Introduction

As discussed in the introduction video for this module, when we begin studying astronomy we have to orient ourselves and the way we view the world in a way that people don’t often consider in their daily lives. One aspect of this orientation is to carefully draw connections between the physical idea of the Solar System that you may have—of the Earth and all the planets revolving on orbits around the Sun, spinning away on rotational axes—and the sky that we view from our perspective on Earth. We will tackle this aspect of astronomical orientation in the later part of the learning material for this module.

To begin with, our goal is to address yet another aspect of astronomical orientation: understanding the place of here and now in relation to the rest of the Universe. That sense of here and now has the amazing property that it can at once make us feel so small, as we live our short lives in our tiny corner of the universe, and yet large in the way that life on planet Earth is connected to the evolution of the Universe. This vast connection between life on Earth and the history of our evolving Universe is what astrophysicist Neil deGrasse Tyson calls the most astounding fact about the Universe.

The discovery of this whole picture of the evolution of the Universe and the origin of the atoms that comprise our bodies began with the planets of our Solar System. It was by learning how to sort out and by sorting out the description of the planets, that people began to understand what is out there in space. The planets were the most difficult problem in ancient astronomy—a problem that took more than two thousand years to solve once people first set their minds to the task. It required the development of the scientific method—the most careful procedure we’ve come up with to probe the inner workings of nature—before it could be solved.

This course is dedicated to the astronomy of planets. In the next two modules, we will examine the history of astronomy, taking an in depth look at the problem of sorting out the planets’ motion. After that, we will explore the methodology of astronomy—how we observe light from the Universe through telescopes and interpret our observations based on our knowledge of the way light interacts with matter. After that, we will examine in detail the things we know about each of the objects of our Solar System, working our way outwards from the Sun. As mentioned above, in the present module we will begin all of this with a basic description of the map of the night sky. However, before even getting to that we’re going to start with a sneak peak at the final results—the picture of the Universe as we know it today.

 

Where Are You?

From an astronomical perspective, one answer to the question “where are you?” might be “on a tiny speck in the middle of nowhere.” To most people, the Earth seems quite large. After all, its circumference is over 40,000 km. That is 20 million times the height of an adult human being. It would take 20 million adult humans laid head-to-toe to circle the Earth once. In contrast, we know that people are composed of atoms so small that about 100 trillion must come together to form a cell that we can see through a microscope. The hydrogen atom—the smallest atom in nature—is around 10^–10 m in diameter. It would take a billion billion tightly packed hydrogen atoms just to circle the Earth once.

But in regard to astronomical scales, the Earth is miniscule. In fact, the Earth is so small that the metre—the unit of measure we use on Earth—becomes meaningless shortly after we leave the Earth’s surface. Instead, astronomers find it more convenient to use a unit such as the light-year to measure distance. The light-year is a unit of distance defined by the fastest-moving thing in the universe—light. One light-year is the distance that light travels in one year. Light moves at a speed of 300,000 km/s. There are therefore 10 million million kilometres in a light-year.

Due to the extreme speed of light, it is able to travel around the Earth nearly ten times in a second. In fact, this is why we can communicate with people all over the world (via phone, internet, etc.) in real time. The distance to the Moon is roughly ten times the circumference of Earth; therefore, it takes just a little over a second for light to travel from the Earth to the Moon—i.e., the distance from the Earth to the Moon is roughly a light-second.

The distance between the Earth and the Sun—another standard unit of measure in astronomy known as the Astronomical Unit (AU)—is about 500 times the distance of the Earth to the Moon, or 5000 times the circumference of the Earth. It takes eight minutes for the Sun’s light to reach the Earth.

There are 150 billion metres in 1 AU. The distance is so large that the metre becomes a meaningless figure. Indeed, how can one imagine stacking 150 billion metres end-to-end.

The radius of Neptune’s orbit is roughly 30 AU, or 4 light-hours. The radius of the Solar System, where comet-like debris remains gravitationally bound to the Sun and temperatures are near absolute zero, is 50 AU—nearly 7 light-hours.

The closest star to the Sun is 4 light-years away. A billion Earths would fill the distance between these two stars. All the stars in the sky exist within a small region of our Milky Way Galaxy roughly 1000 light-years in radius, while the Milky Way Galaxy itself is about 100,000 light-years in diameter. Our Solar System lies about 25,000 light-years from the centre, orbiting it at a speed of 230 km/s. At that speed, it completes one orbit roughly every 200 million years.

