CHAPTER 1 | HERE AND NOW 3
Enlarge your fi eld of view by a factor of 100, and you see a
region 1,600,000 km wide (■ Figure 1-5). Earth is the small blue
dot in the center, and the moon, whose diameter is only one-
fourth that of Earth, is an even smaller dot along its orbit
380,000 km away.
Th ese numbers are so large that it is inconvenient to write
them out. Astronomy is sometimes known as the science of big
numbers, and soon you will be using numbers much larger than
these to discuss the universe. Rather than writing out these num-
bers as in the previous paragraph, it is more convenient to write
them in scientifi c notation. Th is is nothing more than a simple
way to write very big or very small numbers without using lots
of zeros. In scientifi c notation, 380,000 becomes 3.8 × 10^5. If you
are not familiar with scientifi c notation, read the section on
powers of 10 notation in Appendix A. Th e universe is too big to
discuss without using scientifi c notation.
When you once again enlarge your fi eld of view by a factor
of 100, Earth, the moon, and the moon’s orbit all lie in the small
red box at lower left of ■ Figure 1-6. Now you can see the sun
and two other planets that are part of our solar system. Our solar
system consists of the sun, its family of planets, and some smaller
bodies, such as moons and comets.
Earth, Venus, and Mercury are planets, small, spherical,
nonluminous bodies that orbit a star and shine by refl ected light.
Venus is about the size of Earth, and Mercury is just over a third
of Earth’s diameter. On the diagram, they are both too small to
be seen as anything but tiny dots. Th e sun is a star, a self-luminousvisual clues to understanding a concept. Special questions called
“Learning to Look” at the end of each chapter give you a chance
to use your own imagination to connect images with theories
about astronomical objects.
■ Figure 1-
NASA
■ Figure 1-
NASA
Earth MoonEarth MoonEnlarged to show
relative size■ Figure 1-
NASA