## How Fast Are You Moving When Sitting Still?

At the end of the day, there's nothing better than collapsing into a large, comfy chair for some rest and relaxation. But before you get too comfortable, have you ever wondered how much you're moving even when you are trying your best to remain an inert blob? The answer to this question is surprisingly complicated and might surprise you!

Daily Motion

Virtually everyone today knows that the Earth is round and spins once every 24 hours. It is this spinning that causes the daily motion experienced on our planet. If you were standing on the equator of Earth you would move at nearly 1,000 miles/hour (1,600 km/hr), but if you began walking towards the North or South Pole your speed would decrease. If you have trouble understanding why imagine spinning a ball on a stick: as you rotate the stick the "poles" on the stick will remain stationary, while a point in the middle of the ball has to move quickly to make it the whole way around. You don't need to cover as much distance closer to the pole to make one circuit, so your overall speed would be slower.

Although most people on Earth are moving at several hundred miles an hour, we don't feel this rotation much in everyday life. Yet this daily motion is observed very often by scientists in the form of what is known as the Coriolis effect. This is where objects that look like they are traveling in a straight line appear to curve when your frame of reference is rotating. Military artillery units first observed the Coriolis effect in the sixteenth century, when they noticed their cannonballs would veer off target when shot long distances. This is because Earth would rotate under the cannonball in flight, so from the ground the cannonball would appear to drift towards the side. It is this daily spinning which causes the Coriolis effect to be observed on Earth.

The Coriolis effect is also very important in the field of meteorology, as the rotation of the Earth determines how pressure gradients like hurricanes rotate. It differs for the northern and southern hemispheres: a system in the Northern hemisphere spins counter-clockwise, while a system in the Southern hemisphere spins clockwise. There is also an urban legend that the Coriolis effect determines which way water will drain in a bathtub or toilet, but on such a small scale the Coriolis effect is negligible.

A low-pressure system over Iceland spinning counter-clockwise due to the Coriolis Force. Image courtesy NASA

Yearly Motion

The next bit of motion to consider is the yearly motion of the Earth around the Sun. The Earth is 93 million miles (150 million kilometers) away from the Sun and orbits once a year, so it is easy to calculate that we travel around the Sun at the brisk pace of 66,000 mi/hr (107 million km/hr). For comparison, if you were to fly in an airplane at this speed from New York to Los Angeles the journey would only take you just over 2 minutes!

Despite this breakneck speed, however, none of the forces listed from here on are observed at all in daily life on Earth.

The Sun's Motion

The Sun may seem like a stable fixture to life here on Earth, but when you are looking at a scale as large as the galaxy it is obvious that appearances can be deceiving. Our Sun is just one of billions of stars in our Milky Way Galaxy, which has the shape of a flattened disk with a bulge in the middle. Our Sun is approximately 40,000 light years (the distance light travels in one year) away from the galactic center of the Milky Way.

All the stars in the galaxy have an individual motion relative to the other stars in their part of the Galaxy. By calculating the average motion of stars in our stellar neighborhood, astronomers have calculated that the Sun (and Earth!) are moving at 43,000 mi/hr (70,000 km/hr), which is a typical random motional speed for stars in this part of the galaxy. It is worth noting that this speed is actually slower than what Earth undergoes while orbiting the Sun- at this speed, our New York-Los Angeles trip would take three and a half minutes.

When it comes to going fast, though, the Sun's random motion relative to other stars around us pales in comparison to its rotation speed within the galaxy. Our galaxy is rotating like a giant pinwheel and like most other stars our Sun is rotating around the center of the galaxy. It takes the Sun a whopping 225 million years to orbit the galaxy once, a time scale so huge that the Sun has only done it 20 times since its formation. In order to do this, our Sun must be traveling at the incredible speed of 483,000 mi/hr (792,000 km/hr)! At this speed, traveling across the United States would take just 18 seconds.

So what happens to the planets orbiting their parent stars as they travel through the galaxy? If the Sun is going so fast, why isn't the Earth left behind? The answer is that the Earth is gravitationally bound to the Sun, the same way the Moon is gravitationally bound to the Earth. As a result, as the Sun moves all its planets are obliged to travel along with it, the same way the moon continues to orbit the Earth despite the Earth orbiting the Sun.

The Andromeda Galaxy, a nearby spiral galaxy similar to what our own galaxy is thought to look like. Image courtesy APOD

Motion Through the Universe

As you may have guessed, our galaxy's speed must also be taken into account. In order to find out its speed, however, scientists encounter a problem. Up until now we have been careful to describe the various speeds in terms of how fast an object is moving relative to other objects. This is true with the speed of the Earth when compared to the Sun or the Sun compared to the center of the galaxy. But what would be the right frame of reference for the motion of our galaxy? We could always compare the Milky Way's speed to that of another galaxy, but this doesn't work very well because that other galaxy would also be moving.

"It's a tricky problem," admits John Ruhl, a physicist at Case Western Reserve University. Ruhl studies the Cosmic Microwave Background (CMB), which is radiation left over from the early universe just a few thousand years after the Big Bang. Because this radiation fills all space, Ruhl said, "it provides a great frame of reference for calculating relative motion." This means if we compared the Milky Way's movement to the CMB and subtract all the other motion we already know about (such as the Earth orbiting the Sun), what we are left over with is the Milky Way Galaxy's motion through the universe.

When astronomers do this calculation, it turns out that the Milky Way is moving at an astounding 1.3 million mi/hr (2.1 million km/hr) through space. At this speed the New York City-Los Angeles flight would take just under 7 seconds, giving a new meaning to the phrase "zip across the country." Although this may sound astonishingly fast it is still much slower than the ultimate speed limit of the universe, which is the speed of light. A light beam making the same journey would leave everyone else in the dust, as it would make the journey in about a hundredth of a second.

The Milky Way Galaxy is currently rushing toward an area of the sky in the constellations of Leo and Virgo, which is called The Great Attractor by astronomers. It is thought that The Great Attractor is made up of several large groups of galaxies, and many neighboring galaxies are also being pulled in that direction.

The galactic motion is the last piece of the motion puzzle, but this often confuses people who think something is missing. "After all," the musing goes, "I have heard that the universe is expanding, so wouldn't this expansion make things in the Solar System and galaxy further apart and change their speeds as a result?"

It's a good question, but in reality it doesn't quite work that way.

"The universe is expanding, but it only does so on homogeneous and isotropic scales," says Ruhl. This scale is one where everything looks uniformly the same in all directions, and you need to work on a very large scale to make the universe look the same everywhere!

Take a loaf of unleavened raisin bread, where the loaf is the universe, the raisins are the galaxies, and the dough is the empty space between the galaxies. If you put the loaf into the oven it would expand, and the raisins would be further apart, but the raisins themselves would remain the same size. The universe's expansion works similar to this model: as it expands the uniform dough expands, but the specific raisins keep their size and shape.

As an extension of this, if you look at one random part of our Solar System or galaxy it will look very different from another random part. This means fortunately you do not need to worry about the universe's expansion in our daily lives.

So what do all the speeds add up to? If everything were perfectly aligned in the same direction, your final speed would be 1.9 million mi/hr (3 million km/hr). By sitting in your chair you are traveling the distance across the country just a hair under five seconds, which is definitely something to ponder next time you find yourself lounging comfortably. What a pity you can't rack up frequent flier miles for this trip!

Written by Yvette Cendes

Reviewed by Nira Datta, Pooja Ghatalia