BCE and CE, centuries, and how to hold deep time in your head

Welcome to your first lesson in world history. Our goal is to build a strong foundation, step by step. Today, we begin with something very basic but extremely important: how we measure and talk about time in history. This might seem simple, but understanding it deeply will help you make sense of everything else we will learn. We will discuss what BCE and CE mean, how centuries work, and finally, how to truly grasp the immense scale of the past. First, let's talk about the system we use to label years. You will often see years followed by letters like BCE or CE. BCE stands for "Before the Common Era." CE stands for "Common Era." These are the terms widely used today in academic and scientific communities around the world. You might also know them by older terms: BC, which means "Before Christ," and AD, which means "Anno Domini." "Anno Domini" is Latin and means "in the year of the Lord." Both sets of terms refer to the same calendar system, but BCE and CE are preferred in modern scholarship because they are neutral and inclusive, not tied to any specific religion. They allow everyone to discuss history using a common framework. This system, counting years forward and backward from a central point, was developed by a monk named Dionysius Exiguus. A monk is a man who dedicates his life to religious service, often living in a community with other monks. Dionysius lived in the 6th century CE, and his specific task was to calculate the correct date for Easter, a religious holiday. In 525 CE, he developed a system that counted years from what he believed to be the birth of Jesus of Nazareth. This became known as the Anno Domini, or AD, system. It's important to know that Dionysius Exiguus’s calculation of Jesus's birth date is not perfectly accurate by modern historical standards. Most historians now believe Jesus was likely born a few years earlier than 1 CE, possibly between 6 and 4 BCE. But the numbering system itself, starting with year 1 CE, became widely accepted and is still the foundation of our calendar today. There's one crucial detail about this system: there is no year zero. The year 1 CE immediately follows the year 1 BCE. Think of it like a number line with positive and negative numbers, but without a zero in the middle for the years. So, to go from 1 BCE to 1 CE, you move forward one year. If an event happened in 10 BCE and another in 10 CE, there are 19 years between them (10 years before 1 CE, and 9 years after 1 CE, not 20). This no-year-zero concept can sometimes feel a bit strange, but it's essential to remember. Now, let's look at centuries. A century is a period of 100 years. This seems straightforward, but how we label them can sometimes be confusing. The *first* century CE began in the year 1 CE and ended in the year 100 CE. So, the first century includes all the years from 1 to 100. This pattern continues: the second century CE began in 101 CE and ended in 200 CE. This means that the number of the century is always one higher than the "hundreds" part of the year. For example, the 19th century includes years from 1801 CE to 1900 CE. The 20th century covers 1901 CE to 2000 CE. And we are currently in the 21st century CE, which began in 2001 CE and will end in 2100 CE. The same logic applies to centuries BCE. The first century BCE ran from 100 BCE to 1 BCE. The second century BCE ran from 200 BCE to 101 BCE, and so on. Understanding this numbering system is fundamental for putting historical events in their correct sequence. Why do we use BCE and CE instead of BC and AD now? It's a matter of inclusivity and clarity. History is a global subject, studied by people from all cultures and beliefs. Using BCE and CE creates a common, neutral language for everyone. It helps us avoid language that might feel exclusive or religious to some. This shift became widely accepted in academic, scientific, and international contexts during the latter half of the 20th century. While some people still prefer BC and AD, the core system of counting years remains the same. The change in terminology is about fostering a shared, worldwide understanding of the past. Now that we have the basic terms down, we come to the most challenging part of today's lesson: how to grasp what historians call "deep time." Deep time refers to the immense spans of time involved in the history of the Earth, the universe, and indeed, human existence. Our personal lives are measured in decades, maybe a century. Recorded history—history written down by people—goes back a few thousand years. But the actual past stretches back billions of years. Holding this vastness in your mind is difficult, but it is crucial for understanding the true scale of human history. Let's try an analogy. An analogy is a comparison between two things to help explain something complex. Imagine the entire history of the Earth, from its formation to today, as a single calendar year. If the Earth formed on January 1st, then the first very