Cosmic Quiz: How fast are we going?

Seth Jarvis

This week’s Cosmic Quiz winner is Greg McMurray, who asked:

How fast is the Earth moving relative to everything else?

Let’s break it down.

The first motion of Earth that we’re all familiar with is rotation – the movement of the Earth that gives us day and night.  Here in Salt Lake City at about 40 degrees north latitude, Earth’s daily rotation about its axis carries us along at about 760 miles per hour.

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At mid-latitudes, your speed from the rotation of the Earth is better than 700 miles per hour.

Then there’s the next familiar motion of our planet, Earth’s orbit around the Sun each year.  Our distance to the Sun is roughly 93,000,000 miles, which makes the circumference of the circle (yes, I know our orbit is slightly elliptical but for the purposes of this exercise let’s keep it simple) divided by 365.25 days work out to an average speed of  67,000 mph.   That’s like covering the distance between Los Angeles  to New York City in 3 minutes.

Earth orbits the Sun at an average speed of 67,000 mph!

Earth orbits the Sun at an average speed of 67,000 mph!

Next, we have the motion of our solar system, which includes us, through the Milky Way galaxy.  We’re about 27,000 light years from the center of the galaxy, and complete one “orbit” of the galactic center in about 220 million years.  That works out to a speed of about 500,000 miles per hour. How does L.A. to New York in 20 seconds sound?

Our little neighborhood of stars orbits the center of our galazy at 500,000 mph.

Our little neighborhood of stars orbits the center of our galazy at 500,000 mph.

Our Milky Way galaxy is itself moving relative to other galaxies.  We’re actually moving toward the Andromeda Galaxy, our nearest neighboring spiral galaxy, at 200,000 mph.  In a few billion years Andromeda and the Milky Way will merge, and that will be spectacular, but that’s a subject for blogging about at another time.

Finally, our “Local Group” of galaxies (made up of our Milky Way galaxy, the Andromeda galaxy and a few nearby smaller galaxies) is moving towards the “Great Attractor” supercluster of galaxies hundreds of millions of light years from us at better than a million miles per hour.

Our Milky Way Galaxy and the rest of the Local Group of galaxies are headed for the "Great Attractor" supercluster of galaxies at better than a million miles per hour.

Our Milky Way Galaxy and the rest of the Local Group of galaxies are headed for the "Great Attractor" supercluster of galaxies at better than a million miles per hour.

But there’s one more bit of brain-bending universe-in-motion information to consider.  None of the speeds described above can hold a candle to the speed at which space is itself expanding as a result of the Big Bang which began the universe 13.7 billion years ago.

Imagine you’re watching a NASCAR race.  Cars are roaring around the track at 200 mph, jostling for position, some cars gaining on the others and some cars pulling away from the others.

Now imagine that the racetrack itself is expanding in all directions 100 times faster than the fastest car on the track.

Yes, it still matters that car #1 is moving towards or away from nearby car #2, but those motions pale compared to how fast the track itself is getting bigger, and overall the distances between race cars is increasing as the track itself expands.

The most distant galaxies visible from Earth are more than 10 billion light years from us.  We can measure their speed away from us by studying the red-shifting of their light.  It turns out that these galaxies are receding from us at something like one-fourth the speed of light – not because that’s how fast they’re moving through space, but because space itself is expanding and carrying these galaxies with it away from us.

So when your toddler is tearing around the house, seemingly incapable of holding still even for a moment, you can take comfort in the knowledge that even if that adorable child should briefly and blessedly take a break, you’re still screaming through the universe at astonishing speeds, and the concept of “holding still for a minute!” really is just a parental fantasy.

There is actually no such thing as "holding still." The universe decrees it.

There is actually no such thing as "holding still." The universe decrees it.

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9 thoughts on “Cosmic Quiz: How fast are we going?

  1. Hm! Yet they tell us not to speed! Very funny.

    Can the police book the earth? Will the speed camera pick up our
    solar system speed or our galaxy speed? Hm!

