spectrum. Of these, all but the Compton are still in orbit and active. NASA is planning for
another telescope, the James Webb Space Telescope, to serve as a replacement for the aging
Hubble. The James Webb is scheduled to launch no earlier than 2013.
Figure 23.12: NASA’s four space based Great Observatories were designed to view the
universe in different ranges of the electromagnetic spectrum. A. Hubble Space Telescope:
visible light, B. Compton Gamma Ray Observatory: gamma ray, C. Spitzer Space Telescope:
infrared, D. Chandra X-ray Observatory: X-ray. ( 26 )
Observations with Telescopes
Ancient Astronomers
Humans have been studying the night sky for thousands of years. Observing the patterns
and motions in the sky helped ancient peoples keep track of time. This was important to
them because it helped them know when to plant crops. They also timed many of their
religious ceremonies to coincide with events in the heavens.
The ancient Greeks made careful observations of the locations of stars in the sky. They
noticed that some of what they thought were ’stars’ moved against the background of other