Galileo made his telescope by placing a piece of glass called a lens at one end of a long, narrow
tube. A telescope like this is not very powerful, so the view of objects in the sky is not sharp.
And the lens tends to introduce false colors.
Late in the seventeenth century, Isaac Newton, dissatisfied with Galileo's spyglass-type of
design, invented the reflector telescope. Newton's design used a curved mirror to gather light
coming into the tube, providing sharper images of the stars and planets, without the color
distortion of the earlier telescope.
The astronomer William Herschel, who discovered the planet Uranus in 1781, built the largest
reflecting telescopes of his day. He used these instruments to discover new moons around Saturn,
distant double stars, and "island universes" -- what we now call galaxies.
Astronomers found that they could use special instruments to see ultraviolet and infrared
light, as well as radio waves, coming from objects in space. These new ways of looking at the
heavens showed us things that our eyes could not see.
By the twentieth century, large observatories were being built all over the world and
astronomers were making many discoveries. But Earth's atmosphere interferes with viewing. The
Hubble Space Telescope orbits Earth, above our atmosphere.
But telescopes have limits and we wanted to know more. As technology developed, NASA
sent spacecraft to our Moon.... and then to other planets.
Next: The Pioneer Missions 
