The Sky Above: How Ancient People Watched the Stars

Have you ever lain on your back in the grass on a clear summer night and looked up at the sky? If you have, you probably noticed thousands of tiny points of light scattered across the darkness. Maybe you tried to count them. Maybe you noticed that some of these lights were brighter than others. And maybe—if you were very lucky and far away from city lights—you saw a faint, cloudy band stretching across the sky that ancient people called the Milky Way, like a river of light flowing above you.

If you've done this, you've done exactly what humans have been doing for tens of thousands of years. You have looked up at the night sky and wondered: "What's out there? What does it all mean?"

Here's an amazing fact: virtually every human civilization that has ever existed on our planet—from the ancient Chinese to the ancient Greeks, from the Native American tribes to the ancient Egyptians, from the Aboriginal Australians to the Mesopotamians—has looked up at the night sky and tried to make sense of it. This isn't a coincidence. It's because the night sky is one of the most visible, predictable, and mysterious things in our world. It changes with the seasons, it guides travelers, and it has inspired stories, religions, and entire sciences.

In this chapter, we're going to explore how ancient people from different parts of the world all learned to read the night sky. You'll discover that humans in ancient China and ancient Greece—lands separated by thousands of miles, speaking completely different languages, and with no way to communicate with each other—actually noticed many of the same things in the sky and came to similar conclusions. This is what we call a "cosmic connection"—a connection made through the stars that transcend borders, languages, and cultures.

But before we can understand what ancient people saw, we first need to understand what exactly they're looking at. What are all those lights in the sky? Are they all the same thing? The answer, fascinatingly, is no. The night sky contains several different types of objects, and learning to tell them apart is the first step in becoming an astronomer.

What Are Stars, Anyway?

Let's start with the objects you're most likely to notice first: the stars. Those countless points of light that seem to fill the darkness? Almost all of them are stars—enormous balls of hot, glowing gas, much like our own Sun, but so far away that they appear as tiny pinpricks of light in our sky.

Think about it this way: the Sun is a star. It's the closest star to Earth, and that's why it looks so much bigger and brighter than any other star. If you could travel 93 million miles away from the Sun (which is how far Earth is from the Sun), you would see it as just another point of light in the sky, similar to all the other stars you see at night. In fact, if you could look at our solar system from a great distance—say, from the planet Neptune—you would barely be able to see the Sun at all. It would look just like one of the stars you see on a clear night.

The closest star to our Sun (after the Sun itself) is called Proxima Centauri. It sounds close to Earth because its name starts with "Proxima," which means "closest." But in reality, Proxima Centauri is incomprehensibly far away. If you could travel there in a spaceship at the speed of a commercial airplane (about 500 miles per hour), it would take you approximately 5.8 million years to arrive. Even traveling at the speed of light—which is the fastest anything can possibly go—it would take more than four years to reach Proxima Centauri. Light from that star takes four years to travel to your eyes each night.

This is why stars appear as tiny points of light even through the most powerful telescopes. They're just too far away. But what they lack in apparent size, they make up for in actual size. Most stars are much, much larger than Earth. Our Sun is about 864,000 miles across—meaning you could fit about 1.3 million Earths inside it. And some stars are much, much larger than our Sun. A star called Betelgeuse, which appears as a reddish point of light in the constellation Orion, is so large that if you replaced our Sun with Betelgeuse, it would extend past the orbit of Jupiter. It's almost 700 times larger than our Sun.

Stars come in different colors, too. If you look carefully on a clear night, you might notice that some stars appear reddish, some appear white, some appear bluish, and some appear yellowish. The color of a star tells us something important about it: how hot it is. Blue stars are the hottest, white stars are very hot, yellow stars (like our Sun) are moderately hot, and red stars are the coolest. This might seem backwards—you might expect blue to be cool and red to be hot—but in stars, it's the opposite of what we see in everyday life.

> 📚 DID YOU KNOW? > > If you look at the flame of a candle, the hottest part near the bottom where the flame touches the wick is blue. The cooler part at the top is orange or yellow. This is similar to how stars work—the blue color means more energy and heat.

Planets: The Wanderers

Now, not everything you see in the night sky is a star. If you watch the sky carefully over many nights, you'll notice that most of the points of light stay in the same position relative to each other. The stars form patterns that remain fixed (though they do slowly shift with the seasons). But sometimes, you'll see a bright point of light that moves against this background of fixed stars. It wanders from one constellation to another over weeks and months.

Ancient people noticed this, and they named these wandering lights "planets," from the Greek word *planetes*, which means "wanderer." The ancient Chinese astronomers noticed the same thing and called them "moving stars" or *xing dong*, which had a similar meaning.

So what's the difference between a star and a planet? The key difference is that planets don't make their own light the way stars do. Stars are giant balls of hot gas that produce energy through nuclear fusion (we'll learn more about this in Chapter 3). Planets, on the other hand, are much smaller, colder bodies that orbit around stars. They shine in our sky because they're reflecting light from the Sun, just like a mirror reflects light from a lamp.

In our solar system, we have eight planets (astronomers recently "demoted" Pluto from planet to "dwarf planet" because it's too small to be a true planet). These eight planets are divided into two groups: the inner rocky planets (Mercury, Venus, Earth, and Mars) and the outer gas giants (Jupiter, Saturn, Uranus, and Neptune).

The planets visible to the naked eye from Earth are Mercury, Venus, Mars, Jupiter, and Saturn. You might be able to see these with your own eyes on a clear night—they look like bright stars, but unlike stars, they move across the sky over time. Sometimes planets appear very bright because they're close to Earth, and sometimes they're dimmer because they're farther away.

Venus is often the brightest object in the night sky (after the Moon). This is because Venus is covered in thick clouds that reflect sunlight very efficiently, like a mirror. The ancient Chinese called Venus *Tai-bai* (太白) or "the Great White" and associated it with the goddess of war and beauty. The ancient Romans called the same planet Venus after their goddess of love and beauty. Different names, same planet—a cosmic connection across cultures!

Mars is another planet with a strong cultural presence. Because of its reddish color (caused by iron rust in its soil), the ancient Chinese called it *Ying Huo* (荧惑), which roughly means "fire star" or "blinking fire." They thought it looked like fire in the sky. The ancient Romans named it after their god of war because its red color reminded them of blood. Again, we see different civilizations looking at the same object and creating stories around it.

Jupiter, the largest planet in our solar system, was called *Suì Xīng* (岁星) by the ancient Ch