How Fast Are You Moving When You Are Sitting Still?
By Andrew Franknoi
© Copyright 1988, Astronomical Society of the Pacific. Permission to reproduce for non-commercial use is hereby granted.
When, after a long day of running around, you
finally find the time to relax in your favorite
problem. We usually define motion by comparing
the moving object to something at rest. A car
armchair, nothing seems easier than just sitting
moves at 60 M.P.H. relative to the highway sign or
still. But have you ever considered how fast you
lamp-post which is fixed in the ground. But if all
are really moving when it seems that you are not
the stars in the Galaxy are moving (both in random
moving at all?
When we are on a smoothly riding train, we
and in organized ways), what shall be the reference
post to which we can compare our Sun’s motion?
sometimes get the illusion that the train is
Astronomers define a local standard of rest in
standing still and the trees are moving backwards.
our section of the Galaxy by the average motion of
In the same way, because we ride with the spinning
all the stars in our neighborhood. (In using
Earth, it appears to us that the Sun and the stars
are moving as day and night alternate. Actually, it
common words like “local” and “neighborhood,” it is
important to keep in mind that even the nearest
is our planet that turns on its axis once each day –
star is about 25 thousand billion miles away!)
and all of us on the Earth’s surface are turning
Relative to this local standard, our Sun (and with
with it. To make one complete rotation in 24
it, the Earth) is moving at about 3,000 miles per
hours, a point near the equator of the Earth, for
example, must move at close to 1000 miles per
hour toward the bright star Vega in the
constellation of Lyra. This speed, though it seems
hour. Because gravity holds us to the Earth, we
very large to us, is not at all unusual for stars.
(and all other things on its surface) move with the
In addition to the individual motions of the stars
planet and don’t especially notice its rotation.
within it, the entire Galaxy spins on its axis like an
In addition to spinning on its axis, the Earth also
revolves around the start which we call the Sun.
enormous pinwheel. Although the details of the
motion are complicated, our measurements show
We are approximately 93 million miles away from
that any star at the distance of our Sun from the
the Sun and at this distance it takes us one year to
center of the Galaxy takes about 220 million years
go completely around. The full path of the Earth’s
orbit is close to 600 million miles; to go around this
to make one complete revolution. (Some
astronomers call this our “galactic year.”)
immense circle in a mere 365 days requires an
You might think that with so much time for a
average speed of 66,000 mile per hour. That’s
single orbit, the sun’s motion around the Galaxy
speed not even the drivers at the Indianapolis 500
would be quiet slow. But actually, the Galaxy is so
ever dream of!
Now our Sun is just one start among several
huge that to make it around in 220 million years,
our Sun must travel at approximately 600,000
hundred billion others which make up the spiral-
miles per hour. And the Earth, anchored to the
shaped aggregation we call our Milky Way Galaxy.
Sun by its gravitational pull, follows along at the
Within this immense group, each star is itself
same incredible speed.
moving; any planet orbiting a star will share its
motion through space with it.
So, the next time someone criticizes you for just
sitting there, don’t hesitate to point out how fast
If we want to describe the motion of a star like
you are really moving all the time.
our Sun among the other stars, we run up against a
This resource is part of Project ASTRO, a program to enhance science
education through partnerships between teachers and astronomers.
Sponsored by the Astronomic Society of the Pacific (ASP), the project is
funded by a grant from the National Science Foundation.
For more information or the right to reproduce this material, write: Project
ASTRO, ASP, 390 Ashton Ave., San Francisco, CA 94112 or call 415.337.1100;
https://www.astrosociety.org/education/k12-educators/project-astro/