612 APPENDIX B
Observing the sky with the naked eye is as important to
modern astronomy as picking up pretty pebbles is to modern
geology. Nevertheless, the sky is inspiring—a natural wonder
unimaginably bigger than the Grand Canyon, the Rocky
Mountains, or any other site that tourists visit every year. To
neglect the beauty of the sky is equivalent to geologists neglecting
the beauty of the minerals they study. Th is supplement is meant
to act as a tourist’s guide to the sky. You analyzed the universe in
the textbook’s chapters; here you can admire it.
Th e brighter stars in the sky are visible even from the centers
of cities with their air and light pollution. But in the countryside,
only a few miles beyond the cities, the night sky is a velvety
blackness strewn with thousands of glittering stars. From a wil-
derness location, far from the city’s glare, and especially from
high mountains, the night sky is spectacular.
Using Star Charts
The constellations are a fascinating cultural heritage of
our planet, but they are sometimes a bit diffi cult to learn because
of Earth’s motion. Th e constellations above the horizon change
with the time of night and the seasons.
Because Earth rotates eastward, the sky appears to rotate
westward around Earth. A constellation visible overhead soon
after sunset will appear to move westward, and in a few hours it
will disappear below the horizon. Other constellations will rise in
the east, so the sky changes gradually through the night.
In addition, Earth’s orbital motion makes the sun appear to
move eastward among the stars. Each day the sun moves about
twice its own diameter, or about one degree, eastward along the
ecliptic; consequently, each night at sunset, the constellations are
shifted about one degree farther toward the west.
Orion, for instance, is visible in the evening sky in January;
but, as the days pass, the sun moves closer to Orion. By March,
Orion is diffi cult to see in the western sky soon after sunset. By
June, the sun is so close to Orion it sets with the sun and is invis-
ible. Not until late July is the sun far enough past Orion for the
constellation to become visible rising in the eastern sky just
before dawn.
Because of the rotation and orbital motion of Earth, you
need more than one star chart to map the sky. Which chart you
select depends on the month and the time of night.
Two sets of charts are included for two typical locations on
Earth. Th e Northern Hemisphere star charts show the sky as seen
from a northern latitude typical for the United States and
Central Europe. Th e Southern Hemisphere star charts are appro-
priate for readers in Earth’s Southern Hemisphere, including
Australia, southern South America, and southern Africa.
To use the charts, select the appropriate chart and hold it
overhead as shown in ■ Figure B-1. If you face south, turn the
chart until the words Southern Horizon are at the bottom of the
chart. If you face other directions, turn the chart appropriately.Appendix B
Observing the Sky
■Figure B-1
To use the star charts in this book, select the appropriate chart for the sea-
son and time. Hold it overhead and turn it until the direction at the bottom
of the chart is the same as the direction you are facing.