1.2 Scale of the Cosmos
1.2.1 Space Numbers
Fields like planetary geology and astronomy deal with distances and timeframes that far exceed our ordinary experience. Just the distance between Earth and the Moon is 384,400 km (or 238,855 miles), which is the equivalent of 30 Earths away! To make dealing with astronomical numbers a little bit easier, we adopt certain practices and units of measurement. First, we use a system for writing large and small numbers called scientific notation. This system is very appealing because it eliminates the many zeros that can seem overwhelming to the reader. In scientific notation, if you want to write a number such as 500,000,000, you express it as 5 × 108. The small raised number after the 10, called an exponent, keeps track of the number of places we had to move the decimal point to the left to convert 500,000,000 to 5. If you are encountering this system for the first time or would like a refresher, we suggest you watch this brief PBS animation that explains how scientific notation works and why it’s useful.
The second way we try to keep numbers simple is to use a consistent set of units—the metric International System of Units, or SI (from the French Système International d’Unités). The metric system has been adopted in virtually all countries except the United States. Its great advantage is that every unit increases by a factor of ten, instead of the strange factors in the American system. The fundamental units of the metric system are:
- length: 1 meter (m)
- mass: 1 gram (g)
- time: 1 second (s)
You can find a more detailed summary of the metric system in section 6.2.1 of the Developmental Math (NROC) open textbook (attr. CC BY-NC-SA).
Because the scale of the solar system and universe is so vast, even metric system units like kilometers and kilograms can seem arbitrary. What does 1,400,000,000 (or 1.4 billion) kilometers—the distance from the Sun to Saturn—really mean to anyone? Oftentimes, scientists will adopt other handy units not within the metric system to describe distances and sizes. One example is the astronomical unit. An astronomical unit (AU) is equal to approximately 1.49 x 108 km (93 million miles) which is the average distance between the Earth and Sun. Distances within our solar system are commonly describe using AU. For example, we could say that Saturn is 9.5 AU from the Sun rather than 1.4 billion kilometers; the former measurement is much more comprehendible.
Another common unit astronomers use to describe distances in the universe is a light-year, which is the distance light travels during one year. Because light always travels at the same speed, and because its speed turns out to be the fastest possible speed in the universe, it makes a good standard for keeping track of distances. You might be confused because a “light-year” seems to imply that we are measuring time, but this mix-up of time and distance is common in everyday life as well. For example, when your friend asks where the movie theater is located, you might say “about 20 minutes from downtown.”
So, how many kilometers are there in a light-year? Light travels at the amazing pace of 3 × 105 kilometers per second (km/s), which makes a light-year 9.46 × 1012 kilometers. You might think that such a large unit would reach the nearest star easily, but the stars are far more remote than our imaginations might lead us to believe. Even the nearest star (other than the Sun) is 4.25 light-years away—more than 40 trillion kilometers. Other stars visible to the unaided eye are hundreds to thousands of light-years away.
1.2.2 Consequences of Light Travel Time
There is another reason the speed of light is such a natural unit of distance for astronomers. Information about the universe comes to us almost exclusively through various forms of light, and all such light travels at the speed of light—that is, 1 light-year every year. This sets a limit on how quickly we can learn about events in the universe. If a star is 100 light-years away, the light we see from it tonight left that star 100 years ago and is just now arriving in our neighborhood. The soonest we can learn about any changes in that star is 100 years after the fact. For a star 500 light-years away, the light we detect tonight left 500 years ago and is carrying 500-year-old news.
Because many of us are accustomed to instant news from the Internet, some might find this frustrating.
“You mean, when I see that star up there,” you ask, “I won’t know what’s actually happening there for another 500 years?”
But this isn’t the most helpful way to think about the situation. For astronomers, now is when the light reaches us here on Earth. There is no way for us to know anything about that star (or other object) until its light reaches us.
But what at first may seem a great frustration is actually a tremendous benefit in disguise. If astronomers really want to piece together what has happened in the universe since its beginning, they must find evidence about each epoch (or period of time) of the past. Where can we find evidence today about cosmic events that occurred billions of years ago?
The delay in the arrival of light provides an answer to this question. The farther out in space we look, the longer the light has taken to get here, and the longer ago it left its place of origin. By looking billions of light-years out into space, astronomers are actually seeing billions of years into the past. In this way, we can reconstruct the history of the cosmos and get a sense of how it has evolved over time.
This is one reason why astronomers strive to build telescopes that can collect more and more of the faint light in the universe. The more light we collect, the fainter the objects we can observe. On average, fainter objects are farther away and can, therefore, tell us about periods of time even deeper in the past. Instruments such as the Hubble Space Telescope, the Very Large Telescope in Chile, and the James Webb Telescope (Figure 1.7) are giving astronomers views of deep space and deep time better than any we have had before.
1.2.3 Scale Models of the Solar System
We’ve already discussed just how big space is and, for this reason, it can be helpful to visualize such large systems in terms of a scale model. A scale model is a physical representation of a real-world object that is smaller in size but maintains the overall proportions and appearance of the original. In other words, it’s a miniaturized version of the real thing! You may have seen scale models of airplanes, ships, automotives, buildings, or even spacecrafts in museums (Figure 1.8). The first step to making a scale model of any object, is to choose a scale ratio you want to use—in other words, the proportional relationship that determines how a dimension of your scaled representation compares to the actual dimension of the object it represents.
In our imaginations, let us build a scale model of our solar system, adopting a scale ratio of 1:1 billion (109)—that is, reducing the actual Solar System by dividing every dimension by a factor of 109. Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. This Earth-Moon system fits into a standard backpack.
In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters—about one city block—from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is ten blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer Solar System are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.
In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth-grape toward a lemon five kilometers away with an accuracy equivalent to the width of a thread in a spider’s web.
If that model represents the Solar System, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.
By the way, model solar systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm’s huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model solar system is in Washington on the Mall between the White House and Congress (perhaps proving they are worlds apart?). Watch the video below to see a how a group of friends built an amazing scale model of our solar system in the Nevada desert!
Text Attributions
This text of this chapter is adapted from:
- Sections 1.4, 1.5, and Appendix D of OpenStax’s Astronomy 2e (2022) by Andrew Fraknoi, David Morrison, and Sidney Wolff. Licensed under CC BY 4.0. Access full book for free at this link.
- Chapter 17 of An Introduction to Geology (2017) by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, and Cam Mosher. Licensed under CC BY-NC-SA 4.0.
Media References
- Wylie Overstreet, Wylie and Alex Gorosh. “To Scale: THE SOLAR SYSTEM.” YouTube, uploaded by To Scale:, 16 Sept 2015, https://www.youtube.com/watch?v=zR3Igc3Rhfg.
A unit of length based on the average distance between the Earth and the Sun; it's approximately 149.6 million kilometers (93 million miles)
