Motion Sickness

March 16, 2009 11:22 AM

How many directions am I moving in while I'm just sitting here? Any idea of how fast?

I'm not really looking for precision, or discussions about relativity, but rather generalities.

I'm guessing that continental drift and the effects of precession would be negligibly slow.

But, best as I can figure out:
Due to rotation, I'm moving eastwards at roughly 1,600 km/h, around the center of the earth.

Then the earth itself is moving about 108,000 km/hr counterclockwise around the sun.

Then the solar system is moving close to 792,000 km/hr around the galactic center.

And the galaxy itself is moving towards the Great Attractor at about 2,160,000 km/hr.

What have I missed? Where the hell is the Great Attractor moving? Isn't the galaxy also moving towards another galaxy in the local cluster, while we both head towards the Great Attractor?

I'm sick at home with the flu and I can't get these questions out of my head. Maybe I shouldn't drink so much TheraFlu. If you can make a FPP out of any of this, then...uh...godspeed. (How fast is that supposed to be? In what direct--oh nevermind)

posted by zylocomotion to Science & Nature (9 answers total)

You have missed the fact that you are using a different reference point for each motion, and the various motions would cancel or add. Even if you used an arbitrary single reference point, the net motions would constantly change depending upon whether the earth was on one side rather than the other side of its orbit, etc. Each reference point would have a different result, and there is no "true" reference point.
posted by weapons-grade pandemonium at 11:35 AM on March 16, 2009

You are moving in one direction, but it's not a straight line, it's been affected by all those other forces.

Or, if you're the center of the universe (supposing uniform symmetric space-time), you are at rest. (Note: this is not true)
posted by blue_beetle at 11:46 AM on March 16, 2009

I'd vote that the cosmic microwave background provides a nice appropriate reference frame.

Wikipedia's CMB article says we are moving at about 630 km/s relative to this background. But this doesn't add in local motion, which as w-g p says, you'll have to be careful with because you want to use the same reference point for all motion.


I'm sick at home too, but no Theraflu. Sigh.
posted by nat at 11:48 AM on March 16, 2009

Due to rotation, I'm moving eastwards at roughly 1,600 km/h, around the center of the earth

Only if you're at/near the equator. Farther from it, your speed is slower, since you're closer to the earth's axis. For an approximation, take that figure and multiply it by the cosine of your latitude. (Here at 39°N where I am, it's more like 1300 km/h.)
posted by DevilsAdvocate at 11:48 AM on March 16, 2009

I've seen the OP's thought experiment in a children's book before, and it was fairly mind-blowing for an 8-year-old me. The point is not be to pedantic about it and just take note of all the galactic-sized forces working upon you at once, despite you not really noticing any of them happening.

Imagine an ant walking around a sphere which is dropping towards the earth. How many directions is the ant moving? Pretty much one... down

See above about being pedantic.

Imagine you're in a train traveling 60 mph. You stand up and walk toward the rear of the train at 3 mph. Sure, you're still moving "forward" at 57 mph, so it's not terribly important in the bigger scheme of things. But we're just kinda toying with the idea here. "While you're walking at 3 mph, you're also traveling backward at 57 mph ... meanwhile, the earth is spinning..."
posted by Cool Papa Bell at 12:33 PM on March 16, 2009

Of course, there's really no reason to suppose that one reference point is any more accurate than another, so you might as well go with the simplest explanation, which is that you're not moving at all if it feels like you're sitting still. I know you said you weren't looking for any discussions about relativity, but I feel like if I were sitting at home stoned on TheraFlu that's what I would want to be thinking about. I mean, the implications are that Copernicus was no more correct than Ptolemy in his description of the universe. That's wild.
posted by ekroh at 1:17 PM on March 16, 2009

Cool Papa Bell: "I've seen the OP's thought experiment in a children's book before, and it was fairly mind-blowing for an 8-year-old me."

Yep - like the Monty Python guys sing:

Just remember that you're standing on a planet that's evolving
and revolving at 900 miles an hour,
It's orbiting at 19 miles a second, so it's reckoned,
the sun that is the source of all our power.
The Sun and you and me, and all the stars that we can see,
are moving at a million miles a day,
In the outer spiral arm, at 40,000 miles an hour,
of the Galaxy we call the Milky Way...

posted by PontifexPrimus at 2:14 PM on March 16, 2009

If you want to add these up, you need to know something about the directions that you're moving. You'll want a star map and (maybe after some planning) a place where you can see the stars at night.

If you go out at dusk the sun will be in the west. The earth's rotation will carry you away from the setting sun, towards the east. East is fixed with respect to buildings and stuff.

Viewed from above the north pole, the earth rotates counterclockwise (to the east) and also orbits the sun counterclockwise. So at dusk the earth's orbital motion is your local "down," and at dawn the earth's orbital motion is your local "up." How close to the vertical depends on your longitude and the time of year. We're having an equinox with the south pole "leading" the north pole. So it's vertical local downward at dusk if you're on the tropic of Capricorn (22° south) and vertical local up at dawn if you're on the tropic of Cancer (22° north). Where I'm sitting at 36° north at dusk (mid-US), I guess the direction of the earth's orbit is more horizontal than vertical. Recline in a chair at a 30° angle to the ground, facing south; now imagine someone pulling the chair away from you.

At midday, right now at the March equinox, the earth's orbital motion carries you west-by-southwest; at midnight it carries you east-by-southeast. At the equinoxes the midday motion is due east and west at midnight and midday, and vertical from the equator at dusk and at dawn. At the September equinox the north pole leads.

If you can track which way the earth's orbital motion carries you, and you can read a star chart, you can identify the other directions you mentioned. The sun appears to travel through the zodiac, the dozen constellations that lie on the sun-earth plane. At dusk right now the setting sun is in Aries, and Cancer is overhead; the earth is heading in the opposite direction, towards Capricorn, which is overhead at dawn. At the June solstice the sun will be in Cancer, and the earth will be heading toward Aries.

The other motions you mention won't change so often. If I read this diagram right, the orbit around the galaxy is carrying the solar system towards Cephus or Cassiopeia, which is north+overhead in the northern hemisphere and north+underfoot in the southern. But the stars in those constellations are moving in roughly the same direction as the sun, so this is as far as you can go before the "relative to what?" question becomes important.

I smell a science project.

Enjoy your TheraFlu.
posted by fantabulous timewaster at 6:50 PM on March 16, 2009

You have the flu. Lie down. You are now at rest. Repeat to yourself: "I am at rest." Get some rest. Get over the flu.
posted by exphysicist345 at 6:55 PM on March 16, 2009 [1 favorite]