Have you ever taken a long trip, perhaps a road trip lasting for days? How far did you travel?
If you traveled from London to Paris, you would cover 450 km or 300 miles. Maybe
you traveled all the way from New York to Los Angeles, a distance of 4,500 km or 2,700 miles.
These distances may seem large compared to your daily travels,
but they are insignificant when compared to the vast distances through the depths of outer space.
Some of the most remote objects in the universe are unimaginably far away from planet Earth.
Hearing that a star is trillions of miles away gives your mind nothing to compare to;
it is simply a large number with no context. So how do astronomers imagine the unimaginable?
First, let's look at a relatively nearby object that you are familiar with: the moon.
From pictures, movies, or your own eyes, you might have the idea that
the moon is right next to the earth. In reality, it is quite far away.
Let’s use common objects to compare the size and distance of the earth and the moon.
If we use a basketball to represent our earth, then comparatively the moon would be about the
size of a tennis ball. At that scale, we would need to place the tennis ball 7 meters or 24
feet away from the basketball in order to model the distance between the earth and the moon.
Now let’s head farther out into space. If the moon is a tennis ball and the earth a basketball,
then the sun would be the size of a house placed over 3 kilometers or 2 miles away.
And the farthest planet, Neptune, would be over 80 kilometers or 50 miles away!
That means that if you set down your basketball, you'd have to get in a car and drive an
hour away just to get to Neptune, and remember, that's shrinking the entire earth
down to the size of a basketball. And we haven't even left the solar system yet.
To deal with these vast distances, astronomers needed different units of measurement than
kilometers or miles. So, they chose to use the distance from the earth to the sun, and called it
one astronomical unit, or AU for short. Using this unit allows us to compare distances in a simpler
and more meaningful way. For example, saying that Saturn is 886 million miles from the sun
is difficult to comprehend; so instead we say that it is 9.5 AU away. In other words, Saturn
is 9.5 times farther from the sun than the earth. That gives the distance real meaning in our minds.
However, the distances to the stars are so immense that even the astronomical unit is too small.
Instead, astronomers often measure these distances in light-years. A light-year
is the distance that light travels in one year. Other than our own sun,
the nearest star to Earth is Proxima Centauri. It is 4.2 light-years away. That means that if
you were to send a light signal to that star, it would take 4.2 years for the signal to arrive.
In comparison, a light signal takes only 1.3 seconds to reach our moon.
Our galaxy, the Milky Way, is a collection of stars 100,000 light-years across.
The nearby Andromeda galaxy is 2.5 million light-years away. In turn,
the Milky Way and Andromeda galaxies are members of a cluster of galaxies, called the Local Group,
covering almost 10 million light-years. The Virgo Cluster lies 65 million light-years away
from Earth. You can see that on the grand scale of the universe, even light-years become too small.
We've traveled far from our tiny home world. But now let's return,
shrinking our scale from galactic clusters down to our home galaxy the Milky Way,
then in even closer to a perfectly ordinary star, our sun. Zooming in closer we zip past
the other planets and return home at last to our tiny but beautiful world.