Hi Jenny,
Scientists suspect that Stonehenge may have been used as a sort of observatory because of the alignments of some of the stones with the positions of the Sun and the Moon at certain times of the year. The main axis of Stonehenge is roughly aligned with the location where the Sun rises on the day of the summer solstice. The summer solstice was an important time in the religions of many ancient people around the world. However, the alignment of the axis of Stonehenge is not exact. Although Stonehenge could have been an important location for ancient religious rituals related to the motions of the Sun and Moon in the sky, it is unlikely that the people who built Stonehenge used it to make precise and accurate astronomical observations the way scientists do today.
For more information about Stonehenge, follow the links below.
http://www.english-heritage.org.uk/stonehenge/
http://www.windows.ucar.edu/tour/link=/the_universe/uts/stonehenge_astro.html