simple forms of life would not appear until March. Complex, multi-celled life would only emerge around October. Dinosaurs would arrive in mid-December and disappear just a few days later, around December 26th. And what about humans? Modern humans would appear in the last hour of the last day, December 31st, at about 11 PM. All of recorded human history—everything we will study this year—would happen in the last few seconds before midnight on December 31st. This analogy helps us understand a critical concept: human recorded history is an extremely small fraction of the Earth's history. When we talk about "ancient history," even that is a tiny sliver. The Earth itself is about 4.5 billion years old. Life on Earth began about 3.8 billion years ago. The first multi-celled organisms appeared about 600 million years ago. Dinosaurs lived from about 230 million years ago to 65 million years ago. Our own species, *Homo sapiens* – modern humans – appeared in Africa roughly 300,000 to 200,000 years ago. This brings us to a second way to hold deep time in your head: scaling. Scaling means understanding the relative sizes of different time periods. We will be studying history that spans thousands of years. But it is important to remember that these thousands of years are preceded by hundreds of thousands of years of human existence before writing, and millions of years of our earlier human ancestors, and billions of years of life and Earth before that. Let's put some key milestones in perspective. * The Earth forms: about 4,500,000,000 years ago (4.5 billion years) * First life appears: about 3,800,000,000 years ago (3.8 billion years) * Dinosaurs go extinct: about 65,000,000 years ago (65 million years) * Modern humans (*Homo sapiens*) appear: about 300,000 to 200,000 years ago * The development of agriculture: about 12,000 years ago * The first cities and writing: about 5,000 years ago Notice the enormous difference in the numbers: billions, millions, thousands. It's not just about adding more zeros; it's about a fundamentally different scale of time. When we talk about 4.5 *billion* years, we are talking about a span of time so vast it is almost impossible to truly imagine. A billion is a thousand millions. If you count one number every second, it would take you about 32 years to count to a billion. So, how do historians manage this? We break it down. We focus on periods. And for human history, we are often dealing with thousands of years, sometimes tens of thousands. When we look at the very, very distant past, like the time before writing, we rely on archaeology – the study of human history and prehistory through the excavation of sites and the analysis of artifacts and other physical remains. For the past few thousand years, we also have written records. It’s important to understand that our knowledge of deep time is built on scientific evidence from geology, which is the study of the Earth’s physical structure and substance, and from paleontology, which is the study of fossil animals and plants. These sciences provide a framework for the history of life and the planet itself. When we turn to human history, we use different kinds of evidence, like archaeological discoveries and eventually, written documents. Think of time as a very long road. Most of that road is the history of the Earth before humans. Then, for a relatively short stretch, there are early humans, leaving behind tools and bones. And only for the very last part of that road do we find buildings, books, and detailed stories. Our course will mostly focus on this last, relatively short, but incredibly rich and complex, stretch of the road: the period of human history with written records. However, remembering the context of deep time is always important. It helps us appreciate how recent many human inventions are, how quickly some things have changed, and how slowly others have evolved. For instance, agriculture, the practice of farming, only developed about 12,000 years ago. For hundreds of thousands of years before that, humans lived as hunter-gatherers, moving from place to place to find food. The invention of agriculture was a massive turning point, but it's a very recent event in the overall history of our species. In summary, when you see BCE and CE, you now know they are inclusive terms for a standard way of measuring years. You understand that there is no year zero. You know that a century is 100 years, and how to count them correctly. And critically, you have started to grapple with the concept of deep time, recognizing that human history, especially recorded history, is just a tiny fraction of the Earth's vast past. This understanding will help you keep perspective as we dive into the incredible stories of human civilization. Next, we will explore why historians divide the past into different periods, like "ancient," "medieval," and "modern," and what problems can arise from doing so. This will further build your framework for understanding the timeline of world history.