04 March 2013


Speed is a funny thing.  More precisely, it is a relative thing.  If two cars are travelling in the same direction on an interstate highway ~ in adjacent lanes and at the same speed ~ and if driver A looks at driver B, neither appears to be moving relative to the other.  But when driver A returns his/her attention to the road and the passing landscape, suddenly the feeling of speed returns, as apparent motion is visualized with reference to the motionless ground.

Now suppose that both drivers pull over at a rest stop.  Both vehicles are still, relative to each other and relative to the landscape.  But is their speed really zero?  Not really.

Even when you are standing still or lying down, you are hurtling at mind-bending speeds.  How is this possible?  Here's how ~

  • Rotation ~ the Earth rotates (spins on its axis) once per day.  By definition, everything on the Earth's surface matches that speed of rotation.  The Earth is roughly 24,000 miles in circumference.
  • Revolution ~ the Earth revolves (orbits) around the Sun once per year.  All of Earth's passengers are inherently matching Earth's speed as it follows its path around the Sun.  The circumference of Earth's orbit is about 600 million miles.
  • Solar Neighborhood ~ our sun (taking the Earth with it) moves among other stars known as the Local Interstellar Cloud.  The sun's motion is measured, not against that of other stars (which can be variable), but rather against an abstract point called the Local Standard of Rest.
  • The Galaxy ~ our sun also moves in orbit around the center of the Milky Way galaxy, a vast stellar spiral over 100,000 light years in diameter, containing 200-400 billion stars.  Our solar system is located within one of the outer arms of the galaxy, the Orion arm.
  • The Universe ~ you thought keeping all these motions in your head was hard?  Now consider that our galaxy is only one of more than 100 billion galaxies in the observable Universe, across an approximate distance 300 million light years.  On this scale motion is not simply orbital.  The Universe has been expanding since its origin in the Big Bang, nearly 14 billion years ago.  Galaxies and clusters of galaxies are speeding not just away from the point of that titanic explosion, but also away from each other (a simplistic analogy is the movement of painted dots on the surface of an expanding balloon).  So complex is all that stellar motion that it would be meaningless to pick an arbitrary spot against which to measure our speed within the universe.  Rather, astronomers use Cosmic Background Radiance as a frame of reference.  CBR is the collection of microwaves and other radio waves produced by the Big Bang.  It is a medium through which we move, much like a boat travels through water, or a plane through air.
Okay, enough definitions ~ what are the speeds at which we travel, even when standing in one spot on the surface of the Earth?
  • Rotation ~ at the equator, an object on Earth's surface moves at about 1000 mph.  Nearer the poles, less.
  • Revolution ~ Earth orbits around the Sun at about 66,000 mph.
  • Solar Neighborhood ~ the Sun moves relative to the Local Standard of Rest at about 43,000 mph, in the direction of the star Vega.
  • The Galaxy ~ the Sun orbits the center of the Milky Way at about 483,000 mph.
  • The Universe ~ the Milky Way moves relative to the CBR at about 1.3 million mph.
Note that because velocity is paired to a specific reference, it is not meaningful to add all those speeds together.  But who needs to?  Each is fascinating to regard by itself.  Just imagining yourself traveling at Earth's rotation speed is mind-bending.  Each successive higher speed becomes even harder to wrap one's imagination around.

So what's with the interweaving objects in the GIF image at top (click to enlarge)?  That is a sped-up verson of what you might see if you were aboard a spaceship traveling parallel to the sun's path ~ the objects are the planets of the Solar System.  This baroque dance is a far cry from the static concentric orbits usually depicted in illustrations (see below - sizes are to scale, distances are not).  Caveat ~ this image is distorted by speed, and controversial in that it seems to portray the solar system moving in a helical path.  The planets do not trail behind the sun, at least not perceptibly.  Still, it's an interesting exercise in visually separating the motions of the Sun and planets.

For the overall flow of information, I am indebted to the Astronomical Society of the Pacific and their excellent article How Fast Are You Moving When You Are Sitting Still?  I highly recommend reading the article for a fuller narrative.

No comments:

Post a Comment