Scientists have studied the sky for centuries. They've often thought about the distances to the stars and found them to be very great. We measure these distances in "light years", the distance light travels in one year traveling at 186,000 miles per second. Distances in space are very great. Even traveling at the speed of light it takes a photon, one unit of light, over 8 minutes to travel from the Sun to the Earth.

Early Astronomy - To better understand the bright points of light in the night sky, early scientists invented the telescope. When the first optical telescope appeared in the 1570s, the design was simple - one concave and one convex lens fitted inside a tube. The tube acted as a receiver, or 'light bucket'. The lenses bent, or refracted, the light as it passed through the glass and thus made the scene appear 3 to 4 times larger. Galileo improved upon the design and by 1609 had developed a 20-power refracting telescope. Galileo made the telescope famous when he discovered the valleys and mountains of the moon and spotted four of Jupiter's satellites.

Telescopes - Astronomers use different kinds of telescopes to observe the universe in different ways.



They use Refracting Telescopes that uses lenses to focus and magnify images. Refracting Telecsopes are limited by the size of their glass lenses. The lenses can become too large and though their own weight can distort images.

Scientists generally use Reflecting Telescopes which use curved mirrors to focus and gathers light. The mirrors can be made very large which allows for the gathering of more light. Both Refracting and Reflecting Telscopes are Optical Telescopes and are limited by the affect the atmosphere has on their performance. If it's too cloudy, too light because of city lights, or because there is too much pollution in the air, optical telescopes become almost impossible to use. That's why astronomers often locate their telescopes on high mountains away from city lights and in a thinner atmosphere.

Scientists also use Radio Telescopes to observe the electromagnetic raditation given off by distant celestial objects.

How big is big? - To give you an idea about how far apart things are in the universe, Proxima Centauri, the star nearest to Earth, is so far away it's only a pinpoint of light in the largest telescopes on Earth. However, Proxima Centauri is very close to us in the giant scale of our galaxy. It is only 4.2 light years from Earth. However, in terms we can understand better here on Earth, Proxima Centauri is a long way off - about 25 trillion miles away from the Sun. Earth is one of nine planets orbiting a star we call the Sun. In turn, our Sun is one star among 100 billion stars in a galaxy we refer to as the Milky Way. If you could look at the whole galaxy, our Sun and Proxima Centauri might even appear to be lost in the vast cloud of stars that make up the galaxy.