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How Big is the Universe?
Throughout history, humans have used a variety of ways to help them answer the questions 'How far?' and 'How big?' Generations of explorers have looked deeper and deeper into the vast expanse of the Universe. And the journey continues today. We now have new methods. And we are making new discoveries.
The top half of the sun is shown in this image.  It is a bright, glowing disk of orange and red, with a large solar flare arcing away from the star in the right of the image.
Image above: Our sun is the nearest star, 93 million miles away. So far away that it would take the Space Shuttle seven months to fly there. That’s why the sun, which is a million times the size of the Earth, looks so small. Credit: SOHO - ESA & NASA

In the third century B.C., a Greek philosopher asked the question 'How far away is the Moon?' He was able to measure the distance by looking at the shadow of the Earth on the Moon during a lunar eclipse.

It was Edmund Halley who found a way to measure the distance to the Sun and to the planet Venus. That was 300 years ago. (Halley is famous for predicting the return of the comet that is named after him.) He knew that the planet Venus would pass directly between the Earth and the Sun usually every 121 years. Did you catch the last time this happened? It was June 8, 2004. If you missed it, you will have another chance in 2012. After that, there will not be another Venus transit until 2117.
This picture shows a galaxy seen from above at an angle.  The Galaxy is a swirling white and light blue spiral of gas and dust on a black background.  In the corner of the image is a picture of the face side of a quarter coin.
Image above: Imagine that our galaxy is the size of a quarter Credit: Hubble Heritage Team (AURA/STSCI/NASA); US Mint

It was knowing this distance from the Earth to the Sun that helped us find the true scale of the entire Solar system for the first time.

When we leave the Solar System, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars. It is so big that even at the speed of light (300,000 kilometers per second or 186,000 miles per second) it would take 100,000 years to travel across it. All the stars in the night sky and our Sun live this galaxy. There are also of other stars that are too faint to be seen.

Imagine that our entire Solar System were the size of a quarter. The Sun is now a microscopic speck of dust. The nine planets are too. Their orbits are represented by the flat disc of the coin. On this scale, the diameter of our Milky Way galaxy will be about the size of the United States! How far away is the nearest star –Proxima Centauri to our Sun? In our model, it would be the size of a quarter, too. And it would be two soccer fields away!

So how big is the Universe? No one knows if the universe is infinitely large. We are not sure if ours is the only Universe that exists. And other parts of the Universe, very far away, might be quite different from the Universe closer to home. Future NASA missions will continue to search for clues to the ultimate size and scale of our cosmic home.

Go on the full exploration of the size of our universe at: How big is our universe? Beautiful images and straight-forward methods and ideas take you from our solar system, into the realm of the stars, the galaxies, and finally into the vast panorama of the observable universe. You can also download and print a PDF version of these explorations.

The Universe Forum talks more about size and distance in the universe at:

To learn more about the recent transit of Venus, check out:

To learn more about the WMAP satellite, visit: To learn more about WMAP education, visit:

For kids 14 and up, check out investigations into the destiny of our universe at:

NASA's Astronomy Picture of the Day website contains images and links to exciting astronomical discoveries and observations. It also includes a searchable directory and glossary of astronomical phenomena.

Similar to Astronomy Picture of the Day, this site has weekly updates of news and images from telescopes observing in ultraviolet, X-Ray and gamma-ray light.

Adapted from The Universe