Global Positioning System Reference
In-Depth Information
CHAPTER ONE
Earth and Its Orbit
Our subject is navigating over the surface of the earth, and so, to set the table,
I begin with some basic facts about our planet and about the influence of its
nearest neighbors on it.
Getting to Know Your Neighborhood
Looking at the solar system from a spaceship, far away, you can easily pick
out your planet—it's the small blue-green one (volume: about a million
million cubic kilometers), third from the sun. In case you do not know
what it looks like (thanks to modern remote sensing, you almost certainly
do know), a somewhat closer view of the earth is provided in figure 1.1. The
colors, owing to the presence of water and life, are what distinguish Earth
from the other planets. These two features have also sculpted the earth's
surface over eons into the hugely varied landscapes and seascapes that we
see today. Suppose that your spaceship is positioned in space so that you
see the sun rotating clockwise about its axis. Almost all of the planets,
satellites, asteroids, and comets that orbit the sun move in a clockwise
manner. That is, you will see the planets orbiting clockwise about the sun
and each satellite, such as our moon, orbiting clockwise about its planet.
Also, most of the planets rotate about their axis in the same clockwise
direction.
Of course, the common direction of rotation is due to the way that the
solar system formed, about 4,568 million years ago.
1
Over the eons, the
axis of the earth's orbit has changed direction, in a regular and predictable
1. The quoted age of the solar system is a recent estimate; see Bouvier and Wadhwa
(2010).
