The Pale Blue Dot and the Size of Our Galaxy

Pale Blue Dot

The Pale Blue Dot is a photograph of Earth taken in 1990 by the Voyager 1 spacecraft from a distance of about 6 billion kilometers (3.7 billion miles) from Earth. Voyager 1, meanwhile, is almost twice as far away from Earth as it was when this picture was taken. No photograph of our planet has ever been taken from as far away as this.

Earth is shown as a tiny speck of dust, 0.12 pixels in size, against the vastness of space. By 1990, the Voyager 1 spacecraft had completed its primary mission and was leaving the Solar System. At the request of Carl Sagan, NASA directed the spacecraft to turn its camera around and to take a photograph of Earth.

Sagan later used the title of the photograph as the main title of his 1994 book: Pale Blue Dot: A Vision of the Human Future in Space.

To me, this photograph, unlike any others, shows the vastness of space and how little we occupy. All of human history has occurred on that dot. The dinosaurs occupied that dot hundreds of millions of years ago. All seven billion of us humans live there, with our memories, our pains, our tragedies, our joys and our entire lives. It’s all happening on that tiny speck of dust unimaginably far away.

While thinking about the size of space, wow large is our galaxy, the Milky Way? It’s a disk of a diameter of about 100,000 light years, containing between 100 billion and 200 billion stars. Let’s do an exercise to visualize how big that is:

Our sun has a diameter of about 1.3 million kilometers and it’s about 150 million kilometers away. Let’s say we shrink the size of the sun down to the size of a human red blood cell. That’s about 7 micrometers. It would be a really, really tiny speck. If the sun were that small, the entire solar system would have a diameter of about 4.5 centimeters. That would be the size of a tangerine. So Voyager 1 would have taken that picture of the pale blue dot, our Earth, from the edge of the tangerine, with the Earth being right near the sun in the very center. It’s a pretty empty tangerine.

On that scale, the Milky Way would be a disk of about 4,700 kilometers in diameter. That’s a bit bigger than the continental United States. Imagine 100 billion little solar systems, more or less the size of tangerines, floating in a disk the size of the United States. That’s our galaxy.

The next nearest significant galaxy to ours is Andromeda. It would be a disk floating about 100,000 kilometers away. That’s about a third of the distance from here to the moon.

Going on from there, we know that there are at least 100 billion such galaxies.

That puts the pale blue dot into perspective.

Here is a video about the Pale Blue Dot, narrated by Carl Sagan himself:

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