I never told you about the letter Jane Crofut got from her minister when she was sick. He wrote Jane a letter and on the envelope the address was like this: It said: Jane Crofut; The Crofut Farm; Grover's Corner; Sutton County; New Hampshire; United States of America.
What's funny about that?
But listen, it's not finished: The United States of America; Continent of North America; Western Hemisphere; the Earth; the Solar System; The Universe; The Mind of God— that's what it said on the envelope.
What do you know!
And the postman brought it just the same.
What do you know!
—Thorton Wilder, Our Town
In this pod, we will consider where you are positioned within the universe. People have been wondering this question since the beginning of time, and the idea that our place in the Cosmos is a "address" appeared briefly in Thorton Wilder's play "Our Town", one of the most popular plays in American History. Just like you live on a street, which is in a town, which is in a state, which is in a country, you live on a planet, which is in a solar system, which is in a galaxy.
Understanding your 'address,' just like Jane Crofut does, is the first step to understanding the whole universe, the goal of cosmology! Seeing as Jane lived over 100 years ago, we should make some updates for the 21st century. So, here is my address:
An address is logical because each element is inside of the next element. For example, Room 308 is in the building located at 133 Marion Road. Marion Road is inside Mattapoisett, which is inside Massachusetts. The same logical extends to all of the "space" parts of the address! Earth is inside the earth-moon system. The earth moon system is inside the solar system. The solar system is inside the Orion Spit The Virgo Supercluster is inside the Laniakea Supercluster etc etc.
To start, (almost) all humans live on the planet earth. What does it mean to live on a planet?
There are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Though there is an official definition of a planet, let's focus less on what that is and more on how that came to be.
The word "planet" comes from the Greek word planetes for a "wanderer". The Greeks saw int he night sky that, unlike the stars, which stayed in the same place each night, the bright objects named Mercury, Venus, Mars, Jupiter, and Saturn moved across the night sky each night, so they were named "planets" or "wanderers." At this time, nobody considered earth to be a planet!
Over the course of the 16th and 17th century, thanks to the work of astronomers such as Nicolaus Copernicus, Galileo Galilee, and Johannes Kepler, people in Europe came to beleive that earth moved around the sun. At this point, the definition of a planet changed (without anyone thinking about it). It no longer meant an object moving across the sky, but a large spherical mass moving around the sun.
Later on, three more planets: Uranus, Neptune, and Pluto were discovered, as well as many asteroids, comets, and small objects in the far reaches of the solar system called the Kuiper Belt. This raised the question, what really is a planet? Does anything orbiting the sun qualify? Does it need to be big? If so, how big? Astronomers were discovering "planets" in other solar systems as well.
The issue was brough to a head in the early 2000s, when the Astronomer Mike Brown working at Cal Tech discovered a new procedure for discvering planets using a technique called interferometery. He was soon discovering new "planets" so fast that pretty soon, if astronomers didn't clearly draw the line between what was and wasn't a planet, there'd be more planets than preschool children could ever learn!
In the summer of 2006, a truly shocking event occured: the entire world media covered an astronomy conference! There, in a contrversial vote, the International Astronomical Union adopted a new definition of a planet that would eliminate all of the objects Mike Brown had discovered, and with them, Pluto. (which was much smaller and farther away than the other planets).
And why does this matter to cosmology? Because we begin our journey on planet earth, know that the term "planet" is not a clearly and precisely defined thing, but something that has changed in definition over time.
For further reading on the definition of a planet, see How I Killed Pluto and Why It Had It Coming by Mike Brown
NASA Solar System Exploration: https://solarsystem.nasa.gov/
Most people assume the next logical step after earth would be to the solar system, but let's take a smaller step first to the Earth-Moon System.
A system are multiple bodies that are taken together and considered to be a single body or unit. Systems can be reduced to a single point mass located at the center of mass of the system.
A system consisting of the earth and the moon, considered as a single unit.
When people say "The earth revolves around the sun," they are not wrong, but they are incomplete! They should say "The earth-moon system revolves around the sun." The center of mass of the earth-moon systm is inside of the earth but not at the center (you will learn to calculate its precise location in another pod). It is slightly out in the direction of the moon, and because the moon is orbitting, the center of mass of the earth-moon system is always changing.
As we will learn when we study the dynamics of systems, a system, such as the earth-moon system, can be considered to be a point mass located at its center of mass. Therefore, if you imagine a single point with the combined mass of the earth and the moon orbiting the sun, that is the correct idea. Now imagine the earth and the moon existing around that single point. That is the earth-moon system.
It's also good to note that the 'earth-moon system' contains many manmade objects orbitting the earth, such as the International Space Station and innumerable GPS and communication satellites.
The Solar System consists of the sun and all of the objects that orbit the sun, considered as a single unit.
An Astronomical Unit (AU) is the distance from the earth to the sun, about 93 million miles. In this list, all of the different parts of their solar system are listed by Astronomical unit.
The information in this list is derived from the NASA Solar System Exploration Website.
https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/overview/The largest item in the asteroid belt. When it was first discovered in 1801, it was considered a planet, which is why it is named for one of the Olympian Pantheon of Gods. Later it was was demoted to asteroid, and then promoted to dwarf planet once that became an option. It belongs in a different category than other asteroids because it is
https://solarsystem.nasa.gov/planets/saturn/galleries/?page=0&per_page=25&order=created_at+desc&search=&href_query_params=category%3Dplanets_saturn&button_class=big_more_button&tags=saturn&condition_1=1%3Ais_in_resource_list&category=51Made of tiny pieces of ice.
https://solarsystem.nasa.gov/resources/868/titan-and-dione/?category=moons/saturn-moons_titan(The moon Dione also visible in the image).
https://solarsystem.nasa.gov/moons/pluto-moons/charon/in-depth/Charon is almost as large as Pluto, and as a result the center of mass of the Pluto system is outside of Pluto. For this reason, Pluto and Charon are called a binary. (see video)
https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/1p-halley/in-depth/Was last seen from earth in 1986, and will be seen again in 2061. The astronomer Edmund Halley was the first to predict the return of this comet.
The farthest away object for which we have a close up image, taken by the New Horizons Satellite in 2019.
A comet that was visible from earth in 1997, but will take 2534 years to complete a full orbit.
Image Credit: https://i.chzbgr.com/full/6410642688/h74A67039/planning-a-planet
(Did you count the planets? Yes, that's how many there are. All others are not invited to the party. Get over it.)
Stars are not spread evenly throughout the universe, but are located together in large groups of billions of stars called galaxies.
The milky way is a collection of over 200 billion stars,
just like our sun!
Many of these stars are similar to the sun are surrounded by planets like earth,
but there are also stars that are different: much bigger and hotter,
or much cooler, or stars that are still forming, or stars that are dying or are
In addition, there are tons of things that aren't stars at all, like quasars and pulsars.
And at the center of the galaxy is a
Just like the earth moves around the sun together with the moon, the earth moves around the milky way together with the whole solar system, and the 200 billion stars of the milky way move through the universe together.
All of the stars that you see in the night sky are outside of the solar system, but inside of our galaxy. For nearly all of human history, humans would view the stars, but also a band of purple across the center of the night sky, which is where the name "Milky Way" comes from.
Image Credit: https://www.space.com/19915-milky-way-galaxy.html; Tunc Tezel
Only after humans began observing different galaxies did we realize that this band of light was made by the stars of our own galaxy! The band of purple light comes from billions of stars added together, almost all of them so far away we can't make them out individually without a telescope. From our position inside the Milky Way, we cannot actually see what it looks like, but we can view other galaxies to see what a galaxy looks like, and we can use computer modeling to figure out how all of the stars are actually arranged. From those analyses, we learned that the milky way is in a spiral shape:
Image Credit: https://www.nasa.gov/mission_pages/GLAST/science/milky_way_galaxy.html
When humans look at the night sky, they imagine the different stars as different shapes, called "constellations." Many different cultures have named constellations. The most well known of these are the ancient Greek constellations, such as Cygnus, Orion, and Ursa Major.
Many of the objects we can observe within our galaxy are named by the
constellation in which they appear.
For example, the black hole at the center of the galaxy is named
"Sagittarius A*" because it is visible to humans in the constellation
However, it is important to know that the using constellations
is not a part of the
Image Credit: https://spaceplace.nasa.gov/nebula/en/
Image Credit: https://www.noao.edu/image_gallery/html/im0057.html
In one of the most famous observations of all time, in December of 1995, the Hubble Space Telescope was pointed at patch of apparently empty sky and exposed for 10 days. After 10 days, this is what the camera had recorded:
Even empty space is full of unseen objects! Even in dark, blank, empty space, there are countless galaxies stretching as far as we can see!
One of the largest questions in science is "How Big is the Universe?" The answer most scientists believe is "infinitely big." Of course, we cannot see as far as the universe is and can never prove it is infinite, but when the Hubble Deep Field was released, it seemed that, however far humans look in all directions, there appears to be an endless expanse of new worlds.
There is one "edge" of the universe humans must respect, and that is the edge from our own point of view. Because the universe (to the best of our knowledge) is about 14 billion years old, humans cannot see beyond 14 billion light years. If an object 14 billion light years away gave off light 14 billion years ago, at the beginning of the universe, it would reach earth now. If any object farther than that gave off light, it would still not have reached earth. The edge of the observable universe, 14 billion light years from earth, is called the cosmic horizon.
A scene from the movie A Beautiful Mind, directed by Ron Howard and starring Russell Crowe and Jennifer Connelly.
The list below gives you a list of "destinations" in the universe. For each destination, classify it as one of the following:
Based on the information above regrading the solar system, answer the following questions on the relative locations of planets in the solar system.
There isn't one answer to this question! Look at some other desciptions of the cosmic address.