The Milky Way Galaxy is just one of 100 billion galaxies in our observable universe which are all separated by about a million light-years. The farthest distance we are capable of observing is 14 billion light-years. Across that distance you could fit 10 billion billion Earths—more than the number of hydrogen atoms you could circle the Earth with. Roughly speaking, the Earth is to our observable Universe, as a single atom is to the entire Earth. If a being existed with the capacity to perceive our observable Universe in the way that we perceive the Earth, and if that being had a “microscope” that it used to see “small” things in a manner analogous to the way we use microscopes to view human cells, that being would require 100 trillion Earths to collect together in one place before the group would even be perceptible through that instrument.

Just as there is no meaningful way to conceive of the number of atoms that exist at macroscopic scales, so we have no means of truly appreciating the vastness of the Universe. Indeed, as noted above, it takes sunlight 7 hours just to leave our Solar System, while it takes just a tenth of a second for light to circle the Earth. However, “0.1 seconds” and “7 hours” are numbers that we do have experience with—and it turns out that while it pushes the limits of what we are able to appreciate, a scale model can be built which accurately represents the sizes of the planets and their orbits within our Solar System.

 

When Are You?

As we’ve just seen, it is difficult to meaningfully represent the relative distance scales within our Universe. It turns out that the situation with time is little better. We’ve already noted that the furthest distance we can observe within our Universe is 14 billion light years. The reason is that the Universe is 14 billion years old, so light from any greater distance would not have had enough time yet to reach us.

In order to appreciate just how long a time this is, consider that human civilisation has existed for roughly only 10,000 years. That is, the era of human civilisation in the Universe is one-millionth the age of the Universe. If we plotted a timeline of the Universe as a calendar year, human civilisation would show up with about a half a minute to spare on December 31. The discovery that Earth is a planet orbiting the Sun—and all of the amazing discoveries that have followed—would all take place in the very last second. 

While it is true that all of the atoms comprising everything on Earth were fused together from individual protons in the cores of stars billions of years ago, so that the Solar System could form from that stardust some 4.6 billion years ago, life here and now as we know it is occurring on the smallest scales imaginable. We live in a region of the Universe so small as to be analogous to a single hydrogen atom if the Universe were shrunk down to the size of the Earth. And if we compress the entire history of the Universe into a timeframe that we are comfortable thinking about, everything that we know about this Universe would have been learned in just a second.

 

Why Study Astronomy?

Reasons for studying astronomy will vary. It may be that you have always had a passing interest in astronomy, found astronomy-related news articles interesting, and decided to take this course for interest’s sake. It may be that when you look up at the night sky you are struck with awe at the vastness of all that is up there. It may be that you want to know all that we know about the Universe, and, since we are stuck observing the Universe from here and now, you might wonder why we think we know it. It may be that you’re curious about where the atoms in your body come from; that you want to know what, according to modern science, you are. Studying astronomy will help you with these things.

In this course, you won’t see a detailed explanation of where the atoms in your body come from, and you won’t learn about the large-scale structure of our expanding Universe, which is currently driving galaxies apart at an increasing rate due to a presence of something called “dark energy” which we discovered less than 20 years ago. However, what you will learn is that all of these great discoveries that have come over the course of the past century all began with the study of the planets in our Solar System. In fact, I can tell you with absolute certainty that humans would not now know that the Earth orbits the Sun if Mercury, Venus, Mars, Jupiter and Saturn did not exist. I can say with just as much certainty that we would not have understood gravity as we do if not for the planets. Therefore, without the planets we would know nothing about the structure and evolution of the Universe and all the matter it contains, because that all depends on what we know about gravity—which we know because 300–400 years ago we finally solved the problem of planetary motion.

The things you’ll learn in this course really break up into three related categories:

1. How do we know the things we know?
2. What methods do we use to find those things out?
3. What have we learned?

Each course module will present a mixture of each of these questions, but we will always try to ensure that the first question is answered. In general, the reason will be that our discoveries are made as we implement the scientific method. Module 2 will discuss in detail the development of the scientific method in the context of exploring planetary motions. However, it is possible now to outline how the method works.

The things we are able to observe help us to form ideas about what those things truly are. Therefore, by considering everything that informs us about what something might be, we make a guess, called a hypothesis, about that thing’s “true nature.” We then work out the logical consequences of that hypothesis—the things that must happen if that hypothesis is true. This usually involves the development of a mathematical description known as the scientific model. We then go out and perform an experiment to see if the things we expect based on our hypothesis really do occur. If they do, the hypothesis is said to be confirmed, the theory verified, and we try to think of other things we might expect to see so we can verify the theory in another way. If the things we expect to see don’t actually occur, and something else is observed, we say the theory is wrong and we go back to the drawing board to come up with a new guess.

Understanding how science works, and why it must work the way it does, will empower you to think critically about information you encounter in any aspect of your life, and whether statements you run into are truly justified. This is just one reason why studying astronomy will be useful regardless of where life takes you after this class.