Date: February 5, 2009

Title: Motion through Space

Podcaster: James A. Brown

Description: This episode explores the varieties of ways those of us on Earth move through space, from the rotation of the Earth to the motion of our galaxy. It might be a surprise to discover just how fast we really are moving every day.

Bio: I’m James A. Brown from Rockwall, Texas.  I enjoy learning about science, astronomy, and space exploration, and I’m looking forward to seeing humans colonize Mars.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Palomar Observatory, a world-class center of astronomical research that is owned and operated by the California Institute of Technology. Learn more at http://www.astro.caltech.edu/palomar/

Transcript:

Hello, I’m James Brown from Rockwall, Texas, and unless you’re listening to this podcast in a car, or on a train, or on an airplane, you probably think you aren’t moving as you hear this.  It certainly doesn’t feel as if you’re going anywhere.  But in reality, you and everybody else on the planet is actually moving very fast.  We’re also moving in several different directions, all at the same time.  How is that possible?  Let’s start with Planet Earth.

The land mass that you happen to be standing or sitting on is moving.  The surface of the Earth is covered with tectonic plates that slowly glide across the magma underneath.  But tectonic plates are big and heavy.  While over time, that’s enough to slam continents around and push mountains up into the sky, it’s not very fast on a human scale–it’s about as fast as our fingernails grow.  So let’s consider some speeds of motion that are more interesting.

First, the Earth is spinning on its axis, just like a basketball spinning on a player’s finger.  This motion is called rotation. As the earth rotates, we go along for the ride, adhered to the surface of our planet by the force of gravity.  So how fast are we moving in rotation?

Well the answer depends on where you are standing on the Earth’s surface.  If you stand directly on the Earth’s North or South Pole, you aren’t really moving–you’re just spinning around in a circle once every twenty-four hours.  But, what if you stand directly on the equator?  The equator is an imaginary line that is equidistant from both of the Earth’s poles and divides the planet into the Northern and Southern Hemispheres.  Stand on the equator, and you’re clocking speeds of sixteen-hundred seventy kilometers per hour with reference to the background stars.  That’s almost twice as fast as a commercial airliner travels.  If we could fly a plane at that speed, we could travel around the entire planet in a single day.  In fact, that’s what we call a day, a single rotation of the Earth on its axis.

Not only is the Earth spinning on its axis, it’s also moving through space on a huge nearly-circular course around the sun.  This is called a revolution or an orbit, and our planet’s orbit lasts exactly one year.  How fast is the Earth revolving around the sun?

Well, it’s a simple calculation.  The earth’s average distance from the sun is one-hundred-fifty million kilometers.  Multiply that by two times pi, and you come up with a circumference of nine-hundred-forty-two million kilometers.  Divide that by the number of hours in a year, and we come up with over one-hundred-seven-thousand kilometers per hour.  To keep the numbers simpler, think of that as the Earth pushing through space at thirty kilometers every second.

If we could build an airplane that traveled at that speed, we could fly from New York to Los Angeles in less than three minutes.  Or we could fly around the world in less than twenty-two minutes.  And yet the Earth has been cruising at that speed for over four billion years and shows no signs of slowing down.

But that’s not all of the motion we are experiencing.  Not only is the earth moving around the sun, but the sun is moving too.  Our sun is moving through space, dragging its planets, the asteroids, the comets and everything else that orbits the sun along with it.  So how fast is the sun moving through space?

Well, the answer depends on your point of reference, so a better question might be, how fast is the sun moving-compared to what?  In this case, we’re comparing the sun’s motion to stars nearby, located within just a few light years.  Later we’ll take a look at the motion of the galaxy itself, but for now, we’re just comparing our sun’s motion with nearby stars.  To help picture this, you might think of measuring the relative speed of a sailboat in a race against other sailboats.  One sailboat might be sailing slightly faster than the sailboat next to it, but we ignore the motion of the ocean’s current that carries both sailboats along.

Compared to our neighboring stars, the sun is traveling at twenty kilometers every second.  This might seem strange at first glance because that is only about two-thirds the speed of the earth’s orbit.  Why is the earth moving faster than the sun?

Imagine the sun as a king walking down a city street.  Now picture the planets as the king’s servants walking along with him.  The servants may have faster velocities than the king as they attend to him.  One servant may run ahead to remove a branch out of the path, and another servant may run behind the king to lift his cloak out of a mud puddle.  So while the servants may be darting quickly back and forth at high speeds, together as a group they all move forward at the king’s slow stately pace.  Likewise, the planets are spinning around the sun at high speeds–Mercury especially–but together the entire solar system cruises through space at twenty kilometers per second.

But of course, that’s not all.  Just like the Earth is both moving around the sun and being pulled along by the sun’s motion, the sun is also moving in a similar fashion.  The Milky Way Galaxy is comprised of over two hundred billion stars, one of which is our sun.  The Milky Way is a spiral galaxy with multiple arms, and we are located in what’s called the Orion arm.  Our galaxy is spinning around like a pinwheel and as such we are orbiting around the galaxy’s center.  How does that work?

Imagine a fish swimming in a fast-flowing river.  The fish moves through the water a little bit under its own power, but the current of the river moves the fish along even more.  Likewise the sun is both moving in relation to its neighboring stars but also being pulled along as the galaxy rotates.  So how fast are we moving around the galaxy’s center?

Very fast.  Our neck of the galaxy–us included–is cruising along at around 220 kilometers per second.  That’s eleven times faster than the sun’s own motion.  If we could build a rocket that fast, we could fly to the moon is less than half an hour.  It took three days for the Apollo astronauts to reach the moon.

We have one more movement to consider.  Just like the Earth is both rotating on its axis and revolving around the sun, the Milky Way galaxy has a similar dual motion.  The galaxy is spinning like a pinwheel, but it’s also moving through space, orbiting around a common point with our nearby galaxy neighbors in a complicated gravitational dance.  How fast is our galaxy moving?

Compared to the Cosmic Microwave Background, the Milky Way galaxy is trucking along at over five-hundred fifty kilometers per second, which of course dwarfs all the other motions we experience.  So even sitting in a chair, we are really moving at high speeds and in different directions all at the same time.  We don’t feel these speeds because the earth holds us so tightly to its surface.

At this point it would be tempting to simply add up all the motions together and come up with a grand total, but reality is more complicated than that.  All of the motions we experience are in different directions, some of which might cancel each other out when it comes to determining a total distance traveled.  A child swinging back and forth on a swingset might be moving very quickly but not getting anywhere.  Put the swingset on a cruise ship and the child’s total change in motion becomes even more complicated.

All in all, it’s the motion of our galaxy with respect to the Cosmic Microwave Background that has the largest effect, and at five-hundred-fifty kilometers per second, it’s nothing to scoff at.  That’s around the earth in just over a minute.  To the moon and back in less than twelve minutes.  To the sun in just over three days.  Now matter how you think of it, that’s fast.  In fact, by the time this podcast is over, you will have traveled through space over three hundred thousand kilometers.  I sure hope you fastened your seatbelt.

Thanks for listening.

End of podcast:

365 Days of Astronomy
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