Spaceship Earth

16:00 | Etiquetas: astrophysics, cosmic perspective, earth, expansion, galaxy, mastering astronomy, milky way, rotation, solar system, universe

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.

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.

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.

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.

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.