance is about one hundred fifty million kilometers. If Earth were the size of a marble, the Sun would be a large exercise ball about fifteen meters away. At that scale, the Moon would be a tiny bead about thirty centimeters away from Earth. The rest of the solar system would stretch across a small city.Our solar system contains the Sun, eight major planets, and many smaller worlds. The inner planets are Mercury, Venus, Earth, and Mars. They are rocky and relatively small. Farther out are Jupiter, Saturn, Uranus, and Neptune. These are giant planets made mostly of gas and ice. Asteroids orbit between Mars and Jupiter, and countless icy bodies inhabit the distant Kuiper Belt. A cloud of frozen comets, called the Oort Cloud, likely surrounds the entire system.Even the region controlled by the Sun’s gravity is tiny compared with the distances to nearby stars. The nearest star beyond the Sun is called Proxima Centauri. It lies a little over four light years away. A light beam leaving the Sun would need more than four years to reach it. At the speed of a fast spacecraft, the trip would require tens of thousands of years.Imagine shrinking the entire solar system so that Neptune’s orbit fits inside a large room. On that scale, the next nearest star would still be several hundred kilometers away. Stars are not crowded like streetlights in a city. They are scattered through the galaxy with terrifying emptiness between them.Together, hundreds of billions of stars form our home galaxy, the Milky Way. Viewed from outside, it would look like a great flattened disk with spiral arms. Our Sun is located in one of those spiral arms, about halfway from the center to the edge. The diameter of the Milky Way is roughly one hundred thousand light years. That means light takes one hundred thousand years to cross from one side to the other.Within this vast rotating disk, stars orbit the galactic center on enormous paths. Our Sun completes one full orbit in about two hundred million years. During that time, continents drift, species evolve, and entire mountain ranges rise and disappear. Human history spans only a tiny fraction of a single galactic year. Our civilization is very young on galactic timescales.Galaxies themselves are not isolated. They gather into groups and clusters bound by gravity. The Milky Way belongs to a small collection of galaxies called the Local Group. The Andromeda Galaxy is the largest member. It lies about two and a half million light years away. Many smaller galaxies orbit these giants like moons. Together, they drift through space as a gravitationally connected family.Beyond the Local Group, collections of galaxies form larger structures called clusters. These clusters can contain hundreds or even thousands of galaxies. Still larger arrangements produce filaments and sheets spanning hundreds of millions of light years. Between these filaments lie vast voids with very few galaxies. When astronomers map the positions of galaxies on the largest scales, a pattern emerges.The distribution of galaxies resembles a web, often called the cosmic web. Thin filaments of dark matter and galaxies form a network across the observable universe. Intersections of filaments appear as dense clusters. Great nearly empty regions open between them like cosmic bubbles. This web arises naturally from the way gravity amplifies tiny early fluctuations in matter.Our entire observable universe stretches about ninety billion light years across. That number refers to the distance between the most widely separated regions we can currently see. Light from the farthest galaxies began its journey over thirteen billion years ago. The space between us and them has expanded dramatically during that time. We therefore observe them as they were when the universe was young.This huge structure might feel abstract, so it helps to locate ourselves precisely. Astronomers sometimes describe a cosmic address, similar to a postal address. At the smallest familiar level, your address might begin with your street and city. For our purposes, we start with Earth. Your planet is Earth, orbiting a star called the Sun.The Sun belongs to the solar system, which occupies a tiny region of interstellar space. The solar system sits in the Orion Arm of the Milky Way Galaxy. The Milky Way lies inside the Local Group of galaxies. The Local Group is a part of the Virgo Supercluster, a large concentration of galaxy clusters. That supercluster belongs to an even larger region called Laniakea, meaning immense heaven. Laniakea is a vast basin in the cosmic web where galaxies flow together under gravity.This cascading address reveals both our significance and our smallness. We occupy one tiny planet around one ordinary star in a typical spiral galaxy. Our galaxy is just one of countless others in a web that may extend far beyond what we can see. Yet through thought and observation, minds on this small world have traced the structure of the cosmos.Astronomy’s roots stretch deep into human history. Long before telescopes, ancient people watched the sky with the naked eye. They saw regular patterns in the motions of the Sun, Moon, and stars. These patterns helped them track seasons, predict floods, and plan migrations. Astronomy began as practical sky watching tied closely to survival.Many ancient cultures built structures aligned with celestial events. Stone circles, temples, and pyramids often point to sunrise on solstices or equinoxes. In Mesopotamia, careful records of planetary positions were kept on clay tablets. In ancient China, court astronomers monitored the heavens for comets and eclipses. In Mesoamerica, complex calendars tracked the cycles of Venus and the Sun.These early observers produced impressive knowledge using only naked eye observations. They predicted eclipses and traced long term cycles of the Moon and planets. However, their explanations often mixed empirical observation with myth and astrology. The sky was viewed as a theater of gods rather than a physical system governed by universal laws.A major turning point came in ancient Greece. Thinkers such as Thales and Anaximander proposed that natural phenomena might have natural explanations. They suggested that the Sun, Moon, and stars were not divine beings but distant physical objects. Later philosophers attempted to build geometric models describing their motions. They placed spherical Earth at the center of nested celestial spheres.