    Don’t speed, because we are already speeding! Ha, ha.
    Yet our rocket space ships can just manage 50000km/hr or more by gravitational
    pull from planets. Aren’t we modern.

    Will we eventually contract back though because of gravity sucking us back?
    because of mass attraction. Sooner or later F=ma must reduce to nill and
    then we will go to f=mgh. Just a thought! Just a thought!

    Amazing universe isn’t it! Just another thought, no wonder we should worry about asteroids when we are speeding at over 105 000km/hr.

    they are not going to hit us! Its just that we are going so bloody fast.

  2. I have a question, how many miles do we go each day, getting closer to the sun? Does it change with the seasons and/or the times of the year?

  3. On average, and rounding, we can say that Earth is moving along its orbit around Sun at 67,000 mph, or 1,608,000 miles in a day. However, these values are very simply calculated and do not account for the complexities of the many variables that affect how Earth and other objects move in space.

    But, keeping things simple, there is a factor that is worth noting here. Johannes Kepler, a 17th century astronomy and mathematician, taught us some important scientific truths about planetary orbits, now known as Kepler’s Laws of Orbits. The first law states that planets orbit the Sun in ellipses, not perfect circles, with the Sun as one of the two focal points of the ellipse. This means that there is a point when Earth is closest to the Sun (called perihelion) and a point where Earth is farthest from the Sun (called aphelion). Kepler’s second law tells us that as a planet gets closer to the Sun in its orbital path, its orbital speed will increase; and as the distance between the planet and the Sun increases, its orbital speed decreases.

    Interestingly enough, at least for those of us living in the northern hemisphere, Earth is closest to Sun in early January (January 2nd, 2010) and farthest from Sun in early July (July 6th, 2010). However, this change in separation distance is not the cause of seasons on Earth.

    So, to answer your question, yes, the speed of the Earth in its orbit does change with the seasons/times of year, but change in seasons are not the cause or effect of the change in orbital speed.

  4. Here’s a dose of added geekiness to this topic:

    Richard already answered the general question about Earth’s average speed around the Sun, but I wanted to address the question of changes in Earth’s speed around the Sun arising from our slightly elliptical orbit.

    A few days ago Earth passed its perihelion, the point in its orbit at which the Earth-Sun distance is at a minimum (91 million miles).

    Per Kepler’s 2nd law of planetary motion regarding elliptical orbits, that’s also when we’re travelling fastest around the Sun. At aphelion, our greatest distance from the Sun (95 million miles), our orbit is slowest.

    Earth’s orbit around the Sun averages 29.8 kilometers/second, but what’s our speed around the Sun at perihelion (closest to the Sun) and aphelion (most distant from the Sun)?

    This would require some number-crunching, and planetarium staff members Duke Johnson and Robert Bigelow produced the numbers for this question. At perihelion, we’re orbiting the Sun at the rate of 30.3 km/sec, and at aphelion we’ve slowed to 29.3 km/sec.

    That one kilometer per second speed difference between slowest and fastest works out to a difference in speeds of better than 2,200 miles per hour.

    Since we’ve got six months between aphelion and perihelion, the acceleration needed to change our orbital speed by one kilometer per second in that time period is vanishingly small, so you never feel the change in our speed around the Sun.

  5. Lunchtime final moment of geek on this subject –

    Earth is about 3.1 million miles closer to the Sun at perihelion in January than it is at aphelion in July.

    It takes approximately half a year (184 days) for Earth to go from perihelion to aphelion. Doing the simple division, Earth is therefore moving at an average speed of about 700 miles per hour towards (aphelion to perihelion) or away from (perihelion to aphelion) the Sun while zipping around in our orbit at 67,000 miles per hour around the Sun.

  6. Would it really take 3 minutes to get to New York City from Los Angeles going at the speed of 67,000 mph?

  7. Straight line distance from L.A. to NYC = 2,450 miles.

    At 67,000 mph that would take ((2,450 miles/67,000 mph)*60 minutes per hour) = 2.2 minutes.

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