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Looking Back in Time

When you look out into space you are actually looking back in time. This is because of the speed of light. At short distances the light travel time is less than a second. But, the Sun is so far from Earth that its light takes 8 minutes to reach us. So when you look at the sun in the sky (never look at it directly, you'll go blind) you see it as it was 8 minutes ago.

As distances get larger so does this "look-back time." The closest star, Alpha Centauri, is so far away that its light takes 4.3 years to reach us. When we look at the closest spiral galaxy to our own Milky Way, the Andromeda galaxy, we see it as it was 2 million years ago. That's when Humans first began walking the Earth.

The Hubble Space Telescope has taken a picture called the Hubble Deep Field (pictured here). When you look at these galaxies we are seeing a distant part of the universe as it was billions of years ago. That's when Earth was just beginning to form.

Last modified May 6, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF