How is
Earth moving in our Solar system?
Earth rotates
on its axis once each day and orbits the Sun once each year. Earth
orbits at an average distance from the Sun of 1 AU and with an axis
tilt of 23 to a line perpendicular to the ecliptic plane.
As Earth
rotates, your speed around Earth’s axis depends on your location: The closer
you are to the equator, the faster you travel with rotation.
Notice that
Earth rotates from west to east, which is why the Sun appears to rise in the
east and set in the west.
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Earth takes a year to complete an orbit of the Sun, but its orbital speed is still surprisingly fast. Notice that Earth both rotates and orbits
counterclockwise as
viewed from above the
North Pole.
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How is
our solar system moving in the Milky Way Galaxy?
We move randomly relative to other stars in our
local solar neighborhood. The speeds are substantial by earthly standards, but
stars are so far away that their motion is undetectable to the naked eye. Our
Sun and other stars in our neighborhood orbit the center of the galaxy every
230 million years, because the entire galaxy is rotating.
Our Local Solar Neighborhood
The small box shows that stars within the local
solar neighborhood (like the stars of any other small region of the galaxy)
move essentially at random relative to one another. They also generally move
quite fast.
Galactic Rotation
Our solar system, located about 27,000
light-years from the galactic center, completes one orbit of the galaxy in
about 230 million years. Even if you could watch from outside our galaxy, this
motion would be unnoticeable to your naked eye. However, if you calculate the
speed of our solar system as we orbit the center of the galaxy, you will find
that it is close to 800,000 kilometers per hour (500,000 miles per hour).
Stars at different distances from the galactic
center orbit at different speeds, and we can learn how mass is distributed in
the galaxy by measuring these speeds. Such studies indicate that the stars in
the disk of the galaxy represent only the “tip of the iceberg” compared to the
mass of the entire galaxy.
Most of the mass of the galaxy seems to be
located outside the visible disk, in what we call the halo. We don’t
know the nature of this mass, but we call it dark matter because we have
not detected any light coming from it.
Studies of other galaxies suggest that they
also are made mostly of dark matter, which means this mysterious matter must
significantly outweigh the ordinary matter that makes up planets and stars. An
even more mysterious dark energy seems to make up much of the total
energy content of the universe.
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This painting shows an edge-on view of the Milky Way Galaxy. Study of galactic rotation shows that although most visible stars lie in the disk and central
bulge, most of the mass
lies in the halo that surrounds and encompasses the disk. Because this mass emits no light that we have
detected, we call it dark matter.
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How do galaxies move within the universe?
Galaxies move essentially at random within the
Local Group, but all galaxies beyond the Local Group are moving away from us.
More distant galaxies are moving faster, which tells us that we live in an
expanding universe.
Two small galaxies (known as the Large and
Small Magellanic Clouds) apparently orbit our Milky Way Galaxy.
For example, the Milky Way is moving toward the
Andromeda Galaxy at about 300,000 kilometers per hour (180,000 miles per hour).
Despite this high speed, we needn’t worry about a collision anytime soon. Even
if the Milky Way and Andromeda Galaxies are approaching each other head-on, it
will be billions of years before any collision begins.
When we look outside the Local Group, however,
we find two astonishing facts recognized in the 1920s by Edwin Hubble, for whom
the Hubble Space Telescope was named:
1) Virtually every galaxy outside the
Local Group is moving away from us.
2) The more distant the galaxy, the
faster it appears to be racing away.
Natural explanation: The entire
universe is expanding.
Are we ever sitting still?
We are never truly sitting still. We spin
around Earth’s axis and orbit the Sun. Our solar system moves among the stars
of the local solar neighborhood while orbiting the center of the Milky Way
Galaxy. Our galaxy moves among the other galaxies of the Local Group, while all
other galaxies move away from us in our expanding universe.
BOOK: The Essential Cosmic Perspective with MasteringAstronomy (Sixth Edition)